Cysteine synthase (CysM) of Mycobacterium tuberculosis is an O-phosphoserine sulfhydrylase: Evidence for an alternative cysteine biosynthesis pathway in Mycobacteria

Department of Medical Biochemistry and Biophysics, Karolinska Institutet, S-171 77 Stockholm, Sweden.
Journal of Biological Chemistry (Impact Factor: 4.57). 10/2008; 283(46):31567-74. DOI: 10.1074/jbc.M804877200
Source: PubMed


The biosynthesis of cysteine is a crucial metabolic pathway supplying a building block for de novo protein synthesis but also a reduced thiol as a component of the oxidative defense mechanisms that appear particularly vital in the dormant state of Mycobacterium tuberculosis. We here show that the cysteine synthase CysM is, in contrast to previous annotations, an O-phosphoserine-specific cysteine synthase. CysM belongs to the fold type II pyridoxal 5'-phosphate-dependent enzymes, as revealed by the crystal structure determined at 2.1-angstroms resolution. A model of O-phosphoserine bound to the enzyme suggests a hydrogen bonding interaction of the side chain of Arg220 with the phosphate group as a key feature in substrate selectivity. Replacement of this residue results in a significant loss of specificity for O-phosphoserine. Notably, reactions with sulfur donors are not affected by the amino acid replacement. The specificity of CysM toward O-phosphoserine together with the previously established novel mode of sulfur delivery via thiocarboxylated CysO (Burns, K. E., Baumgart, S., Dorrestein, P. C., Zhai, H., McLafferty, F. W., and Begley, T. P. (2005) J. Am. Chem. Soc. 127, 11602-11603) provide strong evidence for an O-phosphoserine-based cysteine biosynthesis pathway in M. tuberculosis that is independent of both O-acetylserine and the sulfate reduction pathway. The existence of an alternative biosynthetic pathway to cysteine in this pathogen has implications for the design strategy aimed at inhibition of this metabolic route.

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    • "The cysteine biosynthetic pathway comprises of two arms critical for response to redox stress [23], [24]. The alternative cysteine pathways utilizes thiocarboxylates for cysteine synthesis, which are more resistant to oxidative stress – the type of environment found within the MΦ [23]. Genes encoding CysM and CysO, which are part of the alternative cysteine pathway, exhibit up regulation upon oxidative stress in both Mtb and Mtb:ΔRv2745c (comp). "
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