Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 83

protected buried active site in oxy-hemocyanin is instead accessible to
phenolic substrates in tyrosinase. Detailed insight into how the peroxo moiety
transfers to phenol substrate still needs to be resolved; recent experiments on
model systems and the protein suggest that tyrosinase may effect a direct
phenol-to-quinone conversion rather than phenol to o -catechol transformation.

This is discussed further below.
Catechol oxidases, ubiquitous plant enzymes containing binuclear
copper centers, catalyze the oxidation of o -diphenols to the corresponding o-

quinones. -quinones can undergo auto-

polymerization to form brown polyphenolic catechol melanins, which protect
the damaged plant from pathogens or insects. A recent x-ray structure 23 of an
oxidized catechol oxidase from sweet potato shows that in the active site each
copper ion is coordinated by three histidine residues and the two copper ions
are 2.9 Å apart with a bridging solvent molecule, most likely a hydroxide ion.
Upon reduction the two copper ions move further apart with a metal-metal
distance of 4.4 Å, while no significant conformational change around the
copper centers is observed. In the reduced state, has a distorted trigonal
pyramidal geometry with three coordinated histidine residues and a water
molecule, while engages in a square planar geometry with one missing
coordination site. Based on biochemical, spectroscopic and structural data,
the proposed catalytic mechanism (Scheme 2), 23 similar to that delineated for
the catecholase activity observed for tyrosinase 12,24 (and even certain

The resulting highly reactive o

hemocyanins) 25,26 involves simultaneous binding of dioxygen and catechol
substrate (CAT) to the reduced enzyme to form catechol