Models and Molecular Orbital Semiempirical Calculations in the Study of the Spectroscopic Properties of Bovine Serum Amine Oxidase Quinone Cofactor

Abstract

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.

Modeling Novel Quinocofactors: An Overview

“…In another set of model studies, Bossa and fellow researchers confirmed that copper is not within bonding distance of the oxidized cofactor (29). These authors performed a detailed analysis of spectroscopic data obtained for low-molecular-weight models of the TPQ cofactor, with the aid of semiempirical molecular orbital calculation methods, in an attempt to gain a deeper insight into the cofactor electronic and basic structural features (29,(78)(79)(80).…”

“…In particular, the investigation of the electronic properties of some adducts of 1,4-dihydroxy-2,5-benzoquinone (26; Fig. 3) with bases and hydrazines revealed that all the electronic transitions examined, although highly sensitive to cofactor ionization and hydrogen bonding, could be accounted for without introducing perturbations due to copper, thus disfavoring the possibility of a direct copper-TPQ interaction (29). These studies and, more recently, the AM1 and PM3 semiempirical calculations performed on the adducts between 26 or 4 and 4-(N,Ndimethyl)aminopyridine (81), also provided clues about the TPQ-protein interactions through hydrogen bonding.…”

“…These studies and, more recently, the AM1 and PM3 semiempirical calculations performed on the adducts between 26 or 4 and 4-(N,Ndimethyl)aminopyridine (81), also provided clues about the TPQ-protein interactions through hydrogen bonding. The ability of TPQ model compounds, namely 26, in hydrogen bonding to a large variety of proton acceptors like substituted pyridines and imidazole, suggested that the hydroxyl group at C4 of the cofactor exists in the deprotonate, anionic form and is engaged in a proton-transfer bond to an acceptor, either a water molecule or a basic protein residue (29). The N-base 4-(N,N-dimethyl)-aminopyridine was selected by Bossa and coworkers as a suitable proton acceptor that could mimic the proton acceptor present at the enzyme active site and needed for the stabilization of the TPQ anion.…”

“…These authors performed a detailed analysis of spectroscopic data obtained for low-molecular-weight models of the TPQ cofactor, with the aid of semiempirical molecular orbital calculation methods, in an attempt to gain a deeper insight into the cofactor electronic and basic structural features (29,(78)(79)(80). In particular, the investigation of the electronic properties of some adducts of 1,4-dihydroxy-2,5-benzoquinone (26; Fig.…”

“…However, the final elucidation of the cofactor structure required the synthesis of authentic analogs, followed by a comparison of physical properties for the synthetic and protein-derived compounds. To this end, a model compound was prepared in which the peptide moiety was tied back into a cyclic, hydantoin structure (1a, 1b; Following the path indicated by Klinman and fellow researchers, several model studies were conducted in order to investigate the mechanistic role of TPQ in enzymecatalyzed oxidation of substrate and reduction of dioxygen (20)(21)(22)(23)(24)(25)(26)(27), the interaction of copper with the cofactor (28,29), and the biogenetic process of TPQ at the active site of AOs (30)(31)(32).…”