Amino acids are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins, of which there are countless varieties. Of the 20 amino acids required for manufacturing the proteins the human body needs, the body itself produces only 12, meaning that we have to meet our requirements for the other eight through nutrition. This is just one example of the importance of amino acids in the functioning of life. Another cautionary illustration of amino acids' power is the gamut of diseases (most notably, sickle cell anemia) that impair or claim the lives of those whose amino acids are out of sequence or malfunctioning. Once used in dating objects from the distant past, amino acids have existed on Earth for at least three billion years—long before the appearance of the first true organisms.
HOW IT WORKS
A "Map" of Amino Acids
Amino acids are organic compounds, meaning that they contain carbon and hydrogen bonded to each other. In addition to those two elements, they include nitrogen, oxygen, and, in a few cases, sulfur. The basic structure of an amino-acid molecule consists of a carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a fourth group that differs from one amino acid to another and often is referred to as the-R group or the side chain. The-R group, which can vary widely, is responsible for the differences in chemical properties.
This explanation sounds a bit technical and requires a background in chemistry that is beyond the scope of this essay, but let us simplify it somewhat. Imagine that the amino-acid molecule is like the face of a compass, with a carbon atom at the center. Raying out from the center, in the four directions of the compass, are lines representing chemical bonds to other atoms or groups of atoms. These directions are based on models that typically are used to represent amino-acid molecules, though north, south, east, and west, as used in the following illustration, are simply terms to make the molecule easier to visualize.
To the south of the carbon atom (C) is a hydrogen atom (H), which, like all the other atoms or groups, is joined to the carbon center by a chemical bond. To the north of the carbon center is what is known as an amino group (-NH2). The hyphen at the beginning indicates that such a group does not usually stand alone but normally is attached to some other atom or group. To the east is a carboxyl group, represented as-COOH. In the amino group, two hydrogen atoms are bonded to each other and then to nitrogen, whereas the carboxyl group has two separate oxygen atoms strung between a carbon atom and a hydrogen atom. Hence, they are not represented as O2.
Finally, off to the west is the R -group, which can vary widely. It is as though the other portions of the amino acid together formed a standard suffix in the English language, such as -tion. To the front of that suffix can be attached all sorts of terms drawn from root words, such as educate or satisfy or revolt —hence, education, satisfaction, and revolution. The variation in the terms attached to the front end is extremely broad, yet the tail end, -tion, is a single formation. Likewise the carbon, hydrogen, amino group, and carboxyl group in an amino acid are more or less constant.
A FEW ADDITIONAL POINTS.
The name amino acid, in fact, comes from the amino group and the acid group, which are the most chemically reactive parts of the molecule. Each of the common amino acids has, in addition to its chemical name, a more familiar name and a three-letter abbreviation that frequently is used to identify it. In the present context, we are not concerned with these abbreviations. Amino-acid molecules, which contain an amino group and a carboxyl group, do not behave like typical molecules. Instead of melting at temperatures hotter than 392°F (200°C), they simply decompose. They are quite soluble, or capable of being dissolved, in water but are insoluble in nonpolar solvents (oil-and all oil-based products), such as benzene or ether.
RIGHT-HAND AND LEFT-HAND VERSIONS.
All of the amino acids in the human body, except glycine, are either right-hand or left-hand versions of the same molecule, meaning that in some amino acids the positions of the carboxyl group and the R -group are switched. Interestingly, nearly all of the amino acids occurring in nature are the left-hand versions of the molecules, or the L-forms. (There-fore, the model we have described is actually the left-hand model, though the distinctions between "right" and "left"—which involve the direction in which light is polarized—are too complex to discuss here.)
Right-hand versions (D-forms) are not found in the proteins of higher organisms, but they are present in some lower forms of life, such as in the cell walls of bacteria. They also are found in some antibiotics, among them, streptomycin, actinomycin, bacitracin, and tetracycline. These antibiotics, several of which are well known to the public at large, can kill bacterial cells by interfering with the formation of proteins necessary for maintaining life and for reproducing.
Amino Acids and Proteins
A chemical reaction that is characteristic of amino acids involves the formation of a bond, called a peptide linkage, between the carboxyl group of one amino acid and the amino group of a second amino acid. Very long chains of amino acids can bond together in this way to form proteins, which are the basic building blocks of all living things. The specific properties of each kind of protein are largely dependent on the kind and sequence of the amino acids in it. Other aspects of the chemical behavior of protein molecules are due to interactions between the amino and the carboxyl groups or between the various R -groups along the long chains of amino acids in the molecule.
NUMBERS AND COMBINATIONS.
Amino acids function as monomers, or individual units, that join together to form large, chainlike molecules called polymers, which may contain as few as two or as many as 3,000 amino-acid units. Groups of only two amino acids are called dipeptides, whereas three amino acids bonded together are called tripeptides. If there are more than 10 in a chain, they are termed polypeptides, and if there are 50 or more, these are known as proteins.
All the millions of different proteins in living things are formed by the bonding of only 20 amino acids to make up long polymer chains. Like the 26 letters of the alphabet that join together to form different words, depending on which letters are used and in which sequence, the 20 amino acids can join together in different combinations and series to form proteins. But whereas words usually have only about 10 or fewer letters, proteins typically are made from as few as 50 to as many as 3,000 amino acids. Because each amino acid can be used many times along the chain and because there are no restrictions on the length of the chain, the number of possible combinations for the formation of proteins is truly enormous. There are about two quadrillion different proteins that can exist if each of the 20 amino acids present in humans is used only once. Just as not all sequences of letters make sense, however, not all sequences of amino acids produce functioning proteins. Some other sequences can function and yet cause undesirable effects, as we shall see.
