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Functional Characterization of a Vitamin B₁₂-Dependent Methylmalonyl Pathway in Mycobacterium tuberculosis: Implications for Propionate Metabolism during Growth on Fatty Acids ^,

MRC/NHLS/WITS Molecular Mycobacteriology Research Unit and DST/NRF Centre of Excellence for Biomedical Tuberculosis Research, School of Pathology, University of the Witwatersrand and the National Health Laboratory Service, Johannesburg, South Africa,¹ Global Health Institute, School of Life Sciences, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland²

Received 7 November 2007/ Accepted 17 March 2008

Mycobacterium tuberculosis is predicted to subsist on alternative carbon sources during persistence within the human host. Catabolism of odd- and branched-chain fatty acids, branched-chain amino acids, and cholesterol generates propionyl-coenzyme A (CoA) as a terminal, three-carbon (C₃) product. Propionate constitutes a key precursor in lipid biosynthesis but is toxic if accumulated, potentially implicating its metabolism in M. tuberculosis pathogenesis. In addition to the well-characterized methylcitrate cycle, the M. tuberculosis genome contains a complete methylmalonyl pathway, including a mutAB-encoded methylmalonyl-CoA mutase (MCM) that requires a vitamin B₁₂-derived cofactor for activity. Here, we demonstrate the ability of M. tuberculosis to utilize propionate as the sole carbon source in the absence of a functional methylcitrate cycle, provided that vitamin B₁₂ is supplied exogenously. We show that this ability is dependent on mutAB and, furthermore, that an active methylmalonyl pathway allows the bypass of the glyoxylate cycle during growth on propionate in vitro. Importantly, although the glyoxylate and methylcitrate cycles supported robust growth of M. tuberculosis on the C₁₇ fatty acid heptadecanoate, growth on valerate (C₅) was significantly enhanced through vitamin B₁₂ supplementation. Moreover, both wild-type and methylcitrate cycle mutant strains grew on B₁₂-supplemented valerate in the presence of 3-nitropropionate, an inhibitor of the glyoxylate cycle enzyme isocitrate lyase, indicating an anaplerotic role for the methylmalonyl pathway. The demonstrated functionality of MCM reinforces the potential relevance of vitamin B₁₂ to mycobacterial pathogenesis and suggests that vitamin B₁₂ availability in vivo might resolve the paradoxical dispensability of the methylcitrate cycle for the growth and persistence of M. tuberculosis in mice.

Published ahead of print on 28 March 2008.

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