Environmental Engineering Reference
In-Depth Information
Also in the
dsr
operons of
Dba. vibrioformis
and
Dbu. rhabdoformis
was
dsr
D
which would encode a polypeptide of 9.8-8.7 kDa [
115
]. While the DsrD function
is unknown, its distribution appears to be only in sulfate-reducing microorganisms.
Very good sequence similarity of DsrD between
Dba. vibrioformis, Dbu.
rhabdoformis, D. vulgaris
H
, Dst. thermocisternum, Ar. fulgidus,
and
Ar.
profundus,
was reported. A gene comparable to
dsrD
appears absent in
Pyb.
islandicum
and
Alc. vinosum.
The
dsr
operons of
Dba. vibrioformis
and
Dbu.
rhabdoformis
contain a gene,
dnrN,
that encodes for a 53 kDa protein of unknown
function. The
dsrN
shows considerable homology to
cbiA
, a gene for amination of
cobyrinic acid to cobyrinic acid a,c-diamine [
118
]. Larsen et al
.
[
115
] suggest a
possible role for DsrN in sulfate reducers as the amidation of siroheme since
siroamide is a prosthetic group present
in sulfite reductase of
Desulfovibrio
species [
110
].
2.4.1.4 Desulfofuscidin-Type Sulfite Reductase
The dark brown-colored protein, desulfofuscidin is the dSiR of thermophilic
eubacterial sulfate reducers such as
Thermodesulfovibrio
strains
hydrogeniphilus
and
yellowstoni
and
Thermodesulfobacterium
(
T.
)
commune
and
T. mobile
[
119
-
121
]. The reduction of sulfite by
T. mobile
and
T. commune
is accomplished
by a sulfite reductase that has the
ʱ
2
ʲ
2
subunit structure.
The tetrameric dSiR from
T. commune
has a molecular mass of 167 kDa and
consists of nonidentical subunits of approximately 47 kDa [
120
], compared to
175 kDa (gel filtration) or 190 kDa (sedimentation equilibrium) found for the
dSiR from
T. mobile
, with subunits of 38-44 kDa [
119
]. Spectral absorption
maxima of dSiR from
T. commune
are at 576, 389, and 279 nm while for dSiR
T. mobile
these are at 578, 392, and 281 nm. Four siroheme groups are found in the
dSiR of
T. mobile
and
T. commune
with four [4Fe-4S] clusters in
T. commune
[
69
]
and eight [4Fe-4S] clusters in
T. mobile
[
119
]
.
The EPR spectrum of
desulfofuscidin exhibits resonances assigned to high-spin Fe(III) heme centers,
with
g
values of 7.02, 4.81, and 1.91 for the enzyme from
T. commune
[
120
], and
7.26, 4.78, and 1.92 for the enzyme from
T. mobile
[
119
].
2.4.1.5 Archaeal-Type Sulfite Reductase
The dissimilatory
Ar. fulgidus
sulfite reductase accounts for 0.5 % of the soluble
proteins and the isolated enzyme occurs as a tetramer with an
ʱ
2
ʲ
2
structure
[
117
]. The oxidized enzyme has an
-absorption maximum at 593 nm; upon
reduction with dithionite, the maximum shifts to 598 nm. In the oxidized protein
an absorption band with a maximum at 715 nm is observed; however, this band
disappears upon dithionite reduction suggesting that the siroheme is in the high-
spin state [
117
,
122
].
ʱ
Search WWH ::
Custom Search