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 258

The D. vulgaris H assimilatory-type sulfite reductase has been studied by
chemical, EPR, and M¨ssbauer techniques [ 143 ]. This protein contains one
siroheme and a single [4Fe-4S] cluster. As purified, the siroheme is in a low-spin
Fe(III) state (S

2.44,
2.36, and 1.77. Hereby, the iron-sulfur cluster is in the [4Fe-4S] 2+ state. Similar to
the hemoprotein subunit of E. coli sulfite reductase, low-temperature M ¨ ssbauer
spectra of D. vulgaris H sulfite reductase also show evidence for an exchange-
coupled siroheme-[4Fe-4S] unit [ 143 ]. The presence of an assimilatory-type sulfite
reductase in D. vulgaris H is surprising because this strain produces large amounts
of sulfide during normal growth on sulfate and also because the enzymes respon-
sible for dissimilatory sulfate reduction are constitutive.
Two low molecular mass hemoproteins with sulfite reductase activity (named
P 590 ) have been purified and characterized from two sulfur reducers: Ms. barkeri
and Drm. acetoxidans [ 87 , 144 , 145 ]. Both monomeric hemoproteins present
visible spectra similar to that of the low-molecular-mass and low-spin assimila-
tory-type sulfite reductase of D. vulgaris H. The Drm. acetoxidans sulfite reductase
has a molecular mass of 23.5 kDa and exhibits absorption maxima at 405, 545, and
587 nm [ 144 ]. The Ms. barkeri P 590 has a molecular mass of 23 kDa and its optical
visible spectrum exhibits maxima at 395, 545 and 590 nm (Table 4 )[ 145 ]. EPR
spectra of the enzyme as isolated show that the siroheme is in a low-spin Fe(III)
state (S

¼ 1/2) which exhibits characteristic EPR resonances at g ¼

1/2) with g-values at 2.40, 2.30, and 1.88 for the Ms. barkeri P 590 enzyme
and g-values at 2.44, 2.33, and 1.81 for the Drm. acetoxidans enzyme [ 144 ].
Chemical analysis shows that both hemoproteins contain one siroheme and one
[4Fe-4S] cluster per polypeptidic chain [ 144 ]. The specific sulfite reductase activity
was 906 mU/mg protein for the Drm. acetoxidans P 590 enzyme and 2,790 mU/mg
protein for the Ms. barkeri enzyme [ 87 , 144 ]. The M s. barkeri P 590 enzyme has a
higher specific sulfite reductase activity than that reported for the high-spin dSiR from
SRB such as desulforubidin, desulfofuscidin, and desulfoviridin [ 25 ]. The two
assimilatory-type sulfite reductases of sulfur reducers contain 5 labile sulfur atoms
and 5 iron atoms; as it is the case for the D. vulgaris H enzyme, it was postulated that
the extra sulfur atom could be the bridging ligand between the [4Fe-4S] center and the
siroheme [ 87 ]. Both hemoproteins catalyze the direct six-electron reduction of sulfite
to sulfide without the formation of free intermediates (thiosulfate and trithionate).

¼

3 Enzymology of Hydrogen Sulfide Production
from Elemental Sulfur

3.1 Eubacterial Sulfur Reductase

3.1.1 Sulfur Reductase in Desulfovibrio and Desulfomicrobium Species

The tetraheme cytochrome c 3 has the function of an elemental sulfur reductase in
several Desulfomicrobium and Desulfovibrio species from which the sulfur