Abstract
On the bases of the crystal structures of flavoenzymes, D-amino acid oxidase (DAO) and acyl-CoA oxidase (AGO), which have recently been solved by us, the control mechanisms of the flavin reactivity toward molecular oxygen in the oxidative half-reaction are discussed. The three-dimensional structure of the purple intermediate of DAO, the requisite intermediate of the oxidative half-reaction, reveals the electrostatic effect on raising the electron density at C (4a) of the anionic reduced flavin. The high electron density at C (4a), the site of electron donation to oxygen, results in enhanced reactivity of reduced flavin toward molecular oxygen. According to the three-dimensional structure of rat liver AGO, the active site cleft is larger than that of acyl-CoA dehydrogenase allowing easy access of molecular oxygen in the oxidative half-reaction and formation of the electron-transfer complex with an electron acceptor protein is effectively blocked, rendering AGO the oxidase rather than the dehydrogenase functionality.
