Chemistry Reference
In-Depth Information
FIGURE 16.6
Lactobacillus plantarum Mn catalase: (a) stereo view of the secondary structure
e
the di-Mn unit as red spheres and (b) the
detailed geometry of the di-Mn centre.
(From
Barynin et al., 2001
.
Copyright 2001 with permission from Cell Press.)
ð-cation. With two Mn ions, each of which can operate between Mn
II
and Mn
IV
, there are five possible oxidation
states for Mn catalases. A combination of spectroscopic techniques has shown that at least four of these are
observed
a reduced Mn
I
2
state, a mixed-valence Mn
II
Mn
III
state, an oxidised Mn
III
2
e
state, and a superoxidised
Mn
III
Mn
IV
state. There is no evidence to date for a Mn
I
2
state.
A catalytic mechanism based on the structure of the active site has been proposed (
Figure 16.7
) involving
distinct pathways of reactivity in the oxidised and reduced half reactions. H
2
O
2
binds terminally to the oxidised
cluster of the oxidative half reaction, replacing the water molecule bound to one of the Mn ions (
Figure 16.7
): this
is also the site at which azide binds in the catalase crystals. Two-electron oxidation of the H
2
O
2
by the dinuclear
Mn(III) results in release of the dioxygen product, facilitated by the oxygen bridges which electronically couple
the Mn ions, allowing them to function as a unit. Glu 178 is proposed to transfer the peroxide protons to active site
bases, most likely the solvent bridges between the metal ions. In the reductive half reaction, H
2
O
2
is proposed to
bind in a bridging position, to give a symmetric
O
bond cleavage and reoxidation of the di-Mn core. Glu 178 could serve to protonate the nonbridging oxygen of the
bound substrate. This mechanism results in replacement of one of the oxygen bridges of the cluster during each
turnover cycle, with retention of a substrate oxygen atom between successive reactions.
m
-bridging peroxide complex which will be activated to O
e
NONREDOX DI-MN ENZYMES e ARGINASE
In addition to the redox di-Mn catalases, there are a number of other enzymes with di-Mn centres, of which
the best characterised are arginases, which catalyse the divalent cation-dependent hydrolysis of L-arginine to form
L-ornithine (
Ash, 2004
) and urea.
