Tetrahydromethanopterin

Tetrahydromethanopterin, abbreviated H4MPT, is a coenzyme in methanogenesis. It is the carrier of the C1 group as it is reduced to the methyl level, before transferring to the coenzyme M.



Tetrahydrosarcinapterin (H4SPT) is a modified form of H4MPT, wherein a glutamyl group linked to the 2-hydroxyglutaric acid terminus.

H4MPT is the main platform for C1 transformations
N-Formylmethanofuran donates the C1 group to the N5 site of the pterin to give the formylH4MPT. The formyl group subsequently condenses intramolecularly to give methenylH4MPT+, which is then reduced to methyleneH4MPT. MethyleneMPT is subsequently converted, using H2F420 as the electron source, to methylH4MPT, catalyzed by F420-dependent methylene-H4MPT reductase. MethylH4MPT is the methyl donor to coenzyme M, a conversion mediated by methyl-H4MPT:coenzyme M methyl-transferase.

Comparison with tetrahydrofolate
H4MPT is related to the better known tetrahydrofolate (H4F). The differences are indicated in red and blue in the figure. The most important difference between H4MPT and H4F is that H4F has an electron-withdrawing carbonyl group on the phenyl ring. As a consequence, methenyl-H4MPT is more difficult to reduce than methenyl-H4F. Reduction is effected by a so-called Iron-sulfur cluster free hydrogenase. The cumbersome name distinguishes this hydrogenase from the so-called Fe-only hydrogenases that do contain Fe-S cluster.