Everything about F430 totally explained
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F
430 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 → CH
4 + CoB-S-S-CoM
Corphin in context
Nature uses many
tetrapyrroles -
hemes,
chlorophyll, and
vitamin B12. F
430 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's 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 F
430 contains Ni(I), analogously to the reduced B
12 cofactors that feature Co(I). Whereas Co(I) is d
8 and
diamagnetic, Ni(I) is d
9 and
paramagnetic. The mechanism by which Nature cleaves the CH
3-S bond in methyl coenzyme M is presently (2006) unclear although it's 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 CH
4. 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.
Further Information
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