Versatility and specificity in flavoenzymes: control mechanisms of flavin reactivity.

Flavoenzymes are characterized by their remarkable versatility and strict specificity. The former can be grasped when flavoenzymes are treated as a whole, while the latter refers to each flavoenzyme in which the broad versatility of flavin is specifically controlled. The versatility stems from the variety of the redox, ionic, and electronic states that the flavin ring system can adopt. Versatility of flavoenzymes is reflected in their classification, which has generally been based on substrates and reactions catalyzed. A different classification is presented according to the number of electrons transferred in the reductive and oxidative half reactions. Specificity of each flavoenzyme is understood in terms of the regulatory mechanism of the broad reactive potentiality of flavin. The elements of this regulatory mechanism include hydrogen-bonding network, electrostatic effect, charge-transfer interaction, positioning between a substrate/ligand and flavin, and modulation of resonance hybridization, each of which is explained with relevant examples provided mainly by studies from the author's group.