Protein, a large, complex organic molecule. Proteins are found in all living cells, making up about one-half of the dry weight of most organisms. They are essential to life. Plants make proteins from chemical elements in the soil and the air. Human beings and other animals make proteins from substances in the foods they eat.

Structure of Proteins

Proteins contain carbon, hydrogen, nitrogen, oxygen, and sometimes sulfur, phosphorus, and iron. A protein molecule is made up of smaller units called amino acids. Amino acids are nitrogen-containing compounds that chemically link together in long chains called polypeptides. Some 20 different kinds of amino acids are found in proteins. The arrangement of the various types of amino acid molecules differentiates one protein from another and governs a protein's function. A typical protein is a single polypeptide containing some 200 amino acid molecules, but the number of amino acid molecules in a protein varies from 50 in a single chain to more than 3,000 in several interlocking chains.

Protein synthesis (the putting together of individual amino acids to form a polypeptide) takes place in the cells of an organism. Proteins are made in a series of steps determined by instructions supplied by the cell's genes. The genes specify the arrangement, or sequence, of the different types of amino acids. The sequence of amino acids in the chain can vary almost without limit.

Proteins are classified by chemical composition as simple (consisting entirely of amino acids) and conjugated (consisting of amino acids plus other substances, such as fatty acids, carbohydrates, and metal-containing compounds). Hemoglobin, for example, is a conjugated protein—it consists of four chains of amino acids and four molecules of heme, which contains iron.

Proteins also are classified by shape as fibrous (having a long, stringy shape) and globular (having a rounded shape, in which the chain or chains of amino acids are tightly folded). Fibrous proteins, such as those found in hair and skin, are insoluble in water. Most globular proteins are readily soluble in water or dilute salt solutions.

Types of Proteins

Living organisms contain thousands of different types of proteins. Each protein has an essential role in the structure or biological activity of a cell.

Contractile Proteins, such as myosin in skeletal muscle, help cells to contract or to change shape.

Defense Proteins defend organisms against invasion by microorganisms or protect against the effects of certain types of injuries, such as uncontrolled bleeding. Antibodies are defense proteins that are made by an organism's immune system and destroy foreign bodies such as bacteria and viruses. Fibrinogen is a defense protein that prevents loss of blood by causing it to clot.

Enzymes are catalysts—they promote chemical reactions in an organism and regulate the rate at which they occur. The enzymes trypsin and pepsin, for example, aid in the digestion of food proteins.

Nutrient Proteins and Storage Proteins provide nourishment for a developing organism. Examples include the protein in a kernel of wheat that nourishes a seedling; and casein, a protein in milk that nourishes a developing mammal.

Regulatory Proteins control certain biological activities. Some hormones, such as insulin (a hormone that regulates the metabolism of glucose), are regulatory proteins. Proteins called nucleoproteins regulate activity within a cell's nucleus.

Structural Proteins provide structure and protection to cells and organisms. Tissues that form hair, nails hooves, horn, and feathers consist mainly of the structural protein keratin. The proteins collagen and elastin are major components of connective tissues, including tendons, cartilage, bones, and the deep layers of skin.

Transport Proteins carry specific molecules or atoms from one organ or tissue to another. An example of a transport protein is hemoglobin, which is found in red blood cells. It forms a bond with oxygen as blood passes through the lungs and carries the oxygen to cells throughout the body.

Protein content of some common foods