Can Mycobacterium tuberculosis be starved to death?
A few million people infected by Mycobacterium tuberculosis die annually around the world. HIV patients become more susceptible to M. tuberculosis infections. With the available drugs, it takes around 6 months to treat the patient. The target is to achieve short duration therapy. The potential areas, which may lead to: (i) novel drugs are – (a) Natural products, (b) Mechanism and structure based chemicals, (ii) Novel targets are – (a) Central C Metabolism, (b) Cholestrol metabolism, (c) Energy metabolism, (d) Iron acquisition and storage, (e) generating reactive oxygen or nitrogen species, (f) Mycobacterial membrane protein large, and (g) Mtb ClpP protease, etc.
Central C Metabolism
The unique feature of this microbe is its ability to survive harsh conditions and evade host immune system. It can withstand low oxygen physiological environments. Human physiology operates its energy generation process via C6 compounds, whereas M. tuberculosis needs C2 compounds. M. tuberculosis on infecting human cell, diverts the tricarboxylic acid (TCA) cycle towards glyoxylate generation. The most important enzymes are: (i) isocitrate lyase (ICL), and (ii) malate synthase (GlcB), which are common to the citrate cycle. The conversion of isocitrate to succinate and glyoxylate takes place via glyoxylate shunt, with the help of ICL and GlcB enzymes. These two enzymes have been identified as attractive and potent drug targets to kill persistent bacteria. The genes icl1 and icl2, which encode for the enzyme ICL allow bacteria to withstand stress conditions caused by C starvation and heat shock.
The source of Glyoxylate
Glyoxylate cycle in M. tuberculosis utilizes acetyl CoA, which gets regenerated via fatty acid oxidation pathway. The major issue is how glyoxylate is generated independent of ICL. Bacteria have the unique ability to develop short cuts as the Glyoxylate shunt or cut short the major metabolic pathway such as Tricrboxylic acid cycle (TCA). Under high C conditions accompanied by limited availability of nutrients like N, K, P, O, Mg, etc., bacteria divert the TCA cycle to polyhydroxyalkanoate (PHA) biosynthetic pathway. PHAs act as storage molecules. TCA cycle and PHA biosynthetic pathway compete for Acetyl-CoA.
Glyoxylate re-generation pathway involves: aceA for ICL, aceB GlcB, along with genes for other enzymes: (i) phaA for betaketothiolase, (ii) phaB for NADPH linked Acetoacetyl CoA reductase, (iii) croR for ((R)-Specific enoyl-CoA hydratase, (iv) ECH for ((S)-Specific enoyl-CoA hydratase, (v) ibd2 for Isobutyryl CoA dehydrogenase, (vi) pccB for Propionyl-CoA carboxylase (β-subunit), (vii) meaB for Methyl malonyl-CoA epimerase, (viii) mcm for methyl malonyl CoA mutase, and (ix) ccrA for Crotonoyl CoA reductase. This pathway was observed to operate in methylotrophs. The PHA cycle shares certain intermediates with glyoxylate cycle and their genes: phaA and phaB. PHA depolymerization provides acyl-CoA and Acetyl-CoA, which get metabolized via TCA cycle.
Mycobacterium album has been shown to produce PHA, where as M. smegmatis does not as it lack phaC gene important for PHA biosynthesis. Sequenced genomes of M. tuberculosis strains CDC1551 and H37Rv, M. smegmatis and M. leprae have conserved domains for phaA and phaB . Completely conserved domain for phaC gene has been found to be present in M. tuberculosis, which also possess the gene for PHA depolymerase.
In addition to ICL and GlcB, it may be pertinent to target PHA biosynthetic pathway for inhibiting the growth of M. tuberculosis. Genomics and Bioinformatics help extensively in searching novel and potential drug targets.
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About Author / Additional Info:
Researcher at Microbial Biotechnology and Genomics at CSIR-IGIB, Delhi.