抄録
Flavocoenzymes are extremely versatile in that they show broad potentiality in catalysis, as manifested in the wide range of the reactions catalyzed by flavoenzymes. In order for each of the flavoenzymes to express its specific function, the versatility of the coenzyme should be elegantly controlled. This control mechanism lies in the structure-reaction linkage. We have recently solved the high-resolution three dimensional structure of D-amino acid oxidase by X-ray crystallography. On the basis of the tertiary structure of the enzyme-substrate analog complex, the enzyme-substrate complex structure was modeled by means of molecular mechanics simulation. The simulated structure was not compatible with the hitherto recognized reaction mechanisms which require an amino acid residue acting as a catalytic base to abstract the subtrate α-proton. We have introduced two new mechanisms after analyzing the structure-reaction linkage based on the substrate-binding mode and the requirements that the reaction mechanism should suffice. One of them is called the "electronproton-electron transfer mechanism" and is characterized by two one-electron transfer processes intervened by a proton transfer, while the other distinguishes itself in ionic intermediates and is thus called the "ionic mechanism." In both of the mechanisms flavin N(5) rather than an amino acid residue acts as a base for abstracting the substrate α-proton and flavin C(4_a) is the site of electron acceptance from the substrate.
