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 264

compound oxidation is best characterized in the GSB Chlorobaculum tepidum and
in the PSB Alc. vinosum .

4.2.1 Oxidation of Sulfide

Enzymes that oxidize sulfide are the membrane-bound SQR and the periplasmic
flavocytochrome c sulfide dehydrogenase (FccAB, EC 1.8.2.3) [ 49 ].

4.2.1.1 Sulfide:Quinone Oxidoreductase

SQR (EC 1.8.5.4) catalyzing sulfide oxidation with an isoprenoid quinone as the
electron acceptor is present in both phototrophic and chemotrophic bacteria
[ 49 ]. SQR plays an important role for the sulfide oxidation in PSB and FccAB
appears less widespread. SQR is the only known sulfide-oxidizing enzyme that is
found in all GSB strains. SQR is a member of the flavin disulfide reductases family.
SQR of Rhodobacter capsulatus is a membrane-bound flavoprotein with its
active site located in the periplasm. The first X-ray structure for an SQR has been
recently determined to 2.6

resolution in A. ambivalens [ 158 ]. This membrane-
bound flavoprotein has two redox active sites: a covalently bound FAD and an
adjacent pair of cysteine residues bridged by a trisulfide bridge between the two
cysteine residues [ 158 ].

Å

4.2.1.2 Flavocytochrome c Sulfide Dehydrogenase

Flavocytochrome c is usually a periplasmic enzyme consisting of a small FccA
cytochrome c subunit (20 kDa) and a larger sulfide-binding FccB flavoprotein
subunit (44 kDa). In vitro , flavocytochromes can catalyze electron transfer from
sulfide to a variety of small c -type cytochromes (such as Alc. vinosum cytochrome
c 550 ) that may then donate electrons to the photosynthetic reaction center [ 49 ]. The
in vivo role of flavocytochrome c is unclear and if indeed the FccAB oxidizes H 2 S
in vivo , both PSB and GSB have alternative sulfide-oxidizing enzyme systems, such
as SQR, that may be quantitatively more important.

4.2.2 Oxidation of Polysulfides

Polysulfides appear to be the primary product of sulfide oxidation in a number of
PSB and GSB. The oxidation of sulfur deposits is the least understood step of sulfur
metabolism. It is currently unknown how polysulfides are converted into sulfur
globules and it could be a purely chemical, spontaneous process as longer
polysulfides are in equilibrium with S 0 [ 49 ].