Chemistry Reference
In-Depth Information
(a)
highly
electrophilic oxo
H
H
Cl
nucleophilic attack
O
O
Ca
O
O
Mn
V
O
Mn
IV
Mn
IV
Mn
IV
O
(b)
H
H
Cl
oxyl radical
attack
O
O
•
Ca
O
O
Mn
IV
O
Mn
IV
Mn
IV
Mn
IV
O
FIGURE 16.4
(a,b) Possible mechanisms for formation of dioxygen during the S4-S0 transition.
(Adapted rom
Barber, 2008
.)
FIGURE 16.5
The protein fold of Mn- or Fe-SODs (left) and the active site (right).
(From
Miller, 2004
.
Copyright 2004 with permission from
Elsevier.)
Catalases play an important protective role, catalysing the disproportionation of toxic hydrogen peroxide into O
2
and H
2
O. In contrast to the haem-containing catalases, which are ubiquitous in aerobic organisms, a broad range of
microorganisms, living in microaerophilic (almost oxygen-free) environments, including the lactic acid bacteria
,
3
have catalases which have a dinuclear manganese centre in their active site (
Figure 16.6
)
(
Wu et al., 2004
). These
'alternative' catalases are 4-helix bundle proteins, with the di-Mn centre located in themiddle of the
helical bundle.
The disproportionation of hydrogen peroxide is thermodynamically favourable; however, rapid reaction
requires a two-electron catalyst. For haem catalases, this is achieved by cycling between Fe
III
and Fe
IV
porphyrin
a
3. We mentioned earlier that this family of bacteria has adapted to its environment to function without iron, using Co and Mn instead.
