Modulation of Escherichia coli serine acetyltransferase catalytic activity in the cysteine synthase complex

In bacteria and plants, serine acetyltransferase (CysE) and O ‐acetylserine sulfhydrylase‐A sulfhydrylase (CysK) collaborate to synthesize l ‐Cys from l ‐Ser. CysE and CysK bind one another with high affinity to form the cysteine synthase complex ( CSC ). We demonstrate that bacterial CysE is activated when bound to CysK. CysE activation results from the release of substrate inhibition, with the K i for l ‐Ser increasing from 4 m m for free CysE to 16 m m for the CSC . Feedback inhibition of CysE by l‐ Cys is also relieved in the bacterial CSC . These findings suggest that the CysE active site is allosterically altered by CysK to alleviate substrate and feedback inhibition in the context of the CSC.