Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 287

these regulatory circuits expression of the DmsA DMSO reductase is induced under
anaerobic conditions but repressed in the presence of nitrate. Molybdate is required
for full expression of the dmsABC operon , and defects in molybdate acquisition
affect nitrate-dependent repression of dmsABC expression [ 134 , 135 ].
While the molecular mechanisms of regulation are quite different for the Dor-
and Dms-type DMSO reductases there appear to be some common themes in the
regulation of DMSOR expression: In both systems anaerobiosis is a main factor
controlling activity of the operons, and this effect appears to be mediated by
FNR-type transcription factors. Anaerobiosis is an important regulatory element
also in other DMSOR systems such as the enzymes from Shewanella and
Halobacterium NRC-1 (see below) . Both systems are responsive to the presence
of molybdate in the growth medium through interactions with the ModE transcrip-
tion factor in E. coli and the ModE homologue MopB in R. capsulatus .
In addition, there are then organism-specific regulatory patterns such as the
integration of redox state and environmental light intensities in the phototrophic
Rhodobacter species, while in E. coli competition with nitrate reduction appears to
be a key aspect of DMSOR regulation. Overall both DMSOR systems are clearly
regulated in response to several environmental parameters which involve integra-
tion of the action of a variety of different transcription factors.

2.1.2 Kinetic and Structural Properties of Dimethylsulfoxide
Reductases

2.1.2.1 Kinetics of Dimethylsulfoxide Reduction

Kinetic parameters have been published for both Dor- and Dms-type enzymes using a
variety of substrates, including the biologically relevant compounds DMSO, methi-
onine sulfoxide (MetSO) and trimethylamine N-oxide (TMAO) (Table 1 ). Compre-
hensive lists of substrates tested and enzyme activities can be found in [ 136 , 137 ].
The standard assay for DMSO/TMAO reductases is carried out under anoxic
conditions and uses either methyl or benzyl viologen as the electron donor to the
enzymes with dithionite being used as the reductant. The substrate added can be

Table 1 Kinetic parameters
of Dor- and Dms-type
DMSO reductases.

DorA RC DorA RS DmsA EC
DMSO K M (mM) 0.0097 0.007 0.18
k cat (s -1 ) 42.9 58 79.9
MetSO K M (mM) n.r. 0.33 0.09
k cat (s -1 ) n.r. 58 61.1
TMAO K M (mM) 0.193 68 20.2
k cat (s -1 ) 134.5 2,300 1,203
DorA RC - Rhodobacter capsulatus DorA DMSO reductase
[ 142 ]; DorA RS - Rhodobacter sphaeroides DorA DMSO
reductase [ 143 ], DmsA EC - Escherichia coli DmsABC DMSO
reductase [ 137 ]