Protein metabolism is the chemical cycle of breaking down protein (catabolism) and using the components to synthesizing (anabolism) new molecules to be used in the body. The process is also known as proteometabolism.
Proteins, fats, and carbohydrates (called macronutrients) are part of a complex metabolic cycle that is essential to life. During digestion food containing these nutrients is chemically broken down into its basic components and absorbed for use in the body. Protein molecules are split into their basic building blocks, called amino acids, which are then chemically re-arranged to synthesize new proteins that the body needs. Fats are broken down into fatty acids and cholesterol, and carbohydrates are split into simple sugars such as glucose and fructose, which provide most of the energy to drive chemical reactions in the body. These smaller, simpler molecules are absorbed in the small intestine and enter the circulatory system. They then pass through the liver where some of these "building block molecules" are synthesized into more complex compounds needed by the body.
In order for cellular metabolism to occur, two reactions happen continuously. Small molecules are build up into larger molecules, a process called anabolism or constructive metabolism, while large molecules are broken down into their component parts during a process called catabolism or destructive metabolism. This building up and breaking down is regulated by a complex set of hormones and enzymes, themselves proteins. For an individual to remain healthy, the processes of anabolism and catabolism must remain in balance.
Catabolism, or the breakdown of nutrients obtained from food, releases energy that drives all metabolic activities in the body. For example, glucose is broken down to provide energy for cellular respiration that allows functions such as muscle movement. Proteins are broken into amino acids then re-synthesized into hormones and enzymes to regulate chemical reactions in the cell, and molecules used for tissue growth and repair. Carbohydrates and fats are the preferred sources of energy for cellular metabolism. When the supply of fats and carbohydrates is insufficient to meet the body' needs, proteins can be broken down to supply energy. This accounts for the loss of muscle seen in prolonged cases of starvation.
Protein is a nitrogen-containing compound found in all plants and animals. There is a continuous need for protein to make hormones, enzymes, antibodies, and to produce new tissue (growth) and repair damaged tissues (maintenance). About 75% of human body tissue is made of protein. From 1-2% of the body's total protein is broken down each day into amino acid and recycled into new proteins. About60-70% of the amino acids the body needs come from this recycling process. The rest must be supplied by diet.
A complex molecule
Proteins are complex nitrogen-containing molecules formed by a combination of about 20 amino acids. These twenty amino acids can be connected in thousands of different combinations to form all the different proteins in the body. During protein formation (anabolism) the amino acids are connected in long chains called polypeptides that fold into three-dimensional shapes. The combinations of amino acids produce proteins with unique shapes that perform specific functions in the body such as catalyzing metabolic reactions, repairing tissue, or stimulating glands to produce other proteins.
The body can synthesize 11 amino acids naturally. The remaining nine must be obtained from dietary protein. The nine amino acids that cannot be synthesized by the body are called essential amino acids. It is an absolute requirement for health that diet include foods that contain an adequate amount of these essential amino acids. Foods that contain all the essential amino acids are called complete proteins. Examples of complete proteins are all meats and fish, eggs, and dairy products. Foods that lack one or more essential amino acid are called incomplete proteins. Most plant proteins are incomplete proteins. Dried beans, lentils, and rice are examples of incomplete proteins. Although some excess amino acids are converted into fat and stored as a future energy source, the body cannot not store amino acids for use in making future proteins. As a result, humans need a steady supply of protein in their diets. Individuals whose diet is completely or primarily vegetarian must make sure to eat combinations of foods that provide all the essential amino acids, for example red beans and rice or corn tortillas and beans.
Digestion of protein
The digestion of proteins begins in the stomach where the hormone pepsin is secreted by the stomach. Pepsin breaks the long polypeptide molecules into smaller peptides. The mechanical churning of the stomach assists digestion by mixing food with gastric (stomach) secretions. When the contents of the stomach reach a certain degree of acidity, the pyloric sphincter, a muscle that separates the stomach from the small intestine, opens. The stomach contents, called chyme, flow into the duodenum, or upper part of the small intestine. As chyme moves through the small intestine, enzymes break the chemical bonds in the peptides, reducing the proteins to their component amino acids. (Breakdown of fats and carbohydrates is occurring simultaneously under the direction of different hormones and enzymes). In the small intestine, intestinal cells and blood vessels are in close proximity, separated only by cell membranes. Amino acids molecules are small enough that they can move though the intestinal wall and into blood where they (along with glucose, fatty acids, and the other products of digestion) are carried by a large blood vessel to the liver.
