Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 290

changing ligand environments and trying to reconcile the observation with
proposed mechanisms of catalysis (for more information see [ 84 , 152 ]).
The most recent crystal structure of a Dor-type DMSO reductase [ 81 ], which had
a resolution of 1.3

, resolved the issue of the apparently conflicting structures of
the Mo center. Li et al. [ 81 ] showed that the Mo site was disordered in the crystals,
and that at the high resolution of 1.3

Å

it was possible to identify two distinct forms
of the Mo center, a five-coordinate version similar to the structure reported by [ 83 ]
and a six-coordinate form that now constitutes the accepted version of the DMSO
reductase Mo center in the oxidized Mo(VI) state. The structure reported by Li
et al. [ 81 ] also explained the seven-coordinate, dioxo Mo center published earlier
[ 150 ] which likely arose from the presence of both five- and six-coordinate Mo
centers in the crystals that could not be resolved at the lower resolution of the earlier
structure. In the high-resolution structures, the four dithiolene ligands to the Mo
center are always attached to the Mo ion, and the coordination sphere of the Mo
consists of an oxo-methyl ligand (Ser147 in R. capsulatus ) and an oxo ligand.
In fact, it was shown by additional studies that the dissociation of one of the MGD
cofactors observed in some of the earlier crystal structures was due to reversible
damage caused by exposure of the enzyme to HEPES buffer under oxic conditions
[ 153 ], and modifications of

Å

the enzyme by DMS and air have also been

documented [ 140 ].
Based on the crystallographic, kinetic, and spectroscopic evidence the current
view of the DMSO reductase reaction mechanism is that in the oxidized,
Mo(VI) state the Mo center contains a serine ligand and a coordinated oxo group
and four sulfur ligands provided by the dithiolene groups of the two MGD cofactors.
On reduction of the center to the Mo(IV) state, the oxo group is lost, presumably to
form water, leaving a five-coordinate Mo(IV) center. The substrate, DMSO then
binds to the Mo(IV) center via its oxygen which remains as an oxo ligand on the
reoxidized Mo center after release of the reaction product, DMS [ 84 , 154 ].
To the best of our knowledge structural analyses of the effect of the substitutions
of the two active site residues that control substrate specificity (see above), Tyr114,
and Trp116 are not available at present [ 77 , 142 , 143 , 155 , 156 ].
A variety of spectroscopic techniques have been applied to the study of the Mo
centers of both Dms and Dor-type DMSO reductases including electron paramag-
netic resonance, resonance Raman, protein film voltammetry, various model com-
pounds and, recently, density functional theory calculations. These studies have
been reviewed in detail elsewhere [ 152 , 157 - 161 ].

2.1.3 Dimethylsulfoxide Reductase-Related Enzymes
and Dimethylsulfoxide Reductases in Unusual Biological Contexts

There is a wealth of sequence data indicating that both Dms- and Dor-type enzyme
systems exist in many microorganisms (see Section 2.3 ), and there have been
various reports of anaerobic, DMSO-based respiration in bacteria, but most of the