DNA (deoxyribonucleic acid), a molecule in all cells that contains genetic codes for inheritance, creates encoded instructions for the synthesis of amino acids. In 1986, American medical scientist Thaddeus R. Dryja (1940-) used amino-acid sequences to identify and isolate the gene for a type of cancer known as retinoblastoma, a fact that illustrates the importance of amino acids in the body.
Amino acids are also present in hormones, chemicals that are essential to life. Among these hormones is insulin, which regulates sugar levels in the blood and without which a person would die. Another is adrenaline, which controls blood pressure and gives animals a sudden jolt of energy needed in a high-stress situation—running from a predator in the grasslands or (to a use a human example) facing a mugger in an alley or a bully on a playground. Biochemical studies of amino-acid sequences in hormones have made it possible for scientists to isolate and produce artificially these and other hormones, including the human growth hormone.
Amino Acids and Nutrition
Just as proteins form when amino acids bond together in long chains, they can be broken down by a reaction called hydrolysis, the reverse of the formation of the peptide bond. That is exactly what happens in the process of digestion, when special digestive enzymes in the stomach enable the breaking down of the peptide linkage. (Enzymes are a type of protein—see Enzymes.) The amino acids, separated once again, are released into the small intestine, from whence they pass into the bloodstream and are carried throughout the organism. Each individual cell of the organism then can use these amino acids to assemble the new and different proteins required for its specific functions. Life thus is an ongoing cycle in which proteins are broken into individual amino-acid units, and new proteins are built up from these amino acids.
ESSENTIAL AMINO ACIDS.
Out of the many thousands of possible amino acids, humans require only 20 different kinds. Two others appear in the bodies of some animal species, and approximately 100 others can be found in plants. Considering the vast numbers of amino acids and possible combinations that exist in nature, the number of amino acids essential to life is extremely small. Yet of the 20 amino acids required by humans for making protein, only 12 can be produced within the body, whereas the other eight—isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—must be obtained from the diet. (In addition, adults are capable of synthesizing arginine and histidine, but these amino acids are believed to be essential to growing children, meaning that children cannot produce them on their own.)
A complete protein is one that contains all of the essential amino acids in quantities sufficient for growth and repair of body tissue. Most proteins from animal sources, gelatin being the only exception, contain all the essential amino acids and are therefore considered complete proteins. On the other hand, many plant proteins do not contain all of the essential amino acids. For example, lysine is absent from corn, rice, and wheat, whereas corn also lacks tryptophan and rice lacks threonine. Soybeans are lacking in methionine. Vegans, or vegetarians who consume no animal proteins in their diets (i.e., no eggs, dairy products, or the like) are at risk of malnutrition, because they may fail to assimilate one or more essential amino acid.
Amino Acids, Health, and Disease
Amino acids can be used as treatments for all sorts of medical conditions. For example, tyrosine may be employed in the treatment of Alzheimer's disease, a condition characterized by the onset of dementia, or mental deterioration, as well as for alcohol-withdrawal symptoms. Taurine is administered to control epileptic seizures, treat high blood pressure and diabetes, and support the functioning of the liver. Numerous other amino acids are used in treating a wide array of other diseases. Sometimes the disease itself involves a problem with amino-acid production or functioning. In the essay Vitamins, there is a discussion of pellagra, a disease resulting from a deficiency of the B-group vitamin known as niacin. Pellagra results from a diet heavy in corn, which, as we have noted, lacks lysine and tryptophan. Its symptoms often are described as the "three Ds": diarrhea, dermatitis (or skin inflammation), and dementia. Thanks to a greater understanding of nutrition and health, pellagra has been largely eradicated, but there still exists a condition with almost identical symptoms: Hartnup disease, a genetic disorder named for a British family in the late 1950s who suffered from it.
Hartnup disease is characterized by an inability to transport amino acids from the kidneys to the rest of the body. The symptoms at first seemed to suggest to physicians that the disease, which is present in one of about 26,000 live births, was pellagra. Tests showed that sufferers did not have inadequate tryptophan levels, however, as would have been the case with pellagra. On the other hand, some 14 amino acids have been found in excess within the urine of Hartnup disease sufferers, indicating that rather than properly transporting amino acids, their bodies are simply excreting them. This is a potentially very serious condition, but it can be treated with the B vitamin nicotinamide, also used to treat pellagra. Supplementation of tryptophan in the diet also has shown positive results with some patients.
SICKLE CELL ANEMIA.
It is also possible for small mistakes to occur in the amino-acid sequence within the body. While these mistakes sometimes can be tolerated in nature without serious problems, at other times a single misplaced amino acid in the polymer chain can bring about an extremely serious condition of protein malfunctioning. An example of this is sickle cell anemia, a fatal disease ultimately caused by a single mistake in the amino acid sequence. In the bodies of sickle cell anemia sufferers, who are typically natives of sub-Saharan Africa or their descendants in the United States or elsewhere, glutamic acid is replaced by valine at the sixth position from the end of the protein chain in the hemoglobin molecule. (Hemoglobin is an iron-containing pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them.) This small difference makes sickle cell hemoglobin molecules extremely sensitive to oxygen deficiencies. As a result, when the red blood cells release their oxygen to the tissues, as all red blood cells do, they fail to re-oxygenate in a normal fashion and instead twist into the shape that gives sickle cell anemia its name. This causes obstruction of the blood vessels. Before the development of a treatment with the drug hydroxyurea in the mid-1990s, the average life expectancy of a person with sickle cell anemia was about 45 years.
Amino Acids and the Distant Past
The Evolution essay discusses several types of dating, a term referring to scientific efforts directed toward finding the age of a particular item or phenomenon. Methods of dating are either relative (i.e., comparative and usually based on rock strata, or layers) or absolute. Whereas relative dating does not involve actual estimates of age in years, absolute dating does. One of the first types of absolute-dating techniques developed was amino-acid racimization, introduced in the 1960s. As noted earlier, there are "left-hand" l-forms and "right-hand" d-forms of all amino acids. Virtually all living organisms (except some microbes) incorporate only the l-forms, but once the organism dies, the l-amino acids gradually convert to the mirror-image d-amino acids.
