Tetrahydromethanopterin as a Two-Carbon Carrier: Formation of N⁵-Ethyl- and N⁵,N¹⁰-Ethylene-Tetrahydromethanopterin in Methanothermobacter marburgensis

Tetrahydromethanopterin (H₄MPT) is the main carbon carrier in methane metabolism, which allows the interconversion of a formyl group into a methyl group. Although typically described as a single carbon (C1) carrier, we identified and characterized N⁵-ethyltetrahydromethanopterin (N⁵-ethyl-H₄MPT) and N⁵,N¹⁰-(1,1-ethylene)tetrahydromethanopterin (N⁵,N¹⁰-ethylene-H₄MPT) in the model methanogen Methanothermobacter marburgensis. The chemical competence of H₄MPT to accept C2 moieties was assayed through the study of the spontaneous formation of N⁵,N¹⁰-ethylene-H₄MPT by the condensation of acetaldehyde with H₄MPT under standard conditions. The rate constant for this reaction was determined to be 1.53 ± 0.05 M^–1 s^–1, and the equilibrium constant was determined to be (8.8 ± 0.5) × 10³ M^–1, which is ca. 35 times higher compared to the analogous reaction with the more common carbon carrier tetrahydrofolate. Biochemical assays with M. marburgensis cell lysate suggest that the observed formation of N⁵-ethyl-H₄MPT relies on the enzymatic reduction of N⁵,N¹⁰-ethylene-H₄MPT. These findings illustrate the chemical competency of H₄MPT to promote biocatalysis with two-carbon moieties and highlight that such reactions might be compatible with established H₄MPT-mediated microbial pathways.