{PDOC00580}
{PS00746; NIFH_FRXC_1}
{PS00692; NIFH_FRXC_2}
{PS51026; NIFH_FRXC_3}
{BEGIN}
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* NifH/frxC family signatures and profile *
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Nitrogenase (EC 1.18.6.1) [1] is  the enzyme system responsible for biological
nitrogen fixation.  Nitrogenase is an oligomeric complex which consists of two
components: component 1  which  contains  the active site for the reduction of
nitrogen to ammonia and component 2 (also called the iron protein).

Component 2 is a homodimer of a  protein (gene nifH) which binds a single 4Fe-
4S iron sulfur cluster [2]. In the  nitrogen fixation  process  nifH  is first
reduced by a protein  such as ferredoxin;  the reduced  protein then transfers
electrons to component 1 with the concomitant consumption of ATP.

Crystal  structure  of  the  nitrogenase  iron  protein  has  been solved (see
<PDB:1NIP>)[2],  and revealed that it is composed of two subunits, each folded
as  a single alpha/beta type domain and connected at one surface by the 4Fe:4S
cluster.  The  shape  of the overall fold could be described as either an iron
butterfly or an iron lung, with the cluster representing the head or the heart
respectively.  At  the  core  of  each subunit is an eight-stranded beta sheet
(with  seven  of the eight beta strands oriented in parallel fashion), flanked
by nine alpha helices. This fold is a common motif found in nucleotide-binding
proteins [2].

A number of proteins  are  known  to  be  evolutionary related to nifH.  These
proteins are:

 - Chloroplast  encoded  chlL  (or  frxC) protein [3].  ChlL is encoded on the
   chloroplast genome of some plant species, its exact function  is not known,
   but it   could   act   as   an   electron  carrier  in  the  conversion  of
   protochlorophyllide to chlorophyllide.
 - Photosynthetic bacteria proteins  bchL  and bchX [4].  These  proteins  are
   also likely to play a role in chlorophyll synthesis.

There  are a number of conserved regions in the sequence of these proteins: in
the  N-terminal section there is an ATP-binding site motif 'A' (P-loop) and in
the  central  section there are two conserved cysteines which have been shown,
in  nifH,  to  be  the  ligands  of  the 4Fe-4S cluster. We have developed two
signature  patterns that correspond to the regions around these cysteines, and
a profile which cover all the conserved regions.

-Consensus pattern: E-x-G-G-P-x(2)-[GA]-x-G-C-[AG]-G
                    [C binds the iron-sulfur center]
-Sequences known to belong to this class detected by the pattern: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Consensus pattern: D-x-L-G-D-V-V-C-G-G-F-[AGSP]-x-P
                    [C binds the iron-sulfur center]
-Sequences known to belong to this class detected by the pattern: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Sequences known to belong to this class detected by the profile: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Last update: October 2004 / Text revised and profile added.

[ 1] Pau R.N.
     "Nitrogenases without molybdenum."
     Trends Biochem. Sci. 14:183-186(1989).
     PubMed=2672439
[ 2] Georgiadis M.M., Komiya H., Chakrabarti P., Woo D., Kornuc J.J.,
     Rees D.C.
     "Crystallographic structure of the nitrogenase iron protein from
     Azotobacter vinelandii."
     Science 257:1653-1659(1992).
     PubMed=1529353
[ 3] Fujita Y., Takahashi Y., Kohchi T., Ozeki H., Ohyama K., Matsubara H.
     "Identification of a novel nifH-like (frxC) protein in chloroplasts of
     the liverwort Marchantia polymorpha."
     Plant Mol. Biol. 13:551-561(1989).
     PubMed=2491672
[ 4] Burke D.H., Alberti M., Hearst J.E.
     "The Rhodobacter capsulatus chlorin reductase-encoding locus, bchA,
     consists of three genes, bchX, bchY, and bchZ."
     J. Bacteriol. 175:2407-2413(1993).
     PubMed=8468299

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