Definition of: protein

(prō′tē·in, -tēn) noun Biochem.
Any of a class of highly complex nitrogenous organic compounds occurring naturally in all living matter, and forming an essential part of animal food requirements. They are composed principally of amino acids in varying combinations, and are usually classified as: simple (hydrolyzed only by enzymes or acids into alpha-amino acids or their derivatives); conjugated (simple proteins combined with non-proteins in a form other than a salt); derived (obtained by the action of heat, enzymes, or reagents upon naturally occurring proteins). Also pro′te·id (-id). [<G <Gk. prōteios chief <prōtos first; so called because the chief constituent of living matter]