Characterization of the iron–sulfur clusters in the nitrogenase‐like reductase CfbC/D required for coenzyme F₄₃₀ biosynthesis

Coenzyme F430 is a nickel‐containing tetrapyrrole, serving as the prosthetic group of methyl‐coenzyme M reductase in methanogenic and methanotrophic archaea. During coenzyme F430 biosynthesis, the tetrapyrrole macrocycle is reduced by the nitrogenase‐like CfbC/D system consisting of the reductase component CfbC and the catalytic component CfbD. Both components are homodimeric proteins, each carrying a [4Fe‐4S] cluster. Here, the ligands of the [4Fe‐4S] clusters of CfbC2 and CfbD2 were identified revealing an all cysteine ligation of both clusters. Moreover, the midpoint potentials of the [4Fe‐4S] clusters were determined to be −256 mV for CfbC2 and −407 mV for CfbD2. These midpoint potentials indicate that the consecutive thermodynamically unfavorable 6 individual “up‐hill” electron transfers to the organic moiety of the Ni2+‐sirohydrochlorin a,c‐diamide substrate require an intricate interplay of ATP‐binding, hydrolysis, protein complex formation and release to drive product formation, which is a common theme in nitrogenase‐like systems.