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Steroid Metabolism Pathways in M. tuberculosis

The World Health Organization estimates that 2 million people die every year from tuberculosis (TB) and that 30% of the world’s population is infected with M. tuberculosis, the causative agent. Drug resistance to the front-line TB drugs rifampicin and isoniazid, as well as second-line drugs, has emerged and new antibiotics with different mechanisms of action are needed in the pharmacopeia to combat the spread of these virulent multi-drug and extensively drug resistant organisms (MDR/XDR TB). M. tb is an intracellular pathogen that uses lipids as the primary source of energy in the host. These lipids range from fatty acids to cholesterol. In vitro, M. tb can utilize cholesterol as an energy (carbon) source. In addition, metabolism of host lipids provides a feedstock of important starting materials like acetate (C2), propionate (C3), and perhaps more complex building blocks, for biosynthesis of secondary metabolites. Fully elucidating the chemical sequence in the M. tb cholesterol metabolic pathway is required to understand how cholesterol contributes to the survival of the pathogen in the host. Elucidation of the molecular functions of genes and enzymes identified as part of the cholesterol metabolome will guide their implementation in drug discovery.



Our long-term goal is the elucidation of the role of the cholesterol metabolic pathway(s) in the ability of M. tb to grow and persist in vivo. Our current work is focused on understanding the biochemical function of mycobacterial enzymes involved in cholesterol metabolism and identifying the metabolites of mycobacterial cholesterol metabolism. We suspect that the cholesterol framework is both degraded to simple (2-3 carbons) metabolites and elaborated to form complex secondary metabolites Several M. tb genes, including the igr operon, and fadA5, have been identified that are required for in vitro growth using cholesterol as a sole carbon source, confirming their role in cholesterol metabolism (Chang et al., 2009; Nesbitt et al., 2010). These genes, and by inference cholesterol metabolism, are important for intracellular survival in the host. The fadA5 gene is required for full virulence of M. tb in the chronic stage of mouse lung infection (Nesbitt et al., 2010), whereas, the igr operon is required for growth in the initial stage of infection (Chang et al., 2007). We are investigating the enzymatic function of these genes as well as the metabolic phenotypes of their mutants to understand the molecular causes of these different phenotypes. Ultimately, understanding their phenotypes at the molecular level will direct drug discovery into targets that attenuate M. tb growth and persistence in vivo.



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