H4MPT

FIGURE 7.56 Partial structures of tetrahydrofolate (H4F) and tetrahydromethanopterin (H4MPT). (From Neilson, A.H. and Allard, A.-S., The Handbook of Environmental Chemistry, Vol. 3R, pp. 1-74, Springer, 2002. With permission.)... 

Apart from specific enzyme systems, the carbon dioxide reduction is mediated by a number of unusual coenzymes that can be categorized into two groups, namely Cj carriers and redox coenzymes [19]. The Cj carriers transport the Cl unit from the substrate carbon dioxide to the end-product methane, while the redox coenzymes provide the electrons that are required for the reduction of carbon dioxide to methane. Members of the first group include methanofuran (MF) (11), tetrahydromethanopterin (H4MPT) (12), and the coenzyme M (CoM-SH) (13), while coenzyme F420 (14) and coenzyme B (CoB-SH) (15) belong to the second group. 

Proposed mechanism for the reversible reaction of N, N -methenyltetrahydromethanopterin (methenyl-H4MPT ) with H2 to N, N -methylenetetrahydromethanopterin (methylene-H4MPT) and a proton catalysed by the metal-free hydrogenase from methanogenic archaea... 

A phylogenetic analysis of H4MPT-dependent enzymes reveals distinct archaeal and bacterial branches of the tree that are clearly related however, the results still do not resolve whether a common ancestor contained H4MPT-dependent enzymes or the methylotrophs acquired genes from the Archaea domain. Nonetheless, analysis of the genomic sequences from the Bacteria domain indicate that only the methylotrophic proteobacteria harbor HiMPT-dependent enzymes are consistent with lateral transfer from archaeal donors (Vorholt et al. 1999). 

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