Kenji Kano, Masato Nakagawa, Kazuyoshi Takagi and Tokuji Ikeda
A kinetic study was performed on the oxidative deamination of
benzylamine (BzAm) catalysed by pyrroloquinoline quinone (PQQ), a
tryptophan tryptophylquinone analogue
[3-methyl-4-(3′-methylindol-2′-yl)indole-6,7-dione;
TTQa], and a topa quinone analogue (6-hydroxydopa quinone;
TPQa) using high-performance liquid chromatography under
aerobic conditions and cyclic voltammetry under anaerobic conditions.
The overall kinetics are second-order in the quinone and BzAm
concentrations and independent of the electron acceptors used (dioxygen
under aerobic conditions and electrode under anaerobic conditions). The
second-order rate constants vs. pH profiles are explained in
terms of the acid dissociation of BzAm and the quinones, yielding
pH-independent second-order rate constants, which are in the order
TTQa > PQQ TPQa
. Half-wave potentials of steady-state catalytic voltammograms are
more positive than the redox potentials of the free quinones, indicating
that the BzAm-reduced quinones are not quinols but the corresponding
aminophenols, which are oxidized to iminoquinones. Iminoquinone forms of
PQQ and TTQa exhibit lower catalytic activity compared with
the quinone forms. The reaction rate of TTQa slows down with
the time, while those of PQQ and TPQa are practically
independent of the reaction time within a 15 min period. The diminished
rate of TTQa is reasonably ascribed to slow hydrolysis of
imino-TTQa to TTQa. Generation of the
iminosemiquinone radical during the catalytic reactions is evidenced by
EPR spectroscopic measurements. The significance of radical generation
is discussed in the light of autoxidation of the aminophenols. All these
results are interpreted well by a transamination mechanism. In addition,
an asymmetric ortho-quinone structure is proposed to be
important in catalytic amine oxidation activity, and the relatively low
catalytic activity of free TPQa is ascribed to the
contribution of a para-quinone-type electronic structure. On
the basis of these results, a role for CuII is proposed as a
transient intramolecular electron acceptor in copper amine oxidases to
enhance the reactivity toward dioxygen through the generation of the
imino-TPQ semiquinone and as a weak ligand in TPQ-containing amine
oxidases making the TPQ cofactor bear an ortho-quinone-type
electronic structure.