Propionate metabolism in Mycobacterium tuberculosis: characterization of the vitamin B12-dependent methylmalonyl pathway

Abstract

Propionyl-CoA is a three-carbon (C3) short-chain fatty acid (SCFA) derivative of branchedchain amino acids, branched- and odd-chain fatty acids and cholesterol. Degradation of propionyl-CoA-generating carbon sources during infection (Pandey and Sassetti, 2008) requires the concomitant ability to oxidise this metabolite as a carbon and energy source, so as to avoid its cytotoxic effects if accumulated. The methylcitrate cycle in Mycobacterium tuberculosis (MTB) has been characterized and is essential for propionate oxidation in vitro, although dispensable for growth and persistence in mice (Muñoz-Elias et al., 2006). This study reveals that MTB possesses an alternative pathway for propionate metabolism, the vitamin B12-dependent methylmalonyl pathway. Specifically, we demonstrate the ability of MTB to utilise propionyl-CoA-generating carbon sources in the absence of the methylcitrate cycle, provided that vitamin B12 is supplied exogenously. This ability is shown to be dependent on methylmalonyl-CoA mutase (MCM; MutAB), which requires the adenosylcobalamin derivative of vitamin B12 for activity. The inability of MTB to synthesise vitamin B12 (Warner et al., 2007) is consistent with the essentiality of the methylcitrate cycle for growth on propionate (Muñoz-Elias et al., 2006). The demonstrated functionality of the methylmalonyl pathway offers an explanation for the dispensability of the methylcitrate cycle for survival of the mycobacterium in vivo where access to vitamin B12 may be unrestricted. Gene expression analysis was used to interpret flux through the two pathways on propionate (C3) and valerate (C5) odd-chain fatty acids. In the presence of a functional methylmalonyl pathway, expression of methylcitrate dehydratase (MCD) and methylcitrate lyase (MCL) was reduced. Consistent with reduced levels of bifunctional isocitrate lyase (ICL)1/ MCL in MTB (Gould et al., 2006; Muñoz-Elias et al., 2006), growth on propionate and valerate was shown to by-pass the requirement for carbon anaplerosis by the glyoxylate cycle when propionyl- CoA was converted to the tricarboxylic acid cycle (TCA) intermediate, succinyl-CoA, through the methylmalonyl pathway. The potential of an autonomous methylmalonyl pathway in MTB is demonstrated which underscores the importance of vitamin B12 in MTB physiology. Alternately, MTB deficient for the methylcitrate cycle was able to grow on heptadecanoate (C17) without vitamin B12 supplementation. In the absence of either propionate oxidizing pathway, derivative propionyl-CoA may be used as a key precursor for the biosynthesis of several cell wall virulence lipids (Jain et al., 2007).