MethyleneH4MPT

MethyleneH4MPT, or methylene-tetrahydromethanopterin, is a one-carbon carrier cofactor used by methanogenic archaea in their central carbon metabolism. It is part of the H4MPT-based C1 pathway and stores a methylene group on the tetrahydromethanopterin (H4MPT) scaffold. In the pathway, H4MPT cycles through several oxidation states, including methenyl-H4MPT, methylene-H4MPT, and methyl-H4MPT, during the reduction of carbon dioxide to methane.

Role in methanogenesis: In methanogenic organisms, CO2 is progressively reduced to methane via a sequence of one-carbon reactions carried on H4MPT. Methylene-H4MPT acts as an intermediate that carries the methylene unit between steps, and it can be interconverted with adjacent oxidation states by specific enzymes. The eventual transfer of a methyl group from methyl-H4MPT to coenzyme M (CoM) by the Mtr complex, and its subsequent reduction to methane by methyl-coenzyme M reductase, completes the methane-forming steps.

Enzymology and cofactors: The interconversion among H4MPT oxidation states involves dedicated H4MPT-dependent enzymes, such as methylene-H4MPT dehydrogenase (Mtd), and relies on associated electron carriers within the pathway. The H4MPT cofactor is structurally a pterin-based carrier and is functionally analogous in role to folate-derived C1 carriers used in other domains of life.

Occurrence and significance: MethyleneH4MPT is found in diverse methanogenic archaea, including major orders within the Euryarchaeota. It underpins a key energy-yielding pathway for these organisms and contributes to global carbon cycling through methane production.