F430

F₄₃₀ is the prosthetic group of the enzyme methyl coenzyme M reductase. It is found only in methanogenic archaea. This enzyme catalyzes the release of methane in the final step of methanogenesis:

CH₃-S-CoM + HS-CoB → CH₄ + CoB-S-S-CoM

Corphin in context

Nature uses many tetrapyrroles - hemes, chlorophyll, and vitamin B12. F₄₃₀ is the most reduced tetrapyrrole in nature with only five double bonds. This particular tetrapyrrole derivative is called a corphin. Because of its relative lack of conjugated unsaturation, it is yellow, not the intense purple-red associated with more unsaturated tetrapyrroles. It is also the only tetrapyrrole derivative found in nature to contain nickel. Ni(II) is too small for the N4 binding site of the corphin, which causes the macrocycle to adopt a ruffled structure.

Proposed mechanism of methanogenesis

The active form of F₄₃₀ contains Ni(I), analogously to the reduced B₁₂ cofactors that feature Co(I). Whereas Co(I) is d⁸ and diamagnetic, Ni(I) is d⁹ and paramagnetic. The mechanism by which Nature cleaves the CH₃-S bond in methyl coenzyme M is presently (2006) unclear although it is known that both coenzyme M and coenzyme B fits into a channel terminated by the axial site on nickel. A plausible mechanism entails electron transfer from Ni(I) (to give Ni(II)), and this electron transfer initiates formation of CH₄. Coupling of the coenzyme M thiyl radical with HS coenzyme B releases a proton and re-reduces Ni(II) by one-electron, regenerating Ni(I).

Its structure was deduced by X-ray crystallography and NMR spectroscopy.

Anaerobic methane oxidation

F430 occurs in very high concentrations in bacteria that are thought to be involved in reverse methanogenesis, where methane is converted to methyl coenzyme M. Organisms that promote this remarkable reaction contain 7% by weight nickel protein.

References