Physicochemical and Structural Analysis fo Functions of Quinonoid Cofactors and Quinoproteins

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1 Physicochemical and spectroscopic properties of tryptophan tryptophylquinone (TTQ), a cofactor bacterial methylamine dehydrogenase, have been elucidated in view of redox and acid-base reactions and semiquinone radical electronic structure. Reactivity of TTQ toward ammonia, as a substrate analog, has been also investigated. Significance of these physicochemical and structural properties have been detailed in relation with its function in the enzymatic reaction.2 Column-electrolytic spectroscopy has been developed in order to obtain spectroelectrochemical property of quinones and related molecules. This method has been successfully applied to the thermodynamic analysis of the five-step electron transfer of a quinohemoprotein, alcohol dehydrogenase.3 Several model compounds of topa quinone, a cofactor of copper amine oxidases, were synthesized. Detailed kinetic analysis has been performed concerning quinone-catalyzed amine oxidation using TTQ and TPQ model compounds as well as pyrroloqu … More inoline quinone (PQQ). A reaction intermediate radical generated during the catalytic reaction has been characterized using electron spin resonance spectroscopy. All these results strongly support a transamination mechanism.4 Relation between the catalytic activity and electronic structure of quinones suggests the significance of ortho-quinone structures in expression of the catalytic amine oxidation activity. Tautomerization of free TPQ model compounds between ortho-and para-quinone-type electronic structures is responsible for their relatively low catalytic activity in the amine oxidation compared with TTQ and PQQ as rigorous ortho-quinones. On the other hand, it has been found that semiquinone radicals participates is autoxidation of reduced quinones.5 On the basis of these results, roles of Cu (II) is proposed as a transient intramolecular electron acceptor in copper amine oxidases to enhance the reactivity toward dioxygen through the generation of the imino-TPQ semiquinone and as a weak ligand in TPQ-containing amine oxidases to make the TPQ cofactor to bear an ortho-quinone-type electronic structure. Less