Enzymes with homology to nitrogenase are essential for the reduction of chemically stable double bonds within the biosynthetic pathways of bacteriochlorophyll and coenzyme F430. These tetrapyrrole-based compounds are crucial for bacterial photosynthesis and the biogenesis of methane in methanogenic archaea. Formation of bacteriochlorophyll requires the unique ATP-dependent enzyme chlorophyllide oxidoreductase (COR) for the two-electron reduction of chlorophyllide to bacteriochlorophyllide. COR catalysis is based on the homodimeric protein subunit BchX2, which facilitates the transfer of electrons to the corresponding heterotetrameric catalytic subunit (BchY/BchZ)2. By analogy to the nitrogenase system, the dynamic switch protein BchX2 contains a [4Fe-4S] cluster that triggers the ATP-driven transfer of electrons onto a second [4Fe-4S] cluster located in (BchY/BchZ)2. The subsequent substrate reduction and protonation is unrelated to nitrogenase catalysis, with no further involvement of a molybdenum-containing cofactor. The biosynthesis of the nickel-containing coenzyme F430 includes the six-electron reduction of the tetrapyrrole macrocycle of Ni2+-sirohydrochlorin a,c-diamide to Ni2+-hexahydrosirohydrochlorin a,c-diamide catalyzed by CfbC/D. The homodimeric CfbC2 subunit carrying a [4Fe-4S] cluster shows close homology to BchX2. Accordingly, parallelism for the initial ATP-driven electron transfer steps of CfbC/D was proposed. Electrons are received by the dimeric catalytic subunit CfbD2, which contains a second [4Fe-4S] cluster and carries out the saturation of an overall of three double bonds in a highly orchestrated spatial and regioselective process. Following a short introduction to nitrogenase catalysis, this chapter will focus on the recent progress toward the understanding of the nitrogenase-like enzymes COR and CfbC/D, with special emphasis on the underlying enzymatic mechanism(s).
Keywords: COR; CfbC; CfbD; Chlorophyll biosynthesis; Chlorophyllide oxidoreductase; Coenzyme F430; Nitrogenase-like enzymes.