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varied but is usually present in millimolar amounts [
138
]. The DMSOR reaction is
reversible and can be assayed using the DMS dehydrogenase assay [
139
], but the
K
M
values for DMS as a substrate are very high, indicating that it is a poor natural
substrate [
140
,
141
].
Both characterized DorA proteins have very low
K
M
values (7-10
ʼ
M) for
DMSO, with the
K
M
for MetSO being reported as 330
M for the
R. sphaeroides
enzyme [
142
,
143
] (Table
1
). In contrast, the DmsABC enzyme had a lower
K
M
for
methionine sulfoxide (MetSO) than for DMSO, although the turnover numbers for
the two substrates were similar (61 and 78 s
1
) (Table
1
).
For the N-oxide substrate TMAO, all three enzymes exhibited increased
K
M
values (Table
1
), with the differences ranging from ~1 order of magnitude for the
R. capsulatus
enzyme to ~2-3 orders of magnitude for the other two enzymes. The
turnover numbers were significantly higher for TMAO than for the S-oxide sub-
strates, again with a ~2 order of magnitude increase for the
R. sphaeroides
and
E. coli
enzymes [
143
] (Table
1
).
The reactivity of the Dor-type enzymes towards S-oxides as well as N-oxides has
been shown to be modulated by two active site residues, Tyr114 and Trp116.
Tyr114 is present in all enzymes with known reactivity towards S-oxides, but is
absent in the TorA TMAO reductase. The nearby Trp116 influences substrate
turnover, but not substrate affinity and is present in both S- and N-oxide reducing
enzymes [
142
,
143
].
Non-steady state kinetic parameters have been reported [
140
,
141
] and the influence
of redox potential on the DMSOR reaction has been studied by protein film
voltammetry. These experiments revealed a potential-dependent switch in the activity
of the
E. coli
DmsA protein that was postulated to have implications for the biological
activity of the enzyme [
144
,
145
]. Similar potential-dependent switches were later
identified in other enzymes of the DMSOR enzyme family (e.g., periplasmic and
membrane-bound nitrate reductases) and are now thought to reflect the structure of
the Mo center of these enzymes as the proteins in question share little homology
[
146
]. Early investigation of the Dor-type DMSOR from
Rhodobacter capsulatus
indicated that this enzyme might not possess this switch [
147
]. The protein film
voltammetry experiments for the
Rhodobacter
enzyme also revealed the redox poten-
tials of the Mo
(VI/V)
and Mo
(V/IV)
couples which were reported as a function of pH.
An interesting observation is that in the Dms-type DMSO reductase from
E. coli
the removal of a serine residue postulated to be equivalent to the Ser147 Mo center
ligand of the
Rhodobacter
enzymes (see below) abolished activity of the enzyme
and altered the redox properties of the Mo center [
148
].
ʼ
2.1.2.2 Crystal Structures of Dimethylsulfoxide Reductases
At present only crystal structures of the Dor-type DMSO reductases are available.
The first crystal structures of the enzymes from two different species of
Rhodobacter
were solved within the same year with additional structures being
published in the following years [
82
,
149
-
151
]. All structures agree very well
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