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 257

absorption maximum at 585 nm, but has no siroheme cofactor. Using reduced methyl
viologen as the electron donor, thiosulfate or trithionate could not replace sulfite;
however, nitrite and hydroxylamine were slowly reduced by the sulfite reductase.
Respiratory-linked tetrathionate reduction is present in soil bacterial communi-
ties [ 132 ] and species of Citrobacter, Proteus, Salmonella, and Pseudomonas
[ 133 ]. In Salmonella ( Sal. ) enterica Serovar Typhimurium, tetrathionate reductase
is encoded on the ttr operon with expression controlled by the global regulator
OxrA. This membrane-bound tetrathionate reductase is induced by tetrathionate
and has a bis-molybdopterin guanine dinucleotide (MGD) cofactor [ 134 ].
Dissimilatory thiosulfate reduction is found in numerous environmental bacteria
including Shewanella oneidensis [ 135 ] and enteric bacterial pathogens [ 125 ] such
as Sal. enterica. Anaerobic thiosulfate reductase activity (reaction 12 ; EC 1.97.1-)
in Sal. enterica is linked to the plasma membrane and the enzyme is a product of the
phsABC operon. Subunit phsA contains the active site and the cofactor MGD
[ 136 ]. The cytochrome b subunit, phsC, is an integral membrane protein that
contains two heme cofactors and a site to interact with the electron donor,
naphthoquinone-8 [ 125 ]. Subunit phsB has four iron-sulfur clusters which transfer
electrons between the subunits phsA and phsC.
Sulfite is reduced to sulfide by a cytoplasmic sulfite reductase and in Sal.
enterica the anaerobic sulfite reductase is a product of the chromosomal asr
(anaerobic sulfite reduction) operon [ 137 , 138 ] with asrA, asrB , and asrC encoding
for peptides of 40, 31, and 37 kDa, respectively. From genome analysis, it is
predicted that a [4Fe-4S] ferredoxin is present in both the asrA and asrC peptides
and a siroheme in the asrC subunit. NADH is proposed to be bound into the asrB
subunit. This anaerobic sulfite reductase forms a large complex of about 360,000
M r . An even larger complex has been detected for the assimilatory sulfite reductase
(670,000 M r ) which has an

ʱ 8 ʲ 4 structure with a flavin moiety in the ʱ subunit and

ʲ

siroheme plus [4Fe-4S] clusters in the subunit [ 139 ].

Dissimilatory reduction of sulfite to sulfide by W. succinogenes is attributed to
an octaheme cytochrome c without the involvement of a coupled siroheme-
[4Fe-4S] cofactor [ 140 ]. The octaheme cytochrome c , MccA, contains a heme
bound by the unique motif of CX 15 CH, and is encoded on the mccABCD gene
cluster. An octaheme cytochrome c , SirA, in Shewanella oneidensis MR-1 also
displays dissimilatory sulfite reductase activity [ 141 ].

2.4.3 Low-Molecular-Mass and Low-Spin Assimilatory-Type Sulfite
Reductase from Desulfovibrio vulgaris H, Desulfuromonas
acetoxidans, and Methanosarcina barkeri

A low-molecular-mass assimilatory-type sulfite reductase has been purified and
characterized from D. vulgaris H[ 67 , 142 , 143 ]. This enzyme has a molecular mass
of 27.2 kDa and its optical spectrum exhibits maxima at 405, 545, and 590 nm
(Table 4 ). This hemoprotein is able to reduce sulfite to sulfide in the presence of
reduced methyl viologen. The specific sulfite reductase activity was 900 mU/mg
protein [ 87 ].