Coenzyme-B sulfoethylthiotransferase

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. 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) 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.

Thus, the two substrates of this enzyme are 2-(methylthio)ethanesulfonate and N-(7-mercaptoheptanoyl)threonine 3-O-phosphate, whereas its two products are CoM-S-S-CoB and methane.

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. Such organisms are methanotrophs.