Protein

Proteins are the building blocks of all forms of life. As an organic compound (meaning that they are based on the element carbon) made up of amino acids (the building blocks of proteins), they are key to the major functions of growth and repair as well as to other important specialized functions. The human body makes proteins by following the coded instructions in its genes.

All living things are made up of cells, and the major ingredient of all cells is protein. As organic compounds, proteins play a major role in many of the functions that take place in living things. Proteins called structural proteins make up all of the parts, or building material, of a plant's or animal's body. They make up the walls of an organism's cells. In a human being, hair and fingernails are made of a structural protein called keratin, and tendons (linking muscle to bone) are made of a different structural protein called collagen. The enzymes, hormones, and antibodies in our bodies are all made of protein. Enzymes are essential to all of the chemical reactions that take place inside us; hormones are the body's chemical messengers; and antibodies help fight foreign substances or infection.

Proteins are made up of amino acids. Humans and most other animals require twenty separate types of amino acids in order to be able to make the many different types of proteins they need. Proteins are amino acids that are linked, or bonded, together like chains. Therefore, it is the order of the individual amino acids in the chain that make proteins different from one another. The twenty different amino acids form different proteins in a way similar to how the twenty-five different letters of the alphabet are used in different combinations to form different words.

Different proteins have different sequences of amino acids. The shape that these long protein chains eventually form has a great deal to do with the properties of a protein. Chains that form themselves into a springlike shape build a material that is flexible and can be stretched, while those that form a sheetlike structure are more rigid. An example often used to illustrate the former is the protein keratin that forms wool. Because keratin has a springlike structure, a length of wool can be stretched to nearly twice its original length. On the other hand, the natural fiber silk is composed of the protein called fibroin whose shape is not coiled. As a result, silk cannot be stretched to anywhere near the length that wool can.

Proteins have important structural uses. They are also a necessary part of an organism's diet. Of the twenty different amino acids commonly found in proteins, only ten can be made in cells. The other remaining ten amino acids (called "essential amino acids") must be obtained from food. Unless all these amino acids are obtained by eating various protein-based foods, the body will not have the necessary building blocks to form new protein molecules. The growth and repair of body cells could be harmed or even halted. Nutritionists have determined that these essential proteins can be obtained from meat, eggs, milk, cheese, and other foods usually derived from animals.

Finally, most proteins are species-specific, meaning that each species has slightly different proteins. Thus, the type of proteins found in the cells of a domestic house cat are different from those of a tiger. The farther apart two species are on the evolutionary ladder, the greater difference there will be in the proteins found in their cells.

[See alsoAntibodies and Antigens; Amino Acids; Enzymes; Hormones; RNA ]