Numerous factors influence the rate of conversion, and though amino-acid racimization was popular as a form of dating in the 1970s, there are problems with it. For instance, the process occurs at different rates for different amino acids, and the rates are further affected by such factors as moisture and temperature. Because of the uncertainties with amino-acid racimization, it has been largely replaced by other absolute-dating methods, such as the use of radioactive isotopes.
Certainly, amino acids themselves have offered important keys to understanding the planet's distant past. The discovery, in 1967 and 1968, of sedimentary rocks bearing traces of amino acids as much as three billion years old had an enormous impact on the study of Earth's biological history. Here, for the first time, was concrete evidence of life—at least, in a very simple chemical form—existing billions of years before the first true organism. The discovery of these amino-acid samples greatly influenced scientists' thinking about evolution, particularly the very early stages in which the chemical foundations of life were established.
WHERE TO LEARN MORE
"Amino Acids." Institute of Chemistry, Department of Biology, Chemistry, and Pharmacy, Freie Universität, Berlin (Web site). <http://www.chemie.fu-berlin.de/chemistry/bio/amino-acids_en.html>.
Goodsell, David S. Our Molecular Nature: The Body's Motors, Machines, and Messages. New York: Copernicus, 1996.
"Introduction to Amino Acids." Department of Crystallography, Birbeck College (Web site). <http://www.cryst.bbk.ac.uk/education/AminoAcid/overview.html>.
Michal, Gerhard. Biochemical Pathways: An Atlas of Biochemistry and Molecular Biology. New York: John Wiley and Sons, 1999.
Newstrom, Harvey. Nutrients Catalog: Vitamins, Minerals, Amino Acids, Macronutrients—Beneficial Use, Helpers, Inhibitors, Food Sources, Intake Recommendations, and Symptoms of Over or Under Use. Jefferson, NC: McFarland and Company, 1993.
Ornstein, Robert E., and Charles Swencionis. The Healing Brain: A Scientific Reader. New York: Guilford Press, 1990.
Reference Guide for Amino Acids (Web site). <http://www.realtime.net/anr/aminoacd.html#tryptophn>.
Silverstein, Alvin, Virginia B. Silverstein, and Robert A. Silverstein. Proteins. Illus. Anne Canevari Green. Brookfield, CT: Millbrook Press, 1992.
Springer Link: Amino Acids (Web site). <http://link.springer.de/link/service/journals/00726/>.
Organic compounds made of carbon, hydrogen, oxygen, nitro gen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins.
The chemical forma tion NH2, which is part of all amino acids.
The area of the bio logical sciences concerned with the chemical substances and processes in organisms.
The formation COOH, which is common to all amino acids.
A substance in which atoms of more than one element are bond ed chemically to one another.
A group of only two amino acids.
Deoxyribonucleic acid, a molecule in all cells and many viruses containing genetic codes for inheritance.
A protein material that speeds up chemical reactions in the bodies of plants and animals.
ESSENTIAL AMINO ACIDS:
Amino acids that cannot be manufactured by the body, and which therefore must be obtained from the diet. Proteins that contain essential amino acids are known as complete proteins.
A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes.
Molecules produced by living cells, which send signals to spots remote from their point of origin and induce specific effects on the activities of other cells.
A group of atoms, usually but not always representing more than one element, joined in a structure. Compounds typically are made up of molecules.
At one time chemists used the term organic only in reference to living things. Now the word is applied to compounds containing carbon and hydrogen.
A bond between the carboxyl group of one amino acid and the amino group of a second amino acid.
Large, chainlike molecules composed of numerous subunits known as monomers.
A group of between 10 and 50 amino acids.
Large molecules built from long chains of 50 or more amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes.
Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.
To manufacture chemically, as in the body.
A group of three amino acids.
"Amino Acids." Science of Everyday Things. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/amino-acids
"Amino Acids." Science of Everyday Things. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/amino-acids
Amino acids are known as the building blocks of protein, and are defined as the group of nitrogen-containing organic compounds composing the structure of proteins. They are essential to human metabolism, and to making the human body function properly for good health. Of the 28 amino acids known to exist, eight of them are considered "essential," defined as those that can be obtained only through food. These essential amino acids are tryptophan, lysine, methionine , phenylalaine, threonine, valine, leucine, and isoleucine. The "non–essential" amino acids include arginine , tyrosine, glycine, serine, glutmamic acid, aspartic acid, taurine, cycstine, histidine, proline, alanine, and creatine , which is a combination of arginine, glycine, and methionine.
The human body, minus water, is 75% amino acids. All of the neurotransmitters (proteins) but one are composed of amino acids; and 95% of hormones are amino acids. Amino acids are key to every human bodily function with every chemical reaction that occurs.
Amino acids occur naturally in certain foods, such as dairy products, meats, fish, poultry, nuts, legumes, and eggs. Those sources are considered more complete than vegetable protein, such as beans, peas, and grains, also considered a good—even if not complete—source of amino acids.
Amino acids became popular as dietary supplements by the end of the twentieth century for various uses, including fitness training, weight loss, and certain chronic diseases. Claims exist in holistic medicine that indicate amino acid supplements taken in the proper dosage can aid also in fighting depression, allergies, heart disease , gastrointestinal problems, high cholesterol , muscle weakness, blood sugar problems, arthritis, insomnia , bipolar illness, epilepsy, chronic fatigue syndrome, autism, attention-deficit hyperactivity disorder (ADHD), and mental exhaustion.
