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 251

intramolecular disulfide linkage of DsrC is reformed by interaction with DsrK, a
subunit of the DsrMKJOP transmembrane complex [ 94 - 96 ]. The DsrD, ʴ subunit,

lacks cysteine residues [ 97 ] and has no function in electron transport.
Five different types of enzymes belonging to the high-spin DSR class
(desulfoviridin, desulfofuscidin, desulforubidin, P-582, and Archaeal) have been
isolated and characterized from different genera of SRP [ 2 , 15 , 16 ]. These five
enzymes differ mainly by the behavior of their siroheme moieties, their major
optical absorption, and EPR spectra [ 2 , 15 , 16 ] (Table 4 ).
Metals as cofactors are important in dSiR with each ʱ 2 ʲ 2 structure having
associated with it two sirohemes, a [4Fe-4S] cluster closely associated with each
siroheme and four ferredoxin-type [4Fe-4S] clusters at some distance from the
sirohemes. The binding of a siroheme and the [4Fe-4S] cluster to the

ʱ subunit and

ʲ subunit would be attributed to the (Cys-X 5 -Cys)-X n -(Cys-X 3 -Cys) arrangement

[ 98 ]. Binding of the ferredoxin-type [4Fe-4S] clusters to the subunits is

predicted to follow the arrangement of Cys-X 2 -Cys-X 2 -Cys that is preceded or
followed by a Cys-Pro sequence [ 99 - 101 ].

ʱ and ʲ

2.4.1.1 Desulfoviridin-Type Sulfite Reductase

The green protein, desulfoviridin, is the dSiR characteristic of the genus
Desulfovibrio but it has also been found in some species of the genera Desulfococcus,
Desulfomonile, Desulforegula, and Desulfonema [ 15 , 90 , 102 - 105 ]. The structure of
the dSiR of D. vulgaris H was reported to be a

ʱ 2 ʲ 2 ʳ 2 structure [ 67 ]andsimilartothe
ʱ 2 ʲ 2 ʳ 2 structure found in D. vulgaris oxamicus (Monticello), D. gigas ,and
D. desulfuricans ATCC 27774 [ 106 ]. DsrA and DsrB in D. vulgaris H are the
products of the dsrAB operon while dsrC , which encodes for DsrC, is located at
another site on the chromosome [ 95 , 96 ].
Associated with the

ʱ 2 ʲ 2 structure are two sirohemes and two sirohydrochlorins
which are positioned at the interface of DsrA and DsrB. The sirohydrochlorin
accounts for the absorption maximum at 628 nm. The sulfur atom from sulfite,
the substrate, is bound to the iron atom of siroheme in DsrA and this region is
surrounded by basic amino acids. The other side of the siroheme is surrounded by
residues from DsrB. Access to this catalytic site is through a positively charged
channel; however, a similar channel is lacking in the region where sirohydrochlorin
is bound to DsrA [ 95 , 96 ]. The D. vulgaris H DsrC subunit is in close proximity to
the cleft formed between DsrA and DsrB. The C-terminus of DsrC contains a
cysteine moiety which may interface with the siroheme and may participate in
the catalytic reaction. Oliveira et al. [ 95 , 96 ] propose that the initial reduction of
sulfite is a four and not a six electron step with S 0 formed as an intermediate
product. S 0 would interact with the terminal cysteine on DsrC to form a persulfide
which would be reduced to sulfide. Reduction of DsrC could be achieved by
heterodisulfide reductase activity of the membrane-bound DsrMKJOP which has
an appropriate iron-sulfur center for this reduction process [ 107 ].