The nitrogenase enzyme system catalyzes the ATP-dependent reduction of dinitrogen to ammonia. This group contains both alpha and beta subunits of component 1 of the three known genetically distinct types of nitrogenase systems: a molybdenum-dependent nitrogenase (Mo-nitrogenase), a vanadium-dependent nitrogenase (V-nitrogenase), and an iron-only nitrogenase (Fe-nitrogenase) and, both subunits of Protochlorophyllide (Pchlide) reductase and chlorophyllide (chlide) reductase. The nitrogenase systems consist of component 1 (MoFe protein, VFe protein or, FeFe protein respectively) and, component 2 (Fe protein). The most widespread and best characterized nitrogenase is the Mo-nitrogenase. MoFe is an alpha2beta2 tetramer, the alternative nitrogenases are alpha2beta2delta2 hexamers whose alpha and beta subunits are similar to the alpha and beta subunits of MoFe. For MoFe, each alphabeta pair contains one P-cluster (at the alphabeta interface) and, one molecule of iron molybdenum cofactor (FeMoco) contained within the alpha subunit. The Fe protein contains a single [4Fe-4S] cluster from which, electrons are transferred to the P-cluster of the MoFe and in turn, to FeMoCo at the site of substrate reduction. The V-nitrogenase requires an iron-vanadium cofactor (FeVco), the iron only-nitrogenase an iron only cofactor (FeFeco). These cofactors are analogous to the FeMoco. The V-nitrogenase has P clusters identical to those of MoFe. Pchlide reductase and chlide reductase participate in the Mg-branch of the tetrapyrrole biosynthetic pathway. Pchlide reductase catalyzes the reduction of the D-ring of Pchlide during the synthesis of chlorophylls (Chl) and bacteriochlorophylls (BChl). Chlide-a reductase catalyzes the reduction of the B-ring of Chlide-a during the synthesis of BChl-a. The Pchlide reductase NB complex is a an N2B2 heterotetramer resembling nitrogenase FeMo, N and B proteins are homologous to the FeMo alpha and beta subunits respectively. The NB complex may serve as a catalytic site for Pchlide reduction and, the ZY complex as a site of chlide reduction, similar to MoFe for nitrogen reduction.
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