Chemistry Reference
In-Depth Information
h
ν
e
-
O
2
h
ν
S
0
S
1
2H
+
H
+
2H
2
O
e
-
S
2
S
4
e
-
H
+
h
ν
S
3
e
-
h
ν
FIGURE 16.2
The S-state cycle model of O
2
generation.
(Adapted from
Voet & Voet, 2004
: pp. 1591.)
(a) with distances less than 3
˚
shown by connecting lines.
Figure 16.3
(c,d) presents a remodelling of the water-
splitting site using the native electron density maps of
Ferreira et al. (2004)
and
Loll, Kern, Saenger, Zouni, and
Biesiadka (2005)
and the Mn-anomalous difference map of
Ferreira et al. (2004)
, keeping the Mn
3
Ca
2
þ
O
4
cubane
of Ferreira et al. but with Mn
4
linked to it via a single 3.3
˚
mono-
-oxo bridge.
Although the precise geometry of the Mn
4
Ca cluster is not yet known precisely, these models provide a basis
for developing chemical mechanisms for water oxidation and dioxygen formation. The location of one Mn ion
(Mn
4
or dangler Mn) adjacent to the Ca
2
þ
and their positioning towards the side chains of several key amino acids,
including the redox active TyrZ, suggests that they provide the 'catalytic' surface for binding the two substrate
water molecules and their subsequent oxidation. Two mechanisms are presented in
Figure 16.4
. In the first, it is
proposed that the substrate water associated with Mn
4
is deprotonated during the S-state cycle and that Mn
4
is in
a high-oxidation state (Mn(V)) by the time the cluster reaches the S
4
-state just prior to O
m
O bond formation. The
other three Mn ions are also driven to high-valency states (Mn(IV)) by S
4
and act as an oxidising battery for the
oxo-Mn
4
complex. In this way, the oxo is highly electrophilic, making it an ideal target for a nucleophilic attack by
the oxygen of the second substrate water bound within the coordination sphere of the Ca
2
þ
(
Figure 16.4
(
a)). The
second mechanism (
Figure 16.4
(
a)) proposes that the deprotonated water molecule on Mn
4
forms an oxyl radical
which attacks an oxygen atom linking Ca
2
þ
with a Mn or the oxygen of the water molecule coordinated to the
Ca
2
þ
to form the O
e
e
O bond.
2D
MN
AND DETOXIFICATION OF OXYGEN FREE RADICALS
Manganese is the cofactor for catalases, peroxidases, and superoxide dismutases which are all involved in the
detoxification of reactive oxygen species (SOD). We consider here the widely distributed Mn-SOD and then
briefly describe the dinuclear Mn catalases.
Mn superoxide dismutases are found in both eubacteria and archaebacteria as well as in eukaryotes, where they
are frequently localised in mitochondria. They (
Figure 16.5
) have considerable structural homology to Fe-SODs:
both are monomers of ~200 amino acid and occur as dimers or tetramers and their catalytic sites are also very
