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 256

Ar. veneficus grows with dissimilatory sulfite or thiosulfate reduction but is
unable to couple growth to the transfer of electrons from the electron donor to
sulfate [ 123 ]. The Dsr gene locus of Ar. profundus has dsrA contiguous to dsrB
[ 112 ]. The DsrD, as product of Ar. profundus dsrD, is projected to be about 9.3 kDa
and would be comparable to the subunit present in the dSiR of Desulfovibrio

species where it functions in sulfite-binding [ 108 ]. Downstream of dsrABD in both
Archaeoglobus species is a gene for ferredoxin and this is significant because
ferredoxin may be the electron donor for enzymatic sulfite reduction.

ʳ

2.4.2 Oxy-Sulfur Reductases in Non-sulfate Reducers

There are several genera of bacteria that are capable of reducing tetrathionate,
thiosulfate, and sulfite to H 2 S (see reactions 11 , 12 , and 13 ), respectively. Sulfite, an
intermediate of dissimilatory sulfate reduction, is readily utilized by SRB and the
metabolism of tetrathionate or thiosulfate with the production of sulfide is covered
in Section 2.1 . In the environment, thiosulfate is found in marine and freshwater
sediments [ 124 ] and in the lumen of mammalian large intestine [ 125 ]. Tetrathionate
is present in soils, where SRB are growing [ 126 ], and is produced in the gut as a
result of inflammatory response [ 127 ]. Several microorganisms that are taxonom-
ically unrelated have enzymes that function under anaerobic conditions as facili-
tating sulfite respiration (reactions 11 , 12 , and 13 )[ 20 ].

S 4 O 2 6 2e 2S 2 O 2

3

E 0'

þ ! ¼ þ 170 mV 41 ½ ð 11 Þ

S 2 O 2 2H þ þ 2e ! SO 2

3

E 0' þ þ H 2 S ¼ 402 mV 41 ½ ð 12 Þ

3

SO 2 3 8H þ þ 6e ! 3H 2 O 0'

þ H 2 S þ ¼ 160 mV 41 ½ ð 13 Þ

The sulfite reductase, Fsr, present in Methanocaldococcus jannaschii (M r
70 kDa) could have arisen from gene fusion since the N-terminal part of the enzyme
is a homolog of the

ʲ

subunit of coenzyme F 420 -reducing hydrogenase (FpoF or
FqoF) while the C-terminal half has homology to DsrA and DsrB of the siroheme-
containing dSiR [ 128 ]. A homolog of this bifunctional sulfite reductase from
Methanocaldococcus jannaschii is also present in Methanopyrus kandleri and
Methanococcoides burtonii [ 129 ].
The dissimilatory sulfite reductase, dSiR, of Bilophila wadsworthia has a ʱ 2 ʲ 2 ʳ n

(n 2) multimeric structure. This enzyme has an absorption maximum at 630 nm

[ 130 ] indicating that it is a desulfoviridin-type enzyme. The ʱ subunit (M r 49 kDa)

contains the siroheme-[4Fe-4S] while the ʲ

subunit is a fusion protein resulting from fused dsrB and dsrD genes. Based on
phylogenic analysis, Bilophila wadsworthia is closely related to D. desulfuricans
Essex 6 and at one time may have been a dissimilatory sulfate reducer [ 130 ].
An inducible dissimilatory sulfite reductase in Clostridium pasteurianum couples
growth to the reduction of sulfite [ 131 ].Theisolatedenzyme(M r 83.6 kDa), has an

ʲ subunit polypeptide is 54 kDa. The