Structures of charge-transfer complexes of flavoenzyme D-amino acid oxidase: a study by resonance Raman spectroscopy and extended Hückel molecular orbital method.

Abstract

The structure of the ligand in a charge-transfer complex of oxidized D-amino acid oxidase (DAO) formed upon aerobic addition of chloropyruvate and ammonium sulfate was determined by resonance Raman spectroscopy. The ligand in the complex is in the enamine form, 2-amino-3-chloracrylate, not in the imine form, 2-imino-3-chloropropionate. This conclusion is consistent with our hypothesis that the ligands in the charge-transfer complexes of oxidized DAO and reduced DAO are generally in the enamine and imine forms, respectively. The calculation of HOMO and LUMO of some enamine and imine forms by the extended Hückel molecular orbital method indicated that the enamine form is a better electron donor than the imine form and the imine form is a better electron acceptor than the enamine form. These results, as well as the information about the structure of enzyme-bound ligand, support the following ideas. (i) In the charge-transfer complex of reduced DAO, the reduced flavin is an electron donor. (ii) In the charge-transfer complex of reduced DAO, the reduced flavin is an electron donor and the ligand is an electron acceptor. Resonance Raman studies on the charge-transfer complexes of oxidized DAO and those of reduced DAO, and calculated results of HOMO and LUMO for some enamine and imine forms, corroborated the structure of the stacking form between the flavin ring and the ligand in the active site of DAO.