{PDOC00314}
{PS00365; NIR_SIR}
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* Nitrite and sulfite reductases iron-sulfur/siroheme-binding site *
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Nitrite reductases (NiR) [1]  catalyze the reduction of nitrite into ammonium,
the second step in  the assimilation of nitrate.   There are two types of NiR:
the higher plant chloroplastic form of NiR (EC 1.7.7.1) is a monomeric protein
that uses reduced ferredoxin as the electron donor; while fungal and bacterial
NiR (EC 1.7.1.4) are  homodimeric proteins  that  uses NAD(P)H as the electron
donor. Both forms of NiR contain a siroheme-Fe and iron-sulfur centers.

Sulfite reductase    (NADPH)  (EC  1.8.1.2)  (SIR) [2] is the bacterial enzyme
that catalyzes  the  reduction  of  sulfite  to  sulfide. SIR is an oligomeric
enzyme with  a subunit composition of alpha(8)-beta(4), the alpha component is
a flavoprotein  (SIR-FP),  while the beta component is a siroheme, iron-sulfur
protein (SIR-HP).

Sulfite reductase (ferredoxin) (EC 1.8.7.1) [3] is  a cyanobacterial and plant
monomeric enzyme that also catalyzes the reduction of sulfite to sulfide.

Anaerobic sulfite  reductase (EC 1.8.1.-) (ASR) [4],  a  bacterial enzyme that
catalyzes  the NADH-dependent   reduction  of  sulfite  to sulfide.  ASR is an
oligomeric enzyme composed of three different subunits.  The C component (gene
asrC) seems to be a siroheme, iron-sulfur protein.

These enzymes share  a region of sequence similarity in their C-terminal half;
this  region which spans about 80 amino acids includes four conserved cysteine
residues. Two of the Cys are grouped together  at the beginning of the domain,
and the  two    others  are grouped in the middle of the domain. The cysteines
are involved in the binding of the iron-sulfur center; the last one also binds
the siroheme group [2].

We have derived a signature  pattern from the region around the second cluster
of cysteines.

-Consensus pattern: [STVN]-G-C-x(3)-C-x(6)-[DE]-[LIVMF]-[GAT]-[LIVMF]
                    [The 2 C's are iron-sulfur ligands]
-Sequences known to belong to this class detected by the pattern: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.
-Last update: April 2006 / Pattern revised.

[ 1] Campbell W.H., Kinghorn K.R.
     "Functional domains of assimilatory nitrate reductases and nitrite
     reductases."
     Trends Biochem. Sci. 15:315-319(1990).
     PubMed=2204158
[ 2] Crane B.R., Siegel L.M., Getzoff E.D.
     "Sulfite reductase structure at 1.6 A: evolution and catalysis for
     reduction of inorganic anions."
     Science 270:59-67(1995).
     PubMed=7569952
[ 3] Gisselmann G., Klausmeier P., Schwenn J.D.
     "The ferredoxin:sulphite reductase gene from Synechococcus PCC7942."
     Biochim. Biophys. Acta 1144:102-106(1993).
     PubMed=8347657
[ 4] Huang C.J., Barrett E.L.
     "Sequence analysis and expression of the Salmonella typhimurium asr
     operon encoding production of hydrogen sulfide from sulfite."
     J. Bacteriol. 173:1544-1553(1991).
     PubMed=1704886

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