In enzymology, coenzyme-B sulfoethylthiotransferase, also known as methyl-coenzyme M reductase (MCR) or most systematically as 2-(methylthio)ethanesulfonate:N-(7-thioheptanoyl)-3-O-phosphothreonine S-(2-sulfoethyl)thiotransferase is an enzyme that catalyzes the final step in the formation of methane.^[1] It does so by combining the hydrogen donor coenzyme B and the methyl donor coenzyme M. Via this enzyme, most of the natural gas on earth was produced. Ruminants (e.g. cows) produce methane because their rumens contain methanogenic prokaryotes (Archaea)^[2][3] that encode and express the set of genes of this enzymatic complex.

The enzyme has two active sites, each occupied by the nickel-containing F₄₃₀ cofactor.^[4]

methyl-CoM2-(methylthio)ethanesulfonate + coenzyme BN-(7-mercaptoheptanoyl)threonine 3-O-phosphate ⇌ CoM-S-S-CoB + methane

The two substrates of this enzyme are 2-(methylthio)ethanesulfonate and N-(7-mercaptoheptanoyl)threonine 3-O-phosphate; its two products are CoM-S-S-CoB and methane. 3-Nitrooxypropanol inhibits the enzyme.^[5]

In some species, the enzyme reacts in reverse (a process called reverse methanogenesis), catalysing the anaerobic oxidation of methane, therefore removing it from the environment.^[6] Such organisms are methanotrophs.

This enzyme belongs to the family of transferases, specifically those transferring alkylthio groups.

This enzyme participates in folate biosynthesis.^{[citation needed]}

Structure

Coenzyme-B sulfoethylthiotransferase is a multiprotein complex made up of a pair of identical halves. Each half is made up of three subunits: α, β and γ,^[7] also called McrA, McrB and McrG, respectively.