Any of the group of highly complex organic compounds found in all living cells. Protein is the most abundant class of all biological molecules, comprising about 50 percent of cellular dry weight. Classified by biological function, proteins include the enzymes, which catalyze cellular reactions; collagen, keratin and elastin, which are structural, or support, proteins; hemoglobin and other transport proteins; casein, ovalbumin and other nutrient proteins; antibodies, which are necessary for immunity; protein hormones, which regulate metabolism; and proteins such as actin and myosin, the contractile muscle proteins that perform mechanical work. Structurally, proteins are large molecules composed of one or more chains of varying amounts of the same 22 amino acids, which are linked by peptide bonds. Each protein is characterized by a unique and invariant amino acid sequence. Protein chains may contain hundreds of amino acids; some proteins also incorporate phosphorus or such metals as iron, zinc and copper. The amino acid sequence also determines the molecule's three-dimensional structure; this so-called native state is required for proper biological function. The information for the syntheses of the specific amino acid sequences from free amino acids is carried by the cell's nucleic acid. See also Peptide; Ribonucleic Acid.
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