F430 is the cofactor (sometimes called the coenzyme) of the enzyme methyl coenzyme M reductase (MCR).[1][2] MCR catalyzes the reaction EC 2.8.4.1 that releases methane in the final step of methanogenesis:
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C 42H 51N 6NiO− 13 | |
Molar mass | 906.58014 |
Appearance | Yellow solid |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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It is found only in methanogenic Archaea[3] and anaerobic methanotrophic Archaea. It occurs in relatively high concentrations in archaea that are involved in reverse methanogenesis: these can contain up to 7% by weight of the nickel protein.[4]
The trivial name cofactor F430 was assigned in 1978 based on the properties of a yellow sample extracted from Methanobacterium thermoautotrophicum, which had a spectroscopic maximum at 430 nm.[5] It was identified as the MCR cofactor in 1982[6] and the complete structure was deduced by X-ray crystallography and NMR spectroscopy.[7] Coenzyme F430 features a reduced porphyrin in a macrocyclic ring system called a corphin.[8] In addition, it possesses two additional rings in comparison to the standard tetrapyrrole (rings A-D), having a γ-lactam ring E and a keto-containing carbocyclic ring F. It is the only natural tetrapyrrole containing nickel, an element rarely found in biological systems.[9]
The biosynthesis builds from uroporphyrinogen III, the progenitor of all natural tetrapyrroles, including chlorophyll, vitamin B12, phycobilins, siroheme, heme, and heme d1. It is converted to sirohydrochlorin via dihydrosirohydrochlorin.[10] Insertion of nickel into this tetrapyrrole is catalysed in reaction EC 4.99.1.11 by the same chelatase, CbiX, which inserts cobalt in the biosynthesis of cobalamin, here giving nickel(II)-sirohydrochlorin.[11]
The ATP-dependent Ni-sirohydrochlorin a,c-diamide synthase (CfbE) then converts the a and c acetate side chains to acetamide in reactions EC 6.3.5.12, generating nickel(II)-sirohydrochlorin a,c-diamide. The sequence of the two amidations is random.[11] A two-component complex Ni-sirohydrochlorin a,c-diamide reductive cyclase (CfbCD) carries out a 6-electron and 7-proton reduction of the ring system in a reaction EC 6.3.3.7 generating the 15,173-seco-F430-173-acid (seco-F430) intermediate. Reduction involves ATP hydrolysis and electrons are relayed through two 4Fe-4S centres. In the final step, the keto-containing carbocyclic ring F is formed by an ATP-dependent enzyme Coenzyme F(430) synthetase (CfbB) in reaction EC 6.4.1.9, generating coenzyme F430.[11][12][13] This enzyme is a MurF-like ligase, as found in peptidoglycan biosynthesis.