Hydrogen Sulfide: A Toxic Gas Produced by Dissimilatory Sulfate and Sulfur Reduction and Consumed by Microbial Oxidation - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 261

The Drm. acetoxidans triheme cytochrome c 7 is rapidly reduced by the
D. vulgaris H [Fe] hydrogenase and it can completely reduce polysulfide with a
very high specific activity of 20 moles of hydrogen consumed/min/mg protein

[ 151 ]. This activity is twice as high as that reported for the purified tetraheme
cytochrome c 3 with the highest specific SR activity from Dsm. norvegicum [ 26 ,
147 ]. These results indicate that cytochrome c 7 is probably the true terminal sulfur
reductase in Drm. acetoxidans but the physiological electron donor for this triheme
protein remains to be found.

μ

3.1.4 Sulfur Oxidoreductase from Sulfurospirillum deleyianum

The S. deleyianum DSM 6946 sulfur oxidoreductase (SR) is a constitutive cyto-
plasmic enzyme. The S. deleyianum SR is energized by a [Ni-Fe] hydrogenase and
is several times more active in crude extracts than in other sulfur-reducing
eubacteria [ 27 ]. Sulfur reduction is enhanced by the presence of thiols and the SR
contains at least one [4Fe-4S] center but no b -or c -type cytochromes [ 27 ].

3.2 Archaebacterial Sulfur Reductase

3.2.1 Membraneous Sulfur Reductase Complex from Acidianus
ambivalens

The acidophilic Acidianus ( A. ) ambivalens DSM 3772 grows on elemental sulfur at
80 C. A hydrogenase-sulfur reductase complex (SR) has been isolated from the
membrane of A. ambivalens and the Sr subunits are similar to those found in
W. succinogenes [ 25 ] (Figure 5 ). SR is a molybdoenzyme belonging to the DMSO
reductase family [ 152 ]. The SR gene cluster consists of 5 subunits. The s reA gene
produces a 110 kDa protein that has binding motifs for a [4Fe-4S] center. The sreB
gene encodes for a protein rich in cysteines and could coordinate a [4Fe-4S] center.
SreC is a hydrophobic protein that stabilizes the SR into the membrane and the role
for SreD is unresolved at this time. The hydrogenase gene cluster consists of 12 genes
and 3 of these encode for structural proteins. The HynL subunit contains nickel and
the subunit HynS contains several [Fe-S] binding motifs, making this a [Ni-Fe]
hydrogenase. The third structural gene, Isp1, is an integral membrane protein and
serves to bind the 2 other subunits into the membrane. The quinone, presumed to be
sulfolobusquinone, transfers electrons from hydrogenase to the SR.

3.2.2 Sulfur-Reducing Complex from Pyrodictium abyssi

Extensive studies on sulfur reduction have been performed with Pyrococcus ( P. )
furiosus , A. ambivalens , Py. abyssi , and Py. brockii as model organisms. The
mechanism of sulfur reduction with H 2 , in some members of the Crenarchaeota