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 259

reductase activity can be copurified with the tetrahemoprotein [ 26 , 146 , 147 ]. An
exposed, low-potential heme of the tetraheme cytochrome c 3 from Dsm.
norvegicum Norway 4 has been proposed to play an important mechanistic role.
The polysulfide chains of colloidal S 0 are attacked by the reduced tetraheme
cytochrome c 3 , leading to a collapse of the micelles with the precipitation of S 8
molecules [ 148 ].
The sulfide produced by polysulfide reduction opens up the S 8 rings by a
nucleophilic attack, leading to the production of new molecules of polysulfides,
which are themselves quickly reduced to sulfide by Dsm. norvegicum tetraheme
cytochrome c 3 [ 148 ]. Membranes isolated from D. gigas and Dsm. norvegicum
contained hydrogenase and c -type cytochromes and catalyzed the dissimilatory
sulfur reduction to sulfide. Sufficient hydrogenase and tetrahemic cytochrome c 3
must be linked with the D. gigas cytoplasmic membrane in the correct conforma-
tion to generate proton translocation sufficient for the chemiosmotic synthesis of
ATP [ 149 ].
3.1.2 Polysulfide Reductase from Wolinella succinogenes
The mechanism of polysulfide respiration has been mainly investigated in the
epsilon proteobacterial subclass W. succinogenes [ 27 , 32 ]. The membrane fraction
isolated from W. succinogenes cells grown with formate and either fumarate or
polysulfide, as electron acceptor, catalyzes the polysulfide reduction by formate or
H 2 . The corresponding electron transport chain consists of 8-methyl-menaquinone,
polysulfide reductase, and either hydrogenase or formate dehydrogenase. The
isolated polysulfide reductase consists of the three subunits predicted by the
psrABC operon, and contains a molybdenum ion coordinated by two molecules
of MGD. A model of this membrane association with coupled proton pumping
during sulfur respiration is given in Figure 5 .
The PsrA subunit is the catalytic unit, PsrB is an iron-sulfur protein, and PsrC
is an integral membrane protein that serves to anchor the other subunits on the
membrane. Energy conservation via polysulfide respiration in Archaea and
Bacteria appears to be similar. Membrane-bound respiratory chains produce a
chemiosmotic potential, which is used by membrane-bound ATP synthases to
form ATP [ 29 , 32 ].
3.1.3 Polysulfide Reductase from Desulfuromonas acetoxidans
The final draft genome of Drm. acetoxidans codes for a “cytochromome” of
47 putative multiheme cytochromes c [ 150 ]. This strain contains several multiheme
c -type cytochromes, the most abundant being the triheme cytochrome c 7 .
Polysulfides are formed in solution from the reaction of elemental sulfur with
sulfide and are probably the in vivo substrate utilized by sulfur-reducing eubacteria.