Amino acid therapy as a supplemental aid to a healthy diet joined the fitness craze in the United States by the end of the 1990s. According to author Brenda Adderly in Better Nutrition, in September of 1999, "The creation of new protein from amino acids and the breaking down of existing protein into amino acids are ongoing processes in our bodies. If, for example, you are working out and developing certain muscles, amino acids come to the rescue with new protein to build muscle cells," Adderly noted. "Similarly, when you eat a complete protein, such as meat or beans and rice, the body breaks down the amino acids in that food for later use." Understanding the balance of amino acids in the body can be often the first clue to understanding why a person suffers many ailments, ranging from depression to upset stomach to obesity . Deficiencies in the proper balance of amino acids is likely to occur in those with poor diets . Because stress , age, infection, and various other factors including the amount of exercise a person does, can also affect the levels of amino acids, people with healthy, nutritious diets could also find that they also suffer deficiencies. Adderly adds that, "Not only are the symptoms of amino acid deficiencies wide ranging, but there are no RDAs (recommended daily allowances) or other guidelines, to help us tell if we are least covering all the bases. Add to that the complicated matter of keeping track of all 28 some with names most of us have never even heard and the situation begins to seem overwhelming."
Essential amino acids
The amino acids, which are derived only from food and that the body cannot manufacture, perform various functions.
- Tryptophan. This is considered a natural relaxant, helps alleviate insomnia; helps in the treatment of migraine headaches; helps reduce the risk of artery and heart spasms; and works with lysine to reduce cholesterol levels.
- Lysine. Aids in proper absorption of calcium ; helps form collagen for bone cartilage and connective tissues; aids in production of antibodies, hormones, and enzymes. Research has indicated it also might be effective against herpes by creating the balance of nutrients that slows the growth of the virus causing it. A deficiency could result in fatigue , lack of concentration, irritability, bloodshot eyes, retarded growth, hair loss, anemia , and reproductive problems.
- Methionine. Properties include providing the primary source of sulfur that can prevent disorders of the hair, skin, and nails; lowers cholesterol by increasing the liver's production of lecithin ; reduces liver fat; protects kidneys; and promotes hair growth.
- Phenylalaine. This serves the brain by producing norepinephrine, the chemical that is responsible for transmitting the signals between the nerve cells and the brain; can maintain alertness; reduces hunger pains; acts as an antidepressant; and improves memory.
- Threonine. Makes up a substantial portion of the collagen, elastin, and enamel protein; serves the liver by preventing buildup; aids the digestive and intestinal tracts to function better; and acts as a trigger for metabolism.
- Valine. Promotes mental energy; helps with muscle coordination; and serves as a natural tranquilizer.
- Leucine. Works with isoleucine to provide for the manufacture of essential biochemical processes in the body that are used for energy, increasing the stimulants to the upper brain for greater mental alertness.
Roles of certain non–essential amino acids
- Glycine. Facilitates the release of oxygen for the cell–making process; key role in manufacturing of hormones and health of immune system.
- Serine. Source of glucose storage by the liver and muscles; provides antibodies for immune system; synthesizes fatty acid sheath around nerve fibers.
- Glutamic acid. Nature's "brain food" that increases mental prowess; helps speed the healing of ulcers; aids in combatting fatigue.
Creatine in the spotlight
One of the most discussed amino acid supplements available on the market is creatine monohydrate. The body produces small amounts of creatine in the kidneys, liver, and pancreas, making it a non-essential acid. With most diets that include red meat or fish, also come a few grams of creatine. It is stored in muscle cells and is used in activities, such as weight lifting and sprinting, providing the necessary thrust of energy for such activities. But the natural supply of creatine produced by the body is quickly depleted. After approximately 10 seconds, when muscle fatigue becomes apparent, the daily production is used.
According to Timothy Gower, writing for Esquire in February of 1998, "Scientists identified creatine 160– odd years ago, but only in the 1980s did they figure out that muscle cells can be 'loaded' with up to 30% more of the compound than they normally carry. Since then, several studies have shown that weight lifters primed on the supplement tire less easily, allowing them to work out longer." Gower also noted that creatine users find that the weight they add on is fat-free, whether that is lean tissue or some is water weight, no one has yet determined, since muscle cells do fill with water during creatine loading. Additionally, while it can add to the burst of the energy a sprinter needs to perform well, creatine does not do anything for the marathon runner going for several hours.
Commercially available since 1993, the long-term effects still remain unknown. One 2002 study did show that creatine use improved rehabilitation for injured athletes and another has shown that using the supplement does not increase risk of injury. It should be noted that some 20–30% of people researched showed no improvement using creatine. One early report indicated that creatine could be beneficial for some people in spurring metabolism, burning calories and helping in weight loss. Those reports were as yet inconclusive.
Amino acid supplements to a healthy diet are used for various purposes. The most common uses include: sustaining strength in weight training to build muscles; improving heart and circulatory problems or diseases, particularly in the aging ; the treatment of chronic fatigue syndrome; treating depression and anxiety ; treating eating disorders, such as bulimia and/or anorexia, along with overeating; increasing memory; building up and sustaining the body's immune system in fighting bacteria and viruses. It is important to note that, while the necessity and role of all amino acids has been verified in the maintenance of optimum health, research is not extensive enough to provide indisputable verification of the touted benefits of such supplements over the long term.
Nonetheless, some members of the scientific medical community would seem to confirm what amino acid proponents have long believed to be true. One such study from the Journal of the American College of Cardiology brought good news for the millions suffering from chronic heart failure. Dr. Rainer Hambrecht and colleagues from the University of Leipzig, (Germany) tested the amino acid L-arginine on 38 heart-failure patients. Knowing that the human body converted it into nitric oxide, a chemical that relaxes blood vessels, the researchers gave one group 8 g of it daily for four weeks; another group simply did forearm exercises; and a third group combined the supplement with the exercise. The people who took the supplement alone increased their blood-vessel dilation by a factor of four, as did the exercise group. Those who took both the supplement and performed the exercise increased it by six. More recent studies on arginine in 2002 found that the supplement may help reduce risk of postoperative infections . Further, arginine may enhance women's sexual function.
