Given our interest in labdane-related diterpenoid biosynthesis, we were intrigued by the report that the important human pathogen Mycobacterium tuberculosis encoded a class II diterpene cyclase, particularly with separately reported genetic evidence that this enzyme was involved in the ability of the pathogen to suppress acidification of the phagosome into which the bacterium is enveloped by macrophages. Building on the additional evidence indicating a similar role for the neighboring gene, we demonstrated that the encoded enzyme will react with the halimadienyl diphosphate product of the class II diterpene cyclase, producing (iso)tuberculosinol (the corrected structure, as reported by us and others). Strikingly, this compound was able to partially block phagosomal acidification on its own. We further carried out biochemical characterization of the class II diterpene cyclase, including finding that a transition state analog acts a sub-nanomolar affinity inhibitor, and the discovery that the enzyme exhibited an unusual biphasic response to increasing concentrations of its divalent magnesium ion co-factor. Interestingly, this later enzymatic behavior may act as a physiologically relevant regulatory mechanism, as trace nutrients such as Mg2+ are pumped out of phagosomes. Indeed, depleting Mg2+ from cultures of M. tuberculosis leads to very significant increases in isotuberculosinol levels.
1. Mann, F.M., Prisic, S., Hu, H., Xu, M., Coates, R.M., Peters, R.J. (2009) “Characterization and inhibition of a class II diterpene cyclase from Mycobacterium tuberculosis: Implications for tuberculosis”, J. Biol. Chem., 284(35):23574-23579. PMCID: PMC2749132.
2. Mann, F.M., Xu, M., Chen, X., Fulton, D.B., Russell, D.G., Peters, R.J. (2009) “Edaxadiene: a new bioactive diterpene from Mycobacterium tuberculosis”, J. Am. Chem. Soc., 131(48):15726-15727. PMCID: PMC2787244.
3. Mann, F.M., VanderVen, B.C., Peters, R.J. (2011) “Magnesium depletion triggers production of an immune modulating diterpenoid in Mycobacterium tuberculosis”, Mol. Microbiol., 79(6):1594-1601. PMCID: PMC3084657.
4. Mann, F.M., Peters, R.J. (2012) “Isotuberculosinol: the unusual case of an immuno-modulatory diterpenoid from Mycobacterium tuberculosis”, Med. Chem. Commun., 3(8):899-904. PMCID: PMC3733278.