CysK2 from Mycobacterium tuberculosis is an O-phospho-L-serine-dependent S-sulfocysteine synthase

J Bacteriol. 2014 Oct;196(19):3410-20. doi: 10.1128/JB.01851-14. Epub 2014 Jul 14.


Mycobacterium tuberculosis is dependent on cysteine biosynthesis, and reduced sulfur compounds such as mycothiol synthesized from cysteine serve in first-line defense mechanisms against oxidative stress imposed by macrophages. Two biosynthetic routes to l-cysteine, each with its own specific cysteine synthase (CysK1 and CysM), have been described in M. tuberculosis, but the function of a third putative sulfhydrylase in this pathogen, CysK2, has remained elusive. We present biochemical and biophysical evidence that CysK2 is an S-sulfocysteine synthase, utilizing O-phosphoserine (OPS) and thiosulfate as substrates. The enzyme uses a mechanism via a central aminoacrylate intermediate that is similar to that of other members of this pyridoxal phosphate-dependent enzyme family. The apparent second-order rate of the first half-reaction with OPS was determined as kmax/Ks = (3.97 × 10(3)) ± 619 M(-1) s(-1), which compares well to the OPS-specific mycobacterial cysteine synthase CysM with a kmax/Ks of (1.34 × 10(3)) ± 48.2. Notably, CysK2 does not utilize thiocarboxylated CysO as a sulfur donor but accepts thiosulfate and sulfide as donor substrates. The specificity constant kcat/Km for thiosulfate is 40-fold higher than for sulfide, suggesting an annotation as S-sulfocysteine synthase. Mycobacterial CysK2 thus provides a third metabolic route to cysteine, either directly using sulfide as donor or indirectly via S-sulfocysteine. Hypothetically, S-sulfocysteine could also act as a signaling molecule triggering additional responses in redox defense in the pathogen upon exposure to reactive oxygen species during dormancy.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Kinetics
  • Lyases / chemistry*
  • Lyases / genetics
  • Lyases / metabolism*
  • Mycobacterium tuberculosis / chemistry
  • Mycobacterium tuberculosis / enzymology*
  • Mycobacterium tuberculosis / genetics
  • Serine / analogs & derivatives
  • Serine / metabolism*
  • Substrate Specificity


  • Bacterial Proteins
  • Serine
  • Lyases
  • S-sulfocysteine synthase