Erratum: Corrigendum: Elucidation of the biosynthesis of the methane catalyst coenzyme F430

Divergent Members of the Nitrogenase Superfamily: Tetrapyrrole Biosynthesis and Beyond

The nitrogenase superfamily constitutes a large and diverse ensemble of two-component metalloenzymes. These systems couple the hydrolysis of ATP to the reduction of disparate substrates from diatomic gases (Mo and alternative nitrogenases) to photosynthetic pigments (protochlorophyllide and chlorophyllide oxidoreductases). Only very recently have the activities of the highly divergent and paraphyletic Group IV nitrogenases begun to be uncovered. This review highlights the first characterized member of this group, which was found to catalyze an unprecedented reaction in the coenzyme F430 biosynthetic pathway, and the catalytic potential of a superfamily that has yet to be fully explored.

Geobiological feedbacks, oxygen, and the evolution of nitrogenase

Biological nitrogen fixation via the activity of nitrogenase is one of the most important biological innovations, allowing for an increase in global productivity that eventually permitted the emergence of higher forms of life. The complex metalloenzyme termed nitrogenase contains complex iron-sulfur cofactors. Three versions of nitrogenase exist that differ mainly by the presence or absence of a heterometal at the active site metal cluster (either Mo or V). Mo-dependent nitrogenase is the most common while V-dependent or heterometal independent (Fe-only) versions are often termed alternative nitrogenases since they have apparent lower activities for N₂ reduction and are expressed in the absence of Mo. Phylogenetic data indicates that biological nitrogen fixation emerged in an anaerobic, thermophilic ancestor of hydrogenotrophic methanogens and later diversified via lateral gene transfer into anaerobic bacteria, and eventually aerobic bacteria including Cyanobacteria. Isotopic evidence suggests that nitrogenase activity existed at 3.2 Ga, prior to the advent of oxygenic photosynthesis and rise of oxygen in the atmosphere, implying the presence of favorable environmental conditions for oxygen-sensitive nitrogenase to evolve. Following the proliferation of oxygenic phototrophs, diazotrophic organisms had to develop strategies to protect nitrogenase from oxygen inactivation and generate the right balance of low potential reducing equivalents and cellular energy for growth and nitrogen fixation activity. Here we review the fundamental advances in our understanding of biological nitrogen fixation in the context of the emergence, evolution, and taxonomic distribution of nitrogenase, with an emphasis placed on key events associated with its emergence and diversification from anoxic to oxic environments.

Enzymatic Systems with Homology to Nitrogenase: Biosynthesis of Bacteriochlorophyll and Coenzyme F430

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).