Supplements are recommended by alternative medical practitioners particularly for those who are not getting a proper diet, especially vegetarians who might not be getting a balance of complete protein, as well as athletes, anyone under severe stress, and anyone whose alcohol intake level is moderate to high.
Supplements of various amino acids are available primarily in capsule, tablet, or powder form. A common way of taking amino acids is in a "multiple" amino acid gel cap. These contain sources of protein from gelatin, soy, and whey. The market for supplements in wholesale, retail, and internet sales was estimated to reach into the millions of dollars, with literally hundreds available. Internet sales were a fast-growing area particularly with the use of such supplements as creatine powder publicized by well-known Olympic stars and professional athletes. Daily usage of creatine as evident from research indicated that usage should be leveled at 5 g of powder in a glass of orange juice, and could be taken up to four times a day during peak athletic training. Maintenance dosages were recommended at 5 g once a day.
Because amino acids are naturally produced substances both in the human body and in the protein derived from animal and dairy products, as well as being present in food combinations such as beans and rice, such supplements are not regulated by the United States Food and Drug Administration (FDA), nor are there any specified daily requirements, and they also do not show up in either drug or urine tests. Amino acid supplements might be classified as having no affect at all. Long-term effects were not yet evident, however, due to the relatively recent phenomenon of use.
Interactions of amino acids with drugs has not been sufficiently studied to determine yet if any adverse effects result from using amino acids with medications.
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Dolby, Victoria. "Anxiety? Send herbs, 5–HTP, and amino acids to the rescue!" Better Nutrition (June 1998). Available from http://web2.infotrac.galegroup.com.
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Teresa G. Odle
"Amino Acids." Gale Encyclopedia of Alternative Medicine. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/amino-acids-0
"Amino Acids." Gale Encyclopedia of Alternative Medicine. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/amino-acids-0
Amino acids are the building blocks of protein . The body has twenty different amino acids that act as these building blocks. Nonessential amino acids are those that the body can synthesize for itself, provided there is enough nitrogen, carbon, hydrogen, and oxygen available. Essential amino acids are those supplied by the diet , since the human body either cannot make them at all or cannot make them in sufficient quantity to meet its needs. Under normal conditions, eleven of the amino acids are nonessential and nine are essential.
All amino acids have a similar chemical structure—each contains an amino group (NH2), an acid group (COOH), a hydrogen atom (H), and a distinctive side group that makes proteins more complex than either carbohydrates or lipids . All amino acids are attached to a central carbon atom (C).
The distinctive side group identifies each amino acid and gives it characteristics that attract it to, or repel it from, the surrounding fluids and other amino acids. Some amino acid side groups carry electrical charges that are attracted to water molecules (hydrophilic), while others are neutral and are repelled by water (hydrophobic). Side-group characteristics (shape, size, composition, electrical charge, and pH ) work together to determine each protein's specific function.
|Essential amino acids||Nonessential amino acids|
The three-dimensional shape of proteins is derived from the sequence and properties of its amino acids and determines its function and interaction with other molecules. Each amino acid is linked to the next by a peptide bond, the name given to the link or attraction between the acid (COOH) end of one amino acid and the amino end (NH2) of another. Proteins of various lengths are made when amino acids are linked together in this manner. A dipeptide is two amino acids joined by a peptide bond, while a tripeptide is three amino acids joined by peptide bonds.
The unique shapes of proteins enable them to perform their various tasks in the body. Heat, acid, or other conditions can disturb proteins, causing them to uncoil or lose their shape and impairing their ability to function. This is referred to as denaturation.
Functions of Proteins
Proteins act as enzymes , hormones , and antibodies . They maintain fluid balance and acid and base balance. They also transport substances such as oxygen, vitamins , and minerals to target cells throughout the body. Structural proteins, such as collagen and keratin, are responsible for the formation of bones, teeth, hair, and the outer layer of skin, and they help maintain the structure of blood vessels and other tissues. In contrast, motor proteins use energy and convert it into some form of mechanical work (e.g., dividing cells, contracting muscle).
Enzymes are proteins that facilitate chemical reactions without being changed in the process. The inactive form of an enzyme is called a proenzyme. Hormones (chemical messengers) are proteins that travel to one or more specific target tissues or organs, and many have important regulatory functions. Insulin , for example, plays a key role in regulating the amount of glucose in the blood. The body manufactures antibodies (giant protein molecules), which combat invading antigens. Antigens are usually foreign substances such as bacteria and viruses that have entered the body and could potentially be harmful. Immunoproteins, also called immunoglobulins or antibodies, defend the body from possible attack by these invaders by binding to the antigens and inactivating them.
Proteins help to maintain the body's fluid and electrolyte balance. This means that proteins ensure that the proper types and amounts of fluid and minerals are present in each of the body's three fluid compartments. These fluid compartments are intracellular (contained within cells), extracellular (existing outside the cell), and intravascular (in the blood). Without this balance, the body cannot function properly.
Proteins also help to maintain balance between acids and bases within body fluids. The lower a fluid's pH, the more acidic it is. Conversely, the higher the pH, the less acidic the fluid is. The body works hard to keep the pH of the blood near 7.4 (neutral). Proteins also act as carriers, transporting many important substances in the bloodstream for delivery throughout the body. For example, a lipoprotein transports fat and cholesterol in the blood.
