Conformational flexibility of the nitrogenase-like reductase CfbC/D involved in coenzyme F₄₃₀ biosynthesis

The nitrogenase-like reductase CfbC/D catalyzes the six-electron reduction of the tetrapyrrole macrocycle of Ni²⁺-sirohydrochlorin a,c-diamide during the biosynthesis of coenzyme F₄₃₀, an essential cofactor for biological methane formation and consumption. The individual dimeric protein components, CfbC₂ and CfbD₂, form a complex to allow for electron transfer from the [4Fe:4S] cluster in CfbC₂ to the [4Fe:4S] cluster in CfbD₂ and onto the substrate. The overall process is ATP-dependent. Although CfbC/D belongs to the family of nitrogenase-like enzymes, there are several intriguing differences between CfbC/D, other nitrogenase-like enzymes and nitrogenase itself. In contrast to the other enzymes, CfbC/D exhibits a simpler molecular architecture, forming a heterotetrameric CfbC₂D₂ assembly. Furthermore, the interaction of the CfbC and CfbD components does not depend on the presence of adenosine triphosphate (ATP). Here we report cryoelectron microscopy structures of the CfbC/D complex prepared in the absence and presence of substrates. These structures not only shed light on the molecular architecture of the CfbC/D complex but also reveal a high degree of structural flexibility within the CfbD₂ component, which undergoes a large conformational change in the presence of an ATP analog and the tetrapyrrole substrate. The interplay of CfbD with its reductase CfbC also strongly suggests an evolutionary pathway toward the modern nitrogenase from methanogenic archaea.