Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 296

The DMS dehydrogenases are part of the Type II enzyme group which is the
group with the greatest diversity of reactions catalyzed by representative enzymes.
Within this group the DMS dehydrogenases form a tightly clustered group of
6 sequences, all of which are of proteobacterial origin, and a second, closely related
group of uncharacterized enzymes from

ʵ

-Proteobacteria and Aquificales may also
be DMS dehydrogenases. The closest relatives of DMS dehydrogenases that have
been characterized are the selenate reductases, and the group of enzymes compris-
ing ethylbenzene and C25 cholesterol dehydrogenases (data not shown).
The DMSO reductases represent an interesting case with the Dor-type enzymes
being representative of the Type III enzyme group, while the Dms-type enzymes
are Type II enzymes and are distantly related to DMS dehydrogenase as indicated
by their subunit structure and the presence of an Fe/S cluster in the catalytic subunit.
Within the Type II enzymes, the DmsA enzymes form a deep branching, indepen-
dent lineage [ 175 , 180 ].
This raises an interesting question regarding the identity of the amino acid ligand
to the Mo center in Dms-type DMSO reductases. The two available crystal struc-
tures of Type II enzymes (NarGHI nitrate reductase and ethylbenzene dehydroge-
nase) [ 89 , 91 , 182 ] as well as sequence alignments (Figure 8 ) indicate that the Type
II enzymes contain an aspartate ligand to the Mo center, and some authors have
suggested that, e.g., the DmsA protein from Halobacterium NRC-1 contains an
aspartate as the Mo ligand [ 174 ].
However, there is also a study that indicates that at least the E. coli DmsA
protein might contain a serine ligand to the Mo center. Trieber et al. identified
Ser176 as a putative conserved Mo ligand in DmsA based on sequence alignments
with Dor/Tor Type III proteins [ 148 ]. They created variants in this amino acid, all of
which abolished the ability of E. coli to grow anaerobically with DMSO as the
electron acceptor, and also abolished DMSO reductase enzyme activity in vitro
[ 148 ]. Changes to the redox properties of the Mo center were also reported, and an
S176H mutation led to a complete loss of the Mo electron paramagnetic resonance
spectrum [ 148 ]. All of these results strongly indicate a role for DmsA-Ser176 in the
function of the E. coli DMSO reductase.
Using sequence alignments containing a variety of sequences for Type II and
Type III DMSOR family enzymes, we found that the alignment of DmsA sequences
with the identified Mo ligands of Type II and Type III enzymes is very sensitive to
the selection of sequences to be used in the alignment (Figure 8 ).
Alignments containing mostly sequences of Dor-type TMAO and DMSO reduc-
tases as well as sequences for a few Type II enzymes strongly resembled the
alignment shown by [ 148 ]where E. coli DmsA-Ser176 clearly aligns with the
known serine ligand of Dor/Tor-type enzymes. However, E. coli DmsA-Ser176 is
located in a 'GDYS' sequence motif, and in this version of the alignment the aspartate
residue located in this motif aligns with the known Asp Mo ligand of the Type II
enzymes. The only exception to this is the Halobacterium DmsA sequence.
In contrast, if alignments are carried out with sequences of Type II enzymes only
(DMS dehydrogenase, chlorate reductase, ethylbenzene dehydrogenase, selenate
reductase, steroid C25 dehydrogenase, and perchlorate reductase) and sequences