Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 282

three subunits. While the catalytic subunits of the various enzyme complexes are
clearly related throughout the enzyme family, the other subunits and especially the
membrane subunits show considerable diversity [ 73 , 84 ].
Another key feature of DMSO reductase family enzymes is that all of them are
of bacterial origin, and share a common active site assembly where a single Mo ion
is coordinated by two molecules of a pyranopterin-dinucleotide cofactor (Figure 3 ).
The pyranopterin ligands provide two dithiolene sulfur ligands to the Mo atom
each, additional Mo ligands including amino acids, oxygen and sulfur atoms may
also be present and will vary depending on the type of enzyme studied [ 84 , 85 ]. The
type of the dinucleotide attached to the pyranopterin cofactor may also vary, with
guanosine and cytosine being the most commonly found derivatives [ 84 , 85 ].
The reactions catalyzed by DMSO reductase family enzymes are very diverse
and so far include both redox and non-redox reactions as well as hydroxylation
reactions. The most common reactions are redox reactions that involve the transfer
of oxygen or sulfur to or from a substrate, depending on whether the enzyme in
question is a reductase or a dehydrogenase. The first of these two electron transfer
reactions uses water as the oxygen donor or reaction product and is a reaction that is
unique to Mo- or W-containing enzymes (eq. 1 )[ 92 ].

2e þ 2H þ Ð

R O þ R þ H 2 O ð 1 Þ

For the sulfur transfer reactions HS or H 2 S replace water in the reaction scheme
(eq. 2 )

2e þ 2H þ Ð R S þ R þ H 2 S

2e þ H þ Ð HS

R S þ R þ ð 2 Þ

A special case is the reaction of the formate dehydrogenase where catalysis leads
to the formation of CO 2 and water from formate. This constitutes a formal hydrogen
atom transfer as oxygen from water is not incorporated into the product. However,
the reaction still

involves a two electron transfer from the substrate to the

enzyme [ 93 ].
In addition to the above reactions that are typical for mononuclear Mo and W
enzymes, some recently identified enzymes of the DMSO reductase family, e.g.,
ethylbenzene dehydrogenase or C25 cholesterol dehydrogenase [ 94 , 95 ], catalyze
hydroxylation reactions (eq. 3 ):

2e þ 2H þ R H þ H 2 O Ð R OH þ ð 3 Þ

A few known members of the DMSO reductase family even catalyze non-redox
reactions (acetylene hydratase, pyrogallol phloroglucinol transhydroxylase) [ 96 -
99 ]. Several recent reviews on enzymes of the DMSO reductase family exist
and cover various aspects of the evolution, structural, spectroscopic, and kinetic
properties of the enzymes in the enzyme family [ 73 , 79 , 84 , 85 , 93 , 100 , 101 ].