δ‐l‐(α‐Aminoadipoyl)‐l‐cysteinyl‐d‐valine synthetase: isolation of l‐cysteinyl‐d‐valine, a ‘shunt’ product, and implications for the order of peptide bond formation

Abstract
L‐Cysteinyl‐d‐valine was isolated from incubations of l‐glutamate, l‐cysteine and l‐valine with δ‐l‐(α‐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 α‐peptide bond between cysteine and valine is formed before