The liver is at the heart of protein metabolism. It has both anabolic and catabolic functions. In the liver, amino acids are synthesized into larger proteins that circulate through the body performing a huge variety of tasks including stimulating production of other proteins. The liver also breaks down proteins, for example, hemoglobin found in dead red blood cells. The liver cleanses the blood by removing cellular debris and processing excess nitrogen that is produced by chemical reactions within cells. This excess nitrogen is initially in the form of ammonia (NH3. If allowed to remain in the blood, it would rapidly become toxic to the body. The liver converts ammonia to non-toxic urea that is removed from the body in urine. This lost nitrogen must then be replaced through diet.
Protein metabolism consists of a cycle of breaking down proteins, synthesizing new ones and removing nitrogenous waste products that result from these reactions. The amount of protein needed to balance this cycle changes throughout an individual' life. Growing children who are creating new muscle and bone, for example, have higher protein needs than adults.
Role in human health
The Daily Reference Intake (DRI) for protein changes with age. The DRI is a United States government-determined measure of the amount of a nutrient an individual should consume daily and replaces the recommended daily amount (RDA) measurement. Current DRI guidelines call for children ages 1-3 to consume 1.1 grams of protein per kilogram of body weight (g/kg body weight) or roughly 17 g/day. Children ages 4-13 should receive 0.95 g/kg body weigh, and children 14-18 should receive 0.85 g/kg body weight. For reference, 3 ounces of lean beef provide about 30 grams of protein; milk provides about 1 g/ounce or 8 grams per cup; an egg contains about 6 grams of protein. Adolescent boys have higher protein requirements than adolescent girls, and pregnant women have higher protein requirements than non-pregnant women.
A negative nitrogen balance occurs when a person loses (through excretion) more protein than is provided through diet. Negative nitrogen balance is often associated with inadequate caloric intake (starvation) and not just inadequate protein intake. However, malnutrition can occur a person's diet does not include food with all of the essential amino acids. This can be a particular problem individuals who eat no animal products. Negative nitrogen balances also may occur after surgery and during advanced stages of cancer.
Common diseases and disorders
Health conditions related to protein metabolism are generally caused by the amount of dietary protein consumed. Inadequate protein intake is rarely an issue in the United States because most people routinely eat food containing far more than the recommended protein DRI. People who eat too much protein are at risk of gaining weight since excess protein is converted to fat. Meat, a good source of protein, also contains fat and cholesterol. A diet with too much protein from fatty meat can result in high cholesterol, and an increased risk for heart disease.
In addition, excessive protein consumption increases the amount of calcium excreted in urine. Calcium is crucial to bone health, so bone strength may be affected by a prolonged high-protein diet. Excess dietary protein can also damage the liver and kidneys. Furthermore, recommended daily intake of protein for healthy individuals is harmful to people with cirrhosis of the liver, therefore, people with liver or kidney damage may be placed on low-protein diets.
Inadequate protein intake can inhibit growth, reduce muscle mass weaken the immune system. Over time, it can strain the heart and cause death. Severe lack of protein, known as protein energy malnutrition, can be caused by eating disorders such as anorexia and bulimia.
Starvation is another cause of low-protein intake. Starvation (intake of inadequate calories) may be the result of a famine, economic conditions, or caregiver abuse. In poor countries, severe protein malnutrition causes a disorder called kwashiorkor. This condition primarily affects children between the ages of 1 and 3. Once these children are weaned and no longer receiving breast milk, their diets consist of foods containing little protein. Symptoms of this condition include impaired growth, a swollen stomach, and fatigue. Treatment consists of slowly reintroducing a balanced, higher calorie diet. If not treated, kwashiorkor can result in liver damage and death.
Calcium— A substance mineral found in bones and teeth.
Cholesterol— A soft, waxy, and fatty substance (lipid) found in animal tissue and fat.
Cell— The basic structural unit of all living organisms, such as animals and plants.
Cirrhosis— A condition where healthy liver cells are replaced by scar tissue. The condition is most often caused by alcoholism.
Enzyme— A protein that helps regulate the speed pf a chemical reaction.
Glucose— A simple of sugar that can be used by the body.
Nitrogen— A chemical element found in all proteins.
Triglycerides— Fatty compounds consisting of three fatty acids and one glycerol molecule.
Garrison, Jr., Robert and Elizabeth Somer. The Nutrition Desk Reference New York: McGraw-Hill, 1998.
Tortora, Gerard, and Sandra Reynolds Grabowski.Introduction to the Human Body New York: JohnWiley & Sons, 2001.
Davis, Carla. "A Question of Protein." Vegetarian Times 331 (May 2005): 26.
Hrastar, Laura M. "New Amines on the Block: Synthetic Amino Acids Aid Understanding of Large Protein Complexes." The Scientist 19 i11 (June 6, 2005): 34.
American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, IL 60606-6995. (800) 877-1600. http://www.eatright.org