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Elucidation of the Role of the Methylene-Tetrahydromethanopterin Dehydrogenase MtdA in the Tetrahydromethanopterin-Dependent Oxidation Pathway in Methylobacterium extorquens AM1

ABSTRACT The methylotroph Methylobacterium extorquens AM1 oxidizes methanol and methylamine to formaldehyde and subsequently to formate, an intermediate that serves as the branch point between assimilation (formation of biomass) and dissimilation (oxidation to CO2). The oxidation of formaldehyde to formate is dephosphotetrahydromethanopterin (dH4MPT) dependent, while the assimilation of carbon into biomass is tetrahydrofolate (H4F) dependent. This bacterium contains two different enzymes, MtdA and MtdB, both of which are dehydrogenases able to use methylene-dH4MPT, an intermediate in the oxidation of formaldehyde to formate. Unique to MtdA is a second enzymatic activity with methylene-H4F. Since methylene-H4F is the entry point into the biomass pathways, MtdA plays a key role in assimilatory metabolism. However, its role in oxidative metabolism via the dH4MPT-dependent pathway and its apparent inability to replace MtdB in vivo on methanol growth are not understood. Here, we have shown that an mtdB mutant is able to grow on methylamine, providing a system to study the role of MtdA. We demonstrate that the absence of MtdB results in the accumulation of methenyl-dH4MPT. Methenyl-dH4MPT is shown to be a competitive inhibitor of the reduction of methenyl-H4F to methylene-H4F catalyzed by MtdA, with an estimated Ki of 10 μM. Thus, methenyl-dH4MPT accumulation inhibits H4F-dependent assimilation. Overexpression of mch in the mtdB mutant strain, predicted to reduce methenyl-dH4MPT accumulation, enhances growth on methylamine. Our model proposes that MtdA regulates carbon flux due to differences in its kinetic properties for methylene-dH4MPT and for methenyl-H4F during growth on single-carbon compounds.