Involved in the biosynthesis of the unique nickel-containing tetrapyrrole coenzyme F430, the prosthetic group of methyl-coenzyme M reductase (MCR), which plays a key role in methanogenesis and anaerobic methane oxidation. Catalyzes both the six-electron reduction of the tetrahydroporphyrin ring system and the gamma-lactamization of the c-acetamide side chain of Ni-sirohydrochlorin a,c-diamide to yield 15,17(3)-seco-F430-17(3)-acid (seco-F430), the last intermediate in the biosynthesis of the coenzyme F430. 3 AH2 + ATP + H2O + Ni-sirohydrochlorin a,c-diamide = 15,17(3)-seco-F430-17(3)-acid + 3 A + ADP + phosphate Homodimer. The Ni-sirohydrochlorin a,c-diamide reductive cyclase complex is composed of a NifH homolog component CfbC and a NifD homolog component CfbD. Belongs to the NifH/BchL/ChlL family.