Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 84

complex, in which the dioxygen binds in a side-on mode with a
distance of 3.8 Å. Electron transfer from the coordinating
catecholate to the peroxide followed by protonation of the peroxide group and
O-O bond cleavage yields -quinone and the hydroxide-bridged dicupric state

of the enzyme. The oxidized enzyme then regenerates itself to the dicuprous
form by oxidizing another molecule of catechol to o -quinone.

o

1.2.2 Amine Oxidases; Galactose Oxidase
Amine oxidases catalyze oxidative deamination reactions involving
functions such as the crosslinking of collagen and elastin and the regulation of
blood plasma biogenic amines. 27,28 A carbonyl-containing cofactor has long
been known to be present, identified by Klinman and co-workers as
topaquinone (TPQ) (B). 29 Recent protein x-ray crystal structures 30-33 confirm
this conclusion, while also verifying the spectroscopically deduced
coordination environment (A) with 3 histidine ligands. The TPQ lies close to
the copper ion but is not coordinated, and it is shown that this group is
conformationally flexible. The enzyme reaction can be divided into two half-
reactions: a reductive half which involves initial Schiff-base formation of
amine substrate with quinone, and an oxidative half which requires copper ion
and dioxygen to recycle the reduced enzyme back to its oxidized resting state,
as well as release of ammonia and hydrogen peroxide. The role of copper is
to facilitate reoxidation of the reduced cofactor by (after reactions with
substrate); a Cu(I) semiquinone intermediate (in equilibrium with Cu(II)-
quinone) has been detected and is presumed to interact directly with (also

see below). 28

Another aspect of considerable interest is the self-processing of TPQ,
known to be derived from a protein tyrosine precursor. Copper ion is
implicated in this cofactor biogenesis and the copper-dioxygen chemistry
involved is discussed below.
Recently, Knowles and co-workers 34 using flash-freezing techniques
obtained x-ray crystal structures of several species related to the oxidative half
reaction of the amine oxidase from Escherichia coli. The structure of the
anaerobically β-phenylethylamine-reduced amine oxidase shows that the
reduced cofactor TPQ exists in the aminoquinol form possessing a hydrogen
bond between the O-2 of the aminoquinol and axial water that coordinates to
the copper ion, while the product phenylacetaldehyde remains bound at the