Английская Википедия:Cofactor F430

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Шаблон:Distinguish Шаблон:Chembox F₄₃₀ is the cofactor (sometimes called the coenzyme) of the enzyme methyl coenzyme M reductase (MCR). MCR catalyzes the reaction Шаблон:EC number that releases methane in the final step of methanogenesis:

Шаблон:Chem + HS–CoB → Шаблон:Chem + CoB–S–S–CoM

Файл:Coenzyme M (CoM).svg Structure of coenzyme M (HS-CoM)

Файл:Coenzyme B (CoB).svg Structure of coenzyme B (HS-CoB)

It is found only in methanogenic Archaea 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.

Structure

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. It was identified as the MCR cofactor in 1982 and the complete structure was deduced by X-ray crystallography and NMR spectroscopy. Coenzyme F₄₃₀ features a reduced porphyrin in a macrocyclic ring system called a corphin. 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.

Biosynthesis

Файл:Uroporphyrinogen III skeletal.svg uroporphyrinogen III

Файл:Dihydrosirochlorin.png dihydrosirohydrochlorin

Файл:SirohydrochlorinCorr.png sirohydrochlorin

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. Insertion of nickel into this tetrapyrrole is catalysed in reaction Шаблон:EC number by the same chelatase, CbiX, which inserts cobalt in the biosynthesis of cobalamin, here giving nickel(II)-sirohydrochlorin.

Файл:Biosynthesis of seco Cofactor F430.svg

Nickel(II)-sirohydrochlorin a,c-diamide is converted to seco-F430. It is traditional to depict only one of four Ni-N bonds.

The ATP-dependent Ni-sirohydrochlorin a,c-diamide synthase (CfbE) then converts the a and c acetate side chains to acetamide in reactions Шаблон:EC number, generating nickel(II)-sirohydrochlorin a,c-diamide. The sequence of the two amidations is random. 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 Шаблон:EC number 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 Шаблон:EC number, generating coenzyme F₄₃₀. This enzyme is a MurF-like ligase, as found in peptidoglycan biosynthesis.