InterPro: IPR000392 Nitrogenase iron protein, subunit NifH/Protochlorophyllide reductase, subunit ChlL

This entry represents members of the NifH/BchL/ChlL family.

Nitrogen fixing bacteria possess a nitrogenase enzyme complex that catalyses the reduction of molecular nitrogen to ammonia [1, 2, 3]. The nitrogenase enzyme complex consists of two components:

• Component I is nitrogenase MoFe protein or dinitrogenase, which contains 2 molecules each of 2 non-identical subunits.
• Component II is nitrogenase Fe protein or dinitrogenase reductase, which is a homodimer. The monomer is encoded by the nifH gene [2].

Component II has 2 ATP-binding domains and one 4Fe-4S cluster per homodimer: it supplies energy by ATP hydrolysis, and transfers electrons from reduced ferredoxin or flavodoxin to component I for the reduction of molecular nitrogen to ammonia [4]. 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) IPR001687 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.

Protochlorophyllide reductase is involved in light-independent chlorophyll biosynthesis. The light-independent reaction uses Mg-ATP and reduced ferredoxin to reduce ring D of protochlorophyllide (Pchlide) to form chlorophyllide a (Chlide). This enzyme complex is composed of three subunits: ChlL, ChlN and ChlB. ChlL is present as a homodimer, and binds one 4Fe-4S cluster per dimer. The conserved domains, including the ATP-binding motif and the Fe-S binding motif found in the three subunits, are similar to those in nitrogenases [5].