Isolation and identification of 5,10-methenyl-5,6,7,8-tetrahydromethanopterin, a coenzyme involved in methanogenesis

Purine metabolism in Methanococcus vannielii

Methanococcus vannielii is capable of degrading purines to the extent that each of these purines may serve as the sole nitrogen source for growth. Results presented here demonstrate that purine degradation by M. vannielii is accomplished by a route similar to that described for clostridia. Various characteristics of the purine-degrading pathway of M. vannielii are described. Additionally, it is shown that M. vannielii does not extensively degrade exogenously supplied guanine if that compound is present at levels near or lower than those required to supply the cellular guanine requirement. Under those conditions, M. vannielii incorporates the intact guanine molecule into its guanine nucleotide pool. The benefits of a purine-degrading pathway to methanogens are discussed.

Determination of the relative configuration of 5,6,7,8-tetrahydromethanopterin by two-dimensional NMR spectroscopy

The relative configuration of the pterin moiety of 5,6,7,8-tetrahydromethanopterin 1, a coenzyme isolated from methanogenic archaea, has been determined by two-dimensional NMR spectroscopy of N5,N10-methenyl-5,6,7,8-tetrahydromethanopterin 2 to be rel-(6R; 7S; 11R). The complete proton resonance assignment of the pterin moiety of N5,N10-methylene-5,6,7,8-tetrahydromethanopterin 3 is described including the relative stereospecific assignment of the C(14a) methylene protons.

Evidence of a common pathway of carbon dioxide reduction to methane in methanogens

The roles of methanofuran and tetrahydromethanopterin as carriers of C1 moieties in the reduction of carbon dioxide to methane were studied in representatives of diverse groups of methanogens, confirming that these roles, first reported for Methanobacterium thermoautotrophicum, are common for methanogenesis in general. Extracts of the methanogens tested converted formyl-methanofuran and methyl-tetrahydromethanopterin to methane; the extractable cofactors derived from the same methanogens, with one exception, complemented a methanofuran- and tetrahydromethanopterin-deficient enzyme system from M. thermoautotrophicum. The amounts of extractable methanofuran and tetrahydromethanopterin were determined for each representative methanogen.

Methanofuran (carbon dioxide reduction factor), a formyl carrier in methane production from carbon dioxide in Methanobacterium

Methanofuran (carbon dioxide reduction factor) became labeled when incubated in cell extracts of Methanobacterium under hydrogen and 14CO2 in the absence of methanopterin. Proton NMR spectroscopy revealed that a formyl group was bound to the primary amine of methanofuran. [14C]Formylmethanofuran was enzymically converted to 14CH4 in the presence of CH3-S-CoM [2-(methylthio)ethanesulfonic acid], hydrogen, and methanopterin, establishing the formyl moiety as an intermediate in methanogenesis. In the absence of methanopterin, a substantial portion of the formyl label was oxidized to 14CO2 rather than reduced to 14CH4, consistent with a model in which the C1 intermediate is first bound to methanofuran and then to methanopterin, during its reduction. When CH3-S-CoM was replaced by HS-CoM (2-mercaptoethanesulfonic acid), most of the formyl label was oxidized to 14CO2, indicating that methyl group reduction by the CH3-S-CoM methylreductase is required for the conversion of formylmethanofuran to methane.

Methanogenesis: surprising molecules, microorganisms and ecosystems

Methanogenesis involves a novel set of coenzymes as one-carbon and electron carriers. Consequently, metabolic processes of methanogens deviate from those present in non-methanogenic bacteria. Methanogenic bacteria can be classified on the basis of substrate utilization. Group I (24 species) grows at the expense of hydrogen plus CO2 and/or formate and group II (7 species) uses methanol and/or acetate. Hydrogen-consuming methanogens are found as epi- or endosymbionts of anaerobic ciliates.

Coenzymes of methanogenesis from hydrogen and carbon dioxide

Methanogenic bacteria gain their energy for growth from the conversion of a number of simple carbon compounds to methane. With a few exceptions all species known to date are able to reduce CO2 at which hydrogen acts as the electron donor. The reduction of CO2 can formally be considered to proceed through the formyl, the formaldehyde and the methyl level of reduction. These C1-units do not occur as free intermediates, but they remain bound to a number of unique coenzymes during the process. In this paper a survey is given of the structures and functions of these compounds; it deals with methanopterin derivatives, carbon dioxide reduction (CDR) factor, factor F430 and coenzyme M derivatives. A model of the process of methanogenesis that integrates previous ones and that allocates a function to the various coenzymes is presented.