ABSTRACT
Proof of covalent binding of a flavin cofactor, flavine adenine dinucleotide (FAD), to an apoenzyme was first obtained in the case of succinate dehydrogenase from beef heart mitochondria and the structure of the covalent bond elucidated. The simultaneous existence, often within the same organism, of covalently and non-covalently linked flavoproteins raises the question of the significance of covalency. A bacterial enzyme containing covalently attached FAD has been termed sarcosine dehydrogenase, but it has not been demonstrated to be a dehydrogenase rather than an oxidase. An enzyme from Penicillium cyclopium has been obtained in homogeneous form that catalyzes the dehydrogenation and cyclization of B-cyclopiazonate to a-cyclopiazonate. Schizophyllum commune, a soil organism, was the source of cholesterol oxidase, an enzyme converting free cholesterol into cholest-4-en-3-one with the concomitant production of H2O2. Acid-base catalysis of hydrolytic reactions such as peptide or glycoside bond cleavage, but also some redox processes involving disulfide-thiol conversion of cysteine/cystine residues of enzymes are well established.