Humans consume many foods that contain proteins or amino acids. One normally need not worry about getting enough protein or amino acids in the typical American diet. Foods from animal sources are typically rich in essential amino acids. These include chicken, fish, eggs, dairy products, beef, and pork. With the increasing emphasis on vegetarian diets, plant sources of protein are gaining in popularity. Such sources include dried beans (black, kidney, northern, red, and white beans), peas, soy, nuts, and seeds. Although plant sources generally lack one or more of the essential amino acids, when combined with whole grains such as rice, or by eating nuts or seeds with legumes , all the amino acids can be obtained.
see also Diet; Fats; Malnutrition; Nutrients; Plant-Based Diets; Protein.
Susan P. Himburg
Insel, Paul; Turner, R.; and Ross, Don (2001). Nutrition. Sudbury, MA: Jones and Bartlett.
Whitney, Eleanor N., and Rolfes, Sharon R. (2002). Understanding Nutrition, 9th edition. Belmont, CA: Wadsworth Group.
"Amino Acids." Nutrition and Well-Being A to Z. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/food/news-wires-white-papers-and-books/amino-acids
"Amino Acids." Nutrition and Well-Being A to Z. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/food/news-wires-white-papers-and-books/amino-acids
Amino acids are described as essential or non-essential. The non-essential ones can be synthesized in the body but the essential amino acids are those which must be present in the diet (phenylalanine, valine, tryptophan, threonine, lysine, leucine, isoleucine, and methionine). If any one of these amino acids is missing from the diet then many proteins which include this essential component cannot be synthesized. Consequently many other amino acids cannot then be used; they are broken down (deaminated) and the nitrogen is excreted as urea and creatinine, leading to a negative nitrogen balance, as more nitrogen is excreted than is taken in as dietary protein.
The adult body cannot absorb whole proteins from the gut, although young babies are able to absorb antibodies, which are proteins, from mother's milk; this provides passive immunity for the first year or so of life. The digestive processes break down dietary protein to amino acids and small peptides (two or more linked amino acids). Carriers, specific for a single amino acid or a group of similar amino acids, are present in the cells lining the intestine and are responsible for the specific uptake into these cells. Some dipeptides (and maybe tripeptides) also have specialized carrier molecules for uptake in the intestine, and the final stage of their digestion to amino acids takes place in these epithelial cells themselves. Thence they move into the circulating blood; thus amino acids from the diet enter the body's amino acid pool, mixing with other amino acids derived from the breakdown of body proteins in the continual turnover associated with growth, repair, and renewal of tissues. Cells of the different tissues take up selectively from the blood whichever amino acids they need for synthesis of their own proteins. The circulating amino acids gained from digestion are in no great danger of excretion via the kidneys: they are filtered at the glomeruli but are mostly reabsorbed into the blood as they pass down the kidney tubules.
Finally, how is the dietary intake of protein linked to the need for amino acids, particularly the essential ones? The linkage need not be a strong one, as connections exist between the metabolism of amino acids and the metabolism of fats and carbohydrates. Further, there can be conversion of one amino acid to another, at least for the non-essential amino acids. These transamination reactions are common in tissues that have been damaged, as repair and resynthesis take place. Thus after a myocardial infarction the level of the relevant enzymes — transaminases — rises in the blood, and this measurement is used for diagnostic purposes. Excess amino acids are subject to oxidative deamination: the amino group is removed and excreted as nitrogen products and the residue converted either to a ketone body, called acetoacetic acid (one of the products also of fat metabolism), or to products readily converted to glucose. Amino acids are there-fore divided into ketogenic or gluconeogenic (conversion to glucose) types.
Nitrogen losses in the urine may be greater than the nitrogen intake in the diet (negative nitrogen balance) not only when the essential amino acids are missing, but also when the calorie intake is adequate but the overall protein content of the diet is too low; this occurs in kwashiorkor, common in poorly nourished children. If the diet is inadequate in calories as well as deficient in protein, body proteins are broken down to form glucose for energy. This can be prevented by giving glucose, which is thus said to be ‘protein-sparing’.
Alan W. Cuthbert
See also peptides; proteins.
"amino acids." The Oxford Companion to the Body. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/amino-acids
"amino acids." The Oxford Companion to the Body. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/amino-acids
In 1953, Harold Urey and Stanley Miller carried out an amazing experiment in which they produced "molecules of life" from a mixture of gases that they proposed existed in a primordial earth. The experiments simulated what would happen when lightning strikes provided energy for chemical reactions in the atmosphere and suggested a hypothesis for how life might have developed on our planet. Amino acids were the vital molecules that formed in this experiment and supported this hypothesis for the origin of life.
An amino acid is a molecule that contains two functional groups , an amine and a carboxylic acid , as shown in Figure 1. In this illustration there is an additional group called the side chain, designated with an R. The variation seen in naturally occurring amino acids arises from differences in this side chain. The twenty naturally occurring molecules are listed in Table 1. In an aqueous solution , this structure may change so that a proton from the COOH transfers to the NH2 and a zwitterion is formed. This structure depends on the pH of the solution. Most physiological systems fall into such a pH range so the zwitterion form of amino acids is the most stable form in the human body.
The stereochemistry of amino acids is also an important concept. The carbon atom marked α in Figure 1 is chiral, so an amino acid can be one of two enantiomers. Note that the structure shown in the illustration has the amine group on the right and the carboxyl group at the top. This configuration is designated the L form, and all naturally occurring amino acids have this form.
Amino acids are categorized into three groups based on the nature of the side chain. Nine of the amino acids have side chains that are nonpolar . Almost 50 percent of the amino acids that are present in proteins have nonpolar side chains. The second category of amino acid contains six different molecules that have polar side chains. Finally, a group of five amino acids have side chains that are not only polar, but charged.
The key chemical characteristic of amino acids is that they link together to form proteins. Because the COOH functional group is an acid and the NH2 functional group is a base, the two ends of amino acids can readily react with each other. Protein synthesis is more complicated than a simple acid-base neutralization, but consider what happens when two amino
|THE TWENTY COMMON AMINO ACIDS FOUND IN PROTEINS|
|Name||One-Letter Abbreviation||Three-Letter Abbreviation|
acids react to form a peptide bond. The new molecule, now a dimer with two amino acid residues still has one end that is an acid and another end that is a base, so it is apparent that the process of forming a peptide bond with another amino acid could be repeated. If many amino acids are strung together, the result is a natural polymer—a protein.
see also Proteins; Urey, Harold; Zwitterion.
Thomas A. Holme
Barrett, G. C., and Elmore, D. T. (1998). Amino Acids and Peptides. New York: Cambridge University Press.
Cantor, C. R., and Schimmel, P. R. (1980). Biophysical Chemistry, Part I. San Francisco: W.H. Freeman.
Jones, J. (2002). Amino Acid and Peptide Synthesis. Oxford: Oxford University Books.
"Amino Acid." Chemistry: Foundations and Applications. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/amino-acid-0
"Amino Acid." Chemistry: Foundations and Applications. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/amino-acid-0
Eleven of the amino acids involved in proteins can be synthesized in the body, and so are called non‐essential or dispensable amino acids, since they do not have to be provided in the diet. They are alanine, arginine, aspartic acid, asparagine, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.
Nine amino acids cannot be synthesized in the body at all and so must be provided in the diet; they are called the essential or indispensable amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. In addition, arginine may be essential for infants, since their requirement is greater than their ability to synthesize it. Two of the non‐essential amino acids are made in the body from essential amino acids: cysteine (and cystine) from methionine, and tyrosine from phenylalanine.
The limiting amino acid of a protein is that essential amino acid present in least amount relative to the requirement for that amino acid. The ratio between the amount of the limiting amino acid in a protein and the requirement for that amino acid provides a chemical estimation of the nutritional value (protein quality) of that protein, termed chemical score. Most cereal proteins are limited by lysine, and most animal and other vegetable proteins by the sum of methionine + cysteine (the sulphur amino acids). In whole diets it is usually the sulphur amino acids that are limiting.
A number of other amino acids also occur in proteins, including hydroxyproline, hydroxylysine, γ‐carboxyglutamate and methylhistidine, but are nutritionally unimportant since they cannot be re‐utilized for protein synthesis. Other amino acids occur as intermediates in metabolic pathways, but are not required for protein synthesis, and are nutritionally unimportant, although they may occur in foods. These include homocysteine, citrulline, and ornithine.
The amino acids can be classified by the chemical nature of the side‐chain. Two are acidic: glutamic acid (glutamate) and aspartic acid (aspartate), with a carboxylic acid (—COOH) group in the side‐chain. Three, lysine, arginine, and histidine, have basic side‐chains. Three, phenylalanine, tyrosine, and tryptophan, have aromatic side‐chains. Three, leucine, isoleucine, and valine, have a branched side‐chain. These three have very similar metabolism, and a rare genetic disease affecting their metabolism results in maple syrup urine disease. Two, methionine and cysteine, contain sulphur in the side‐chain; although cysteine is not an essential amino acid, it can only be synthesized from methionine, and it is conventional to consider the sum of methionine plus cysteine (the sulphur amino acids) in respect to protein quality.
An alternative classification of the amino acids is by their metabolic fate; whether they can be utilized for glucose synthesis or not. Those that can give rise to glucose are termed glucogenic (or sometimes antiketogenic); those that give rise to ketones or acetate when they are metabolized are termed ketogenic. Only leucine and lysine are purely ketogenic; isoleucine, phenylalanine, tyrosine, and tryptophan give rise to both ketogenic and glucogenic fragments; the remainder are purely glucogenic.
"amino acids." A Dictionary of Food and Nutrition. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/amino-acids
"amino acids." A Dictionary of Food and Nutrition. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/amino-acids
Amino acids are simple organic compounds made of carbon, hydrogen, oxygen, nitrogen, and, in a few cases, sulfur. Amino acids bond together to form protein molecules, the basic building blocks of all living things. Amino acids can vary widely. Only about 20 amino acids are common in humans and animals, with 2 additional ones present in a few animal species. There are over 100 lesser known amino acids found in other living organisms, particularly plants.
The first few amino acids were discovered in the early 1800s. Although scientists determined that amino acids were unique compounds, they were unsure of their exact significance. Scientists did not understand their importance in the formation of proteins—chemical compounds responsible for the structure and function of all cells—until the first part of the twentieth century.
An important characteristic of amino acids is their ability to join together in chains. The chains may contain as few as 2 or as many as 3,000 amino acid units. Amino acids become proteins when 50 or more are bonded together in a chain.
All the millions of different proteins in living things are formed by the bonding of only 20 amino acids. Like the 26 letters of the alphabet that join together to form different words, the 20 amino acids join together in different combinations and sequences to form a large variety of proteins. But whereas most words are formed by about 10 or fewer letters, proteins are formed by 50 to more than 3,000 amino acids. Because each amino acid can be used many times along the chain and because there are no restrictions on the length of the chain, the number of possible combinations for the formation of protein is truly enormous.
The order of amino acids in the chain, however, is extremely important. Just as not all combinations of letters make sense, not all combinations of amino acids make functioning proteins. Some amino acid combinations can cause serious problems. Sickle-cell anemia is a serious, sometimes fatal disease caused by a single amino acid being replaced by a different one at the sixth position from the end of the protein chain in the hemoglobin molecule, the oxygen-carrying particle in red blood cells.
Human need for amino acids
The 20 amino acids required by humans for making protein are necessary for the growth and repair of tissue, red blood cells, enzymes, and other materials in the body. Twelve of these amino acids, called non-essential amino acids, can be made within the body. The other eight, called the essential amino acids, cannot be made by the body and must be obtained from the diet. Proteins from animal sources—meat, eggs, milk, cheese—contain all the essential amino acids. Except for soybeans, vegetable proteins do not have all the essential amino acids. Combinations of different vegetables, however, form a complete source of essential amino acids.
Words to Know
Essential amino acid: Amino acid that cannot be produced by the human body and must be obtained from the diet.
Non-essential amino acid: Amino acid that is produced within the human body.
Proteins: Organic substances consisting of amino acids and other elements that form the basis of living tissues.
[See also Nutrition; Proteins ]
"Amino Acid." UXL Encyclopedia of Science. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/amino-acid-1
"Amino Acid." UXL Encyclopedia of Science. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/amino-acid-1
Amino acids are molecules that have both an amino group (-NH2) and a carboxylic acid group (-COOH), hence the name. The most common amino acids are the α-amino acids, the building blocks of proteins . These have the amino group, the carboxylic acid group, a hydrogen, and a characteristic side chain all attached to one carbon atom, designated the α-carbon. Each type of α-amino acid has a unique side chain that determines its properties and its role in proteins. The side chains (or "R" groups) can range from a hydrogen atom, as in glycine, to the more complicated side chains of tryptophan or arginine.
The α-carbon atom has four different groups attached to it arranged at the points of a tetrahedron. This arrangement is asymmetric and can occur in two different forms, or enantiomers, that are related to each other in the same way as an object and its image in a mirror. These two enantiomers are called L and D. Only L-amino acids occur in proteins made by living systems. D-amino acids and amino acids other than α-amino acids occur in biological systems but are not incorporated into proteins.
Many organisms can synthesize all of the amino acids they require from compounds present in the metabolic pathways they use for energy production. Humans, however, are not able to synthesize all of the necessary amino acids, and a number of them must be obtained from the diet.
The major use of amino acids is to construct proteins. A protein is a linear chain of amino acids linked together by peptide bonds . A peptide bond is formed when the amino group attached to the α-carbon of one amino acid is joined to the carboxyl group of a second amino acid with the elimination of water. The side chain of each amino acid residue protrudes from the polypeptide backbone. The sequence of amino acids in the chain is determined by the deoxyribonucleic acid (DNA) sequence of the gene that codes for that protein.
The three-dimensional structure and the properties of a specific protein, and therefore its biological role, are determined by the sequence of amino acid side chains. In proteins, acidic amino acid side chains are negatively charged, and basic ones are positively charged. The polar and charged amino acids are hydrophilic, meaning they like to interact with water (or are water-loving). The nonpolar, aromatic , and sulfur-containing amino acid side chains prefer to interact with themselves or each other (they are hydrophobic, or water-avoiding).
A protein folds so that nonpolar side chains tend to be buried within the protein while polar and charged side chains tend to be exposed to the water around the protein. The biological function of a protein is generally highly dependent on its three-dimensional structure.
see also Enzymes; Protein Structure
Wayne F. Anderson
Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Publishing, 2000.
Stryer, Lubert. Biochemistry, 4th ed. New York: W. H. Freeman and Company, 1995.
"Amino Acid." Biology. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/amino-acid
"Amino Acid." Biology. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/amino-acid
amino acid (əmē´nō), any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins. They are characterized by the presence of a carboxyl group (COOH) and an amino group (NH2) attached to the same carbon at the end of the compound. The 20 amino acids commonly found in animals are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. In addition to these 20, scientists have synthesized more than 70 artificial amino acids that are not found in animals, and more than 100 less common amino acids also occur in biological systems, particularly in plants. Every amino acid except glycine can occur as either of two optically active stereoisomers, d or l; the more common isomer in nature is the l-form. When the carboxyl carbon atom of one amino acid covalently binds to the amino nitrogen atom of another amino acid with the release of a water molecule, a peptide bond is formed. Amino acids are released in the intestinal tract by the digestion of food proteins and are then carried in the bloodstream to the body cells, where they are used for growth, maintenance, and repair. Cellular catabolism breaks amino acids down into smaller fragments. Many of the amino acids necessary in metabolism can be synthesized in the human or animal body when needed; these are called nonessential. Others cannot be synthesized in sufficient quantities; these are termed essential and must be provided in the diet. Synthetic amino acids have been used by scientists as markers to track biological processes and as components of disease treatments; artificial amino acids can in some cases increase the effectivenesss of treatments by slowing the normal breakdown of the hormone or other biologic into which they are incorporated.
"amino acid." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/amino-acid
"amino acid." The Columbia Encyclopedia, 6th ed.. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/amino-acid
Plants and many microorganisms can synthesize amino acids from simple inorganic compounds, but animals rely on adequate supplies in their diet. The essential amino acids must be present in the diet whereas others can be manufactured from them.
"amino acid." A Dictionary of Biology. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/amino-acid-3
"amino acid." A Dictionary of Biology. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/amino-acid-3
"amino acid." World Encyclopedia. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/amino-acid
"amino acid." World Encyclopedia. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/amino-acid
The amino acids occurring in proteins
"amino acid." A Dictionary of Nursing. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/amino-acid
"amino acid." A Dictionary of Nursing. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/amino-acid
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"amino acid." A Dictionary of Plant Sciences. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/amino-acid-1
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"amino acid." A Dictionary of Earth Sciences. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/amino-acid
"amino acid." A Dictionary of Earth Sciences. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/amino-acid
"amino acid." A Dictionary of Zoology. . Encyclopedia.com. (October 18, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/amino-acid-2
"amino acid." A Dictionary of Zoology. . Retrieved October 18, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/amino-acid-2