A complex, high polymer containing carbon, hydrogen, oxygen, nitrogen, and usually sulfur, and composed of chains of amino acids connected by peptide linkages (—CO•NH—). Proteins occur in the cells of all living organisms and in biological fluids (blood plasma, protoplasm). They are synthesized by plants largely through the nitrogen-fixing ability of certain soil bacteria. Their molecular weight may be as high as 40 million (tobacco mosaic virus). They have many important functional forms: enzymes, hemoglobin, hormones, viruses, genes, antibodies, and nucleic acids. They also serve as the basic component of connective tissue (collagen), hair (keratin), nails, feathers, skin, etc. Some have been synthesized in the lab.The sequence of amino acids;in the polypeptide chain is of critical importance in genetics. Proteins can be hydrolyzed to their constituent amino acids and can be broken down into simpler forms by proteolytic enzymes. They form colloidal solutions, and behave chemically as both acids and bases simultaneously (amphoteric). They are denatured by changes in pH, and by heat, UV radiation, and many organic solvents.Simple proteins contain only amino acids, conjugated proteins contain amino acids plus nucleic acids, carbohydrates, lipids, etc. On the basis of solubility, they can be classified as albumins (water soluble), globulins (insoluble in water but soluble in aqueous salt solutions), and prolamins (soluble in alcohol-water mixture but not in alcohol or water alone). A number of proteins have been synthesized, notably the hormone insulin. Proteins are an essential component of the diet, occurring chiefly in meat, eggs, milk, and fish. Edible proteins suitable for human food as well as cattle feed can be produced from microorganisms grown in carbonaceous or nitrogenous media to form yeastlike materials. Paraffinic hydrocarbons (methane) and petroleum-derived ethanol can be used as growth media for protein biosynthesis.Industrial applications of proteins include plastics, adhesives, and fibers derived from casein and soybean protein, but these have been declining in recent years. Special forms in which proteins are commercially available include textured proteins for food products, and protein hydrolyzate and liquid predigested protein, both for medical use.See Ribonuclease; Deoxyribonucleic Acid; Nutrition; Amino Acid; Protein, Textured; Protein, Single-cell; Polypeptide.
