Figure 4.

The transformation of sirohydrochlorin into coenzyme F₄₃₀. The steps involved in the biosynthesis of F₄₃₀ from sirohydrochlorin are outlined. Initially, sirohydrochlorin is chelated with nickel by the enzyme CfbA to give nickel sirohydrochlorin. Next, the two acetic acid side chains on rings A and B, the a and c side chains, are amidated in a reaction catalyzed by CfbB that also requires glutamine and ATP as substrates. This generates nickel sirohydrochlorin a,c-diamide, which acts as the substrate for the reductase system that is catalyzed by CfbC and -D. The reductase removes three double bonds from the macrocycle, which also spontaneously results in the formation of the lactam ring E, thereby generating seco-F₄₃₀. The final step, mediated by CfbE, results in the formation of the cyclic hexanone ring F in another ATP-requiring process. The shaded box for coenzyme F₄₃₀ coordinates with other pathway figures and the summary depiction in Fig. 14.