Functional characterization of a vitamin B12-dependent methylmalonyl pathway in Mycobacterium tuberculosis: implications for propionate metabolism during growth on fatty acids
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 (C(3)) 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 B(12)-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 B(12) 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 C(17) fatty acid heptadecanoate, growth on valerate (C(5)) was significantly enhanced through vitamin B(12) supplementation. Moreover, both wild-type and methylcitrate cycle mutant strains grew on B(12)-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 B(12) to mycobacterial pathogenesis and suggests that vitamin B(12) availability in vivo might resolve the paradoxical dispensability of the methylcitrate cycle for the growth and persistence of M. tuberculosis in mice.