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been characterized. The
Rhodovulum sulfidophilum
DMS dehydrogenase is
expressed during phototrophic growth of the bacteria in the presence of DMS
which can serve as an electron donor to the photosynthetic apparatus via its electron
acceptor, a cytochrome
c
2
[
139
,
175
-
177
]. DMS dehydrogenases essentially catalyze
the reverse of the reaction of DMSO reductases (Figure
2
), and like the DMSO
reductases are capable of catalyzing both the forward and the reverse reaction [
139
].
The
R. sulfidophilum
DdhABC DMS dehydrogenase is a soluble, trimeric,
periplasmic protein that consists of a catalytic Mo subunit (DdhA), that also
contains an Fe/S cluster, a subunit with 4 Fe/S clusters (DdhB), and a subunit
with a single heme
b
group (DdhC) [
139
,
175
,
177
]. The operon encoding DMS
dehydrogenase,
ddhABDC
, also encodes a cytoplasmic protein, DdhD, that is likely
to be a molecular chaperone involved in maturation of the enzyme, and upstream of
this operon a gene (
ddhS
) encoding a signal kinase with homology to the DorS
sensor kinase was identified [
175
], but no molecular studies of the regulation of the
DMS dehydrogenase have been published so far, and a genome sequence is
currently not available, although a sequencing project for
R. sulfidophilum
appears
to exist (Source:
www.ncbi.nlm.nih.gov/
)
.
DMS dehydrogenase activity assays contain dichlorophenol-indophenol,
phenazineethosulfate, and DMS as the substrate [
139
]. The purified DMS dehydro-
genase had a maximal activity around pH 8, with p
K
a
values of 7.7 and 8.9 being
reported, and
K
M
values for both DMS and cytochrome
c
2
were in the low
micromolar range with values of 52
M, respectively [
178
]. Further
analyses showed that DMS dehydrogenase uses a two-site ping-pong catalytic
mechanism [
178
]. Cyclic voltammetry revealed a pH-dependent shift in the heme
redox potential of approximately 20 mV/pH unit, and electron paramagnetic reso-
nance was used to determine the redox potentials of the Mo, heme, and four of the
Fe-S redox centers present in the enzyme at pH 8. The redox potentials of the two
Mo couples were 55
9 and 21
2
ʼ
10 mV
versus
SHE for Mo
V/IV
and 123
13 mV
versus
SHE for the Mo
VI/V
couple [
179
]. A comparison of the redox potentials of the
various centers present in the enzyme showed that the potential range and variation
was very similar to those observed in ethylbenzene dehydrogenase from
Aromatoleum aromaticum
and the NarG-type nitrate reductase [
179
].
þ
þ
/
/
2.2.2 Dimethylsulfide Dehydrogenases: Environmental Distribution
Analysis of DdhA-related sequences present in public databases (
www.ncbi.nlm.
-Proteobacteria
R. sulfidophilum, Sagittula stellata
,and
Citreicella
sp. SE45 that occurs together with
a second group of sequences originating from the
ʱ
-Proteobacteria
Thiorhodococcus
drewsii
,
Pseudomonas chloritidismutans
,and
Halomonas jeotgali
.Thesesixspecies
also contain genes encoding proteins with high homologies to DdhB and DdhC and
can thus be regarded as likely being true DMS dehydrogenases, despite the fact
that the enzyme from
P. chloritidismutans
is annotated as a putative chlorate reduc-
tase (ClrABC). Another group of sequences originating from members of the
γ
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