Elucidation Of The Role Of MtdA In The H4MPT Oxidation Pathway

A fundamental question in biology entails the understanding of how metabolic pathways are regulated in response to changes in substrate availability.  Methylobacterium extorquens AM1 is a methylotroph that oxidizes one-carbon compounds to formaldehyde, a highly toxic intermediate, and subsequently to formate, an intermediate that serves as the main branch point between assimilation (formation of biomass) and dissimilation (oxidation to CO₂).The conversion of formaldehyde to formate is H₄MPT-dependent while the assimilation of carbon into biomass is H₄F-dependent.  M. extorquens has two different Mtd enzymes, MtdA and MtdB, both of which are dehydrogenases and are able to use methylene-H₄MPT; however, only MtdA is able to generate 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 the H₄MPT dependent pathway, as well as its apparent enzymatic redundancy with MtdB is not well understood.  Using phenotypic studies we were able to show that MtdA is able to partially substitute for MtdB in vivo.   In vitro analysis showed that when MtdB is absent, MtdA activity increases resulting in the accumulation of methenyl-H₄MPT and causing a flux bottleneck in the H₄F-dependent assimilation.  Our model proposes that MtdA is able to regulate carbon flux due to subtle differences in its kinetic properties for methylene-H₄MPT and for methenyl-H₄F during growth on single-carbon compounds.