Metal Coordinated Phenoxyl Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 286

OH

O

H

H

O

II

O

II

HisN
HisN

H

HisN
HisN

Cu

Cu

H

O O

H

S S

H

O

H

H 2 O

OH

O

H

H

II

O

O

HisN
HisN

Cu

II

H

HisN
HisN

Cu

OH

O

S

S

OH

H 2 O 2

Cu I

HisN

H

O 2

HisN

OH O

S

Figure8.5 Active site and free radical catalysis by GO. (Adapted with permission from: [53] Copyright 1993
AmericanChemicalSociety;[54]Copyright1998AmericanChemicalSociety;[55]Copyright1998Elsevier.)

position (Figure 8.5). His581, His496, Tyr272 and an exogenous ligand (H 2 O or acetate, replacing the
substrate) coordinate to the copper in equatorial positions. 47,48 A striking feature of this complex is the
cross-linking of Tyr272 to Cys228 through a thioether bond at the ortho position of the hydroxly group,
presumably to lower the tyrosyl/tyrosine redox potential. The enzyme can exist in three well defined
oxidation levels: the copper(II) - tyrosyl radical oxidized form, an intermediate copper(II) - tyrosinate form
and the reduced copper(I) - tyrosine form, with only the former and the latter being catalytically active.
The tyrosyl/tyrosinate redox couple has been estimated at

0.01 V vs Fc + /Fc at pH 7 and extensive
spectroscopic studies on the active oxidized form have shown that the radical is mainly harbored by the
equatorially-bound Tyr272 residue. The enzyme is found to be relatively stable in this radical form with a
t 1
/ 2

+

of 7.2 days for its self-decomposition in absence of substrate. 49 The radical leads to an unusual optical

spectrum for GO, with intense bands at 445 nm and 800 nm 50 mainly attributed to charge transfer and

π * transitions of the tyrosyl group. Strong antiferromagnetic exchange coupling between the tyrosyl

radical and the paramagnetic copper(II) ion induces a diamagnetic ground state, with a singlet - triplet
splitting greater than 200 cm − 1 . 51,52
Catalysis of galactose oxidase proceeds in a ping-pong turnover reaction (Figure 8.5). 53-55 In the first
half-reaction the alcohol binds to the active site and the tyrosyl radical abstracts a hydrogen atom from the

- π