Delta-L-(alpha-aminoadipoyl)-L-cysteinyl-D-valine synthetase: isolation of L-cysteinyl-D-valine, a 'shunt' product, and implications for the order of peptide bond formation.

L-Cysteinyl-D-valine was isolated from incubations of L-glutamate, L-cysteine and L-valine with delta-L-(alpha-aminoadipoyl)-L-cysteinyl-D-valine synthetase and identified by 1H NMR and electrospray ionization MS. This is entirely consistent with our prior proposal (Shiau, C.-Y., Baldwin, J.E., Byford, M.F., Sobey, W.J. and Schofield, C.J. (1995) FEBS Lett. 358, 97-100) that the alpha-peptide bond between cysteine and valine is formed before the delta-peptide bond between alpha-aminoadipate and cysteine. The inclusion of L-glutamate, an analogue of L-alpha-aminoadipate, did not result in a detectable amount of tripeptide product, but did increase apparent yields of L-cysteinyl-D-valine. Conceivably, formation of the L-glutamyladenylate stimulates synthesis of the cysteinyl-valine dipeptide indirectly via a conformational change in the enzyme.