ENTRY       EC 1.18.6.1
NAME        Nitrogenase
CLASS       Oxidoreductases
            Acting on reduced ferredoxin as donor
            With dinitrogen as acceptor
SYSNAME     Reduced ferredoxin:dinitrogen oxidoreductase (ATP-hydrolysing)
REACTION    3 Reduced ferredoxin + 6 H+ + N2 + n ATP = 3 Oxidized ferredoxin
            + 2 NH3 + n ADP + n Orthophosphate;
            8 H+ + 8 e- + 16 ATP + N2 + 16 H2O = H2 + 2 NH3 + 16 ADP +
            16 Orthophosphate;
            16 ATP + 16 H2O = 8 H+ + 8 e- + 16 ADP + 16 Orthophosphate;
            2 H+ + 2 e- + N2 = Diimine;
            2 H+ + 2 e- + Diimine = Hydrazine;
            2 H+ + 2 e- + Hydrazine = NH3;
            2 H+ + 2 e- = Hydrogen
SUBSTRATE   Reduced ferredoxin
            H+
            Acetylene
            N2
            ATP
PRODUCT     Oxidized ferredoxin
            NH3
            ADP
            Orthophosphate
INHIBITOR   Oxygen
COFACTOR    Iron-sulfur
            Molybdenum
            Homocitrate
COMMENT     An iron-molybdenum protein. Acetylene can also act as acceptor,
            in the absence of other acceptors. H+ is reduced to H2; n is
            about 12-18. Formerly EC 1.18.2.1. Consits of two proteins.
            Fe-protein is a dimer of identical subunits that contain one 
            [4Fe-4S] cluster and two ATP binding sites. Cluster and binding
            site is separated in a distance too large for direct coupling 
            between electron transfer and ATP hydrolysis.  MoFe-protein is a
            tetramer of a2b2 that contain iron and molybdenum. Each ab dimer
            contains two redox centers. (1) P-cluste pair, which consists of
            two linked [4Fe-4S] clusters bridged by two Cys thiol ligands and
            a SS-bond between an S atom in each of the two clusters. 
            (2) FeMo-cofactor, which consists of a [4Fe-3S] cluster and a 
            [Mo-3Fe-3S] cluster bridged by three nonprotein ligands; two
            sulfide ions, and an unknown third ligand. Mo atom is approxi-
            mately octahedrally coordinated by 3 S, a His imidazol N, and
            2 O from homocitrate.  Nitrogen is supposed to be oriented such
            that N=N bond is perpendicular to the axis between the two Fe
            in the FeMo-cofactor.  The sequence of the electron transfer is
            Fe-protein -> P-cluster pair -> FeMo-cofactor -> Nitrogen.
            Rapidly inactivated by oxygen. To escape from the inactivation,
            nitrogen fixation is carried out in heterocysts which is lack of
            photosystem II, or under the presence of leghemoglobin.
PATHWAY     PATH: MAP00910  Nitrogen metabolism
MOTIF       PS: PS00090  [STANQ]-[ET]-C-x(5)-G-D-[DN]-[LIVMT]-x-[STAGR]-
                         [LIVMFYST]
            PS: PS00692  D-x-L-G-D-V-V-C-G-G-F-[AG]-x-P
            PS: PS00699  [LIVMFYH]-[LIVMFST]-H-[AG]-[AGSP]-[LIVMNQA]-[AG]-C
            PS: PS00746  E-x-G-G-P-x(2)-[GA]-x-G-C-[AG]-G
GENES       MJA: MJ0685 MJ0823 MJ0879(nifH)
            MTH: MTH1120 MTH1482 MTH1560(NifH) MTH1563(NifD) MTH1564 MTH1565
                 MTH1566 MTH1711 MTH643
            SYN: slr0749(chlL)
STRUCTURES  PDB: 2MIN  3MIN  
DBLINKS     University of Geneva ENZYME DATA BANK: 1.18.6.1
            WIT (What Is There) Metabolic Reconstruction: 1.18.6.1
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