Environmental Engineering Reference
In-Depth Information
Figure 4 Genes for dissimilatory sulfate reduction in
D. vulgaris
Hildenborough and
D. gigas
DSM 1382. (a) Loci of genes for adenylyl sulfate reductase (AprAB), sulfate adenylyl transferase
(Sat), and heterodisulfide reductase (QmoABC). (b) Loci of genes for dissimilatory sulfite
reduction (DsrABD) and cobyrinic acid a,c-diamide synthase (C-synthase). Draft sequence is
used for
D. gigas
.
2.4 Sulfite Reductases
Two classes of sulfite reductases can be defined in SRP on the basis of their
physiological function. The first class comprises the low-molecular-mass and
low-spin sulfite reductases, also called assimilatory-type sulfite reductases (aSiR)
(EC 1.8.99.1). They have a molecular mass around 27 kDa, consist of a single
polypeptide chain and contain one [4Fe-4S] cluster coupled to a siroheme in a
low-spin state [
87
]. The second class is constituted by the high-spin dissimilatory
sulfite reductases (EC 1.8.99.3) dSiR, which possess a molecular mass around
200 kDa and a complex molecular architecture hosting siroheme centers and
[4Fe-4S] clusters. While dSiR is preferred it had been customary to refer to
dissimilatory sulfite reductase as Dsr and currently the genes for dSiR are desig-
nated as
dsr
and their gene products as Dsr. Both aSiR and dSiR enzymes have iron-
sulfur clusters and an iron-chelating sirohydrochlorin referred to as siroheme [
88
].
2.4.1 Dissimilatory High-Spin Sulfite Reductase
High-spin dSiR has either an
ʱ
2
ʲ
2
structure, as in the case of
Ar. fulgidus
[
89
] while
some SRB have an
ʱ
2
ʲ
2
ʳ
m
ʴ
n
multimeric structure, with
ʱ
50 kDa,
ʲ
45 kDa,
ʳ
11 kDa, and
ʴ
8 kDa [
90
,
91
]. Catalytic activity of dSiR is attributed to the
ʱ
2
ʲ
2
structure and the third protein,
subunit (DsrC), is an independent protein that
functions as a sulfur carrier protein to facilitate the release of H
2
S following
reduction of sulfite (Figure
3
)[
92
,
93
]. The reduced sulfur atom forms a persulfide
with a conserved cysteine on DsrC, the
ʳ
ʳ
subunit, and with the release of sulfide, the
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