Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 283

2.1.1.1 Dimethylsulfoxide Reductases - Basic Types and Properties

Within the DMSO reductase enzyme family there are two known types of DMSO
reductases (DMSOR), the Dor- and the Dms-type. Both types of enzymes are located
in the bacterial periplasm, i.e., in an extracytoplasmic compartment, and use quinol
as the electron donor for their reaction. However, while the Dms-type enzymes are
membrane-bound via additional subunits (DmsB, DmsC), the Dor-type enzymes are
soluble and only transiently interact with the membrane-bound DorC-type cyto-
chromes that act as electron donors for these systems (Figure 4 ).

Figure 4 Cellular localization and reactions catalyzed by DMSO reductases of the Dor- and
Dms-type. The enzymes shown are based on the structures of the Rhodobacter DorA DMSO
reductase and the Escherichia coli DmsABC DMSO reductase. MQ/MQH 2 , menaquinone/
menaquinol; UQ/UQH 2 , ubiquinone/ubiquinol; broken arrows indicate transient protein

interactions.

Both types of enzyme systems are part of the bacterial respiratory chains where
they function as alternative terminal reductases mediating anaerobic respiration and
energy conservation in the absence of oxygen. However the primary sequences of
these two types of enzymes only show 26 % amino acid (aa) identity (48 % aa
similarity) which clearly indicates that they are not closely related (see Section 2.3
below).

2.1.1.2 Organization of Dimethylsulfoxide Reductase-Encoding Operons

The core operons encoding Dms-type DMSO reductases usually consist of three
genes, dmsABC , where dmsA encodes the catalytic subunit of the enzyme that
contains the Mo cofactor and an N-terminal iron-sulfur cluster that is usually
designated as FS0. The dmsB gene encodes a ferredoxin-type electron transfer
protein with four [4Fe-4S] clusters (FS1-4), and dmsC encodes a membrane subunit
with 8 transmembrane helices (Figures 4 and 5 ).
For the maturation of the DmsABC protein complex a fourth protein, the DmsD
molecular chaperone is required [ 102 ]. The DmsD protein is a so called 'REMP', a