Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 289

regarding the overall architecture of the enzymes which comprises four distinct
domains and a substrate access channel between domains II and III [ 82 , 149 - 151 ]
(Figure 6 ).

Figure 6 Crystal structure of the Rhodobacter capsulatus DMSO reductase (PDB 1EU1). The
four domains of the protein are labeled (N-term Dom IV) and colored in

deepening shades of grey, bis-MGD cofactor shown as space-filling model.

¼ Dom I; C-term ¼

The substrate access channel in the DMSO reductases is quite wide which allows
a variety of substrates to enter the active site, an observation that is consistent with
the ability of the enzymes to reduce a variety of S- and N-oxide substrates. Solution
of the crystal structure of the R. capsulatus enzyme in complex with DMSO showed
that a polypeptide loop near the active site acts as a 'lid' for the active site during
catalysis [ 149 ].
Regarding the structure of the catalytic Mo center (Figure 3 ), however, clear
differences were observed in these initial structures. While the two pyranopterin-
guanidine dinucleotide cofactors and the central Mo atom were clearly identified in
both enzymes, the structures did not agree concerning the number and nature of
the Mo ligands, where variously between 5 and 7 ligands were reported [ 82 ,
149 - 151 ]. As direct ligands to the Mo center either one or two oxo ligands were
identified, and either one or both of the MGD cofactors were reported as being
ligated to the Mo center. A variety of spectroscopic investigations followed the
publication of the crystal structures, in attempts to resolve the issues caused by the