Alternative Route for Glyoxylate Consumption during Growth on Two-Carbon Compounds by Methylobacterium extorquens AM1

Department of Chemical Engineering, University of Washington, Seattle, WA 98195-2180, USA.
Journal of bacteriology (Impact Factor: 2.81). 04/2010; 192(7):1813-23. DOI: 10.1128/JB.01166-09
Source: PubMed


Methylobacterium extorquens AM1 is a facultative methylotroph capable of growth on both single-carbon and multicarbon compounds. Mutants defective in a pathway involved in converting acetyl-coenzyme A (CoA) to glyoxylate (the ethylmalonyl-CoA pathway) are unable to grow on both C(1) and C(2) compounds, showing that both modes of growth have this pathway in common. However, growth on C(2) compounds via the ethylmalonyl-CoA pathway should require glyoxylate consumption via malate synthase, but a mutant lacking malyl-CoA/beta-methylmalyl-CoA lyase activity (MclA1) that is assumed to be responsible for malate synthase activity still grows on C(2) compounds. Since glyoxylate is toxic to this bacterium, it seemed likely that a system is in place to keep it from accumulating. In this study, we have addressed this question and have shown by microarray analysis, mutant analysis, metabolite measurements, and (13)C-labeling experiments that M. extorquens AM1 contains an additional malyl-CoA/beta-methylmalyl-CoA lyase (MclA2) that appears to take part in glyoxylate metabolism during growth on C(2) compounds. In addition, an alternative pathway appears to be responsible for consuming part of the glyoxylate, converting it to glycine, methylene-H(4)F, and serine. Mutants lacking either pathway have a partial defect for growth on ethylamine, while mutants lacking both pathways are unable to grow appreciably on ethylamine. Our results suggest that the malate synthase reaction is a bottleneck for growth on C(2) compounds by this bacterium, which is partially alleviated by this alternative route for glyoxylate consumption. This strategy of multiple enzymes/pathways for the consumption of a toxic intermediate reflects the metabolic versatility of this facultative methylotroph and is a model for other metabolic networks involving high flux through toxic intermediates.

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    • "The dehydrogenase gene from most aerobic organisms is devoid of redox-active centers [65] and together with the hydrogen dehydrogenase gene (EC: forms a system previously known as formate hydrogenlyase. Glyoxylate is a toxic intermediate which in humans undergoes oxalate formation [66, 67] with severe consequences for the tissues involved. The glyoxylate cycle is thought to be present in bacteria, protists, plants, fungi, and nematodes but not in other Metazoa [68]. "
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    • "This was done by setting to zero the flux from crotonyl-CoA to propionyl-CoA and by adding the ICL reaction. Malate synthase, the enzyme of the glyoxylate cycle that catalyzes the condensation of acetyl-CoA and glyoxylate into malate, was not added since M. extorquens can use a combination of two enzymes to achieve the same reaction [39], as described also in R. sphaeroids [40]. As expected, the glyoxylate cycle was essential for methanol growth and was found in all assimilatory EFMs. "
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    • "In the latter case, genes for the EMCP are present in the genomes but not the genes for the serine cycle. It is interesting to point out that the EMCP functions as a cycle during growth on C1 compounds (regenerating a molecule of glyoxylate per each molecule of methylene-H 4F and each molecule of CO2 assimilated; Chistoserdova et al., 2009), while during growth on C2 compounds, it functions as a liner pathway (Erb et al., 2010; Okubo et al., 2010). Some methylotrophs, however, opt to use the glyoxylate shunt instead of the EMCP. "
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