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 263

The initial step of A. ambivalens sulfur oxidation involves a cytoplasmic sulfur
oxygenase reductase (SOR) (EC 1.13.11.55) catalyzing the oxygen-dependent
sulfur disproportionation to form sulfide plus hydrogen sulfite [ 155 ] (reaction 14 ):

4S 0 2 HSO 3 þ 2H þ þ O 2 þ 4H 2 O ! 2H 2 S þ ð 14 Þ

Then thiosulfate is likely produced from sulfur and hydrogen sulfite in a
non-enzymatic reaction.
SOR of A. ambivalens is a homo-oligomer composed of 24 identical monomers
and the catalytic pocket of each subunit contains a low-potential mononuclear
non-heme iron center and three conserved cysteinyl residues. The iron center is
likely the site for both sulfur reduction and oxidation [ 156 ]. The three products of
the A. ambivalens sulfur oxidation step (sulfite, thiosulfate, and sulfide) are pre-
sumably further oxidized to sulfate for energy conservation [ 29 , 155 ].
Two sulfite oxidation pathways are present in A. ambivalens : (a) A membrane-
bound sulfite:acceptor oxidoreductase as part of the Sox complex; (b) An alterna-
tive indirect soluble sulfite oxidation pathway coupled to substrate-level phosphor-
ylation via APS reductase and APS:phosphate adenylyltransferase [ 155 ].
Two membrane-bound complexes are involved in A. ambivalens thiosulfate
oxidation: (a) The terminal aa 3 -type quinol oxidase which shuttles electrons from
the caldariellaquinone pool to O 2 and consists of three subunits, which are encoded
in a single operon in the A. ambivalens genome. (b) The membrane-bound
tetrathionate-forming thiosulfate:quinone oxidoreductase which oxidizes thiosul-
fate to form tetrathionate with caldariellaquinone as electron acceptor [ 155 , 157 ].

4.2 Eubacterial Inorganic Sulfur Compound Oxidation

The main enzymes or multienzyme complexes involved in sulfur compound
oxidation are present both in CSB and phototrophic sulfur bacteria [ 49 ]. PSB and
GSB use various combinations of sulfide, elemental sulfur, sulfite, and thiosulfate
as electron donors in CO 2 fixation during anoxygenic photosynthetic growth
[ 49 ]. Genetic and biochemical analyses show that the dissimilatory sulfur metab-
olism of the phototrophic organisms is very complex and still incompletely under-
stood. We will only describe here the oxidative sulfur metabolism of anoxygenic
phototrophic bacteria (GSB, PSB, and PNSB).
A variety of enzymes catalyzing inorganic sulfur oxidation reactions have been
purified and biochemically and genetically characterized from PSB and GSB
[ 49 ]. Complete genome sequence data are currently available for one strain of
PSB and for ten strains of GSB. A number of genes potentially involved in the
oxidative sulfur metabolism are present both in GSB and PSB: for example, genes
for the sulfide:quinone oxidoreductase (SQR) and the sulfide-oxidizing enzyme
flavocytochrome c [ 49 ]. On a molecular biochemical and genetic level, sulfur