MODELS AND MOLECULAR-ORBITAL SEMIEMPIRICAL CALCULATIONS IN THE STUDY OF THE SPECTROSCOPIC PROPERTIES OF BOVINE SERUM AMINE OXIDASE QUINONE COFACTOR

The electronic properties of 2,4,5-trihydroxyphenylalanine quinone (TPQ), the cofactor of bovine serum amine oxidase [Janes, S. M., Mu, D., Wemmer, D., Smith, A. J., Kaur, S., Maltby, D., Burlingame, A. L., & Klinman, J. P. (1990) Science 248, 981-987], and some adducts with hydrazines were investigated by means of low-molecular-weight models and semiempirical molecular orbital calculation methods. The enzyme visible band was assigned to the first pi --> pi* transition of the cofactor in p-quinonic form, with the C-4 hydroxyl ionized and hydrogen bonded either to a water molecule or to a basic protein residue. The spectra of the protein adducts with some substituted hydrazines were well accounted for by assuming the inhibitor bound to the C-5 carbonyl, usually in azo form. The adduct with the unsubstituted hydrazine was instead assigned an o-quinone hydrazone form, stabilized by an internal hydrogen bond between the amino group and the ortho carbonyl oxygen, a larger electron delocalization, and formation of a hydrogen bond at the C-6 ionized hydroxyl. On the basis of these assignments, the reaction of the protein with benzylhydrazine [Morpurgo, L., Agostinelli, E., Muccigrosso, J., Martini, F., Mondovi, B., & Avigliano, L. (1989) Biochem. J. 260, 19-25] was rewritten. All examined electronic transitions, though highly sensitive to cofactor ionization and hydrogen bonding, could be accounted for without introducing perturbations due to copper. This confirms that copper is not within bonding distance of the oxidized cofactor.