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.