Enzyme Function
Methanogens rely on such enzymes to catalyze the reduction of CO2, to methane. One step in methanogenesis entails conversion of a methenyl group (formic acid oxidation state) to a methylene group (formaldehyde oxidation state).
Among the hydrogenase family of enzymes, Hmd is unique in that it does not directly reduce CO2 to CH4. The natural substrate of the enzyme is the organic compound methenyltetrahydromethanopterin. The organic compound includes a methenyl group bound to two tertiary amides. The methenyl group originated as CO2 before being incorporated into the substrate, which is catalytically reduced by H2 to methylenetetrahydromethanopterin as shown. Eventually the methylene group is further reduced and released as a molecule of methane.
The hydride transfer has also been shown to be stereospecific. Given that the substrate is planar the hydride originating from H2 is always added to the pro-R face. In the reverse reaction stereospecificity is maintained and the highlighted hydride is removed.
Read more about this topic: 5,10-methenyltetrahydromethanopterin Hydrogenase
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