Protein – Any of a group of complex organic macromolecules that contain carbon, hydrogen, oxygen, nitrogen, and usually sulfur and are composed of one or more chains of amino acids. Proteins are fundamental components of all living cells and include many substances, such as enzymes, hormones, and antibodies, that are necessary for the proper functioning of an organism. They are essential in the diet of animals for the growth and repair of tissue and can be obtained from foods such as meat, fish, eggs, milk, and legumes.
Proteins are macromolecules consisting of long sequences of α-amino acids [H2N-CHR-COOH] in peptide (amide) linkage (elimination of H2O between the α-NH2 and α-COOH of successive residues). Protein is three fourths of the dry weight of most cell matter and is involved in structures, hormones, enzymes, muscle contraction, immunologic response, and essential life functions. The amino acids involved are generally the 20 α-amino acids (for example, glycine, L-alanine) recognized by the genetic code. Crosslinks yielding globular forms of protein are often effected through the -SH groups of two L-cysteinyl residues, as well as by noncovalent forces (hydrogen bonds, lipophilic attractions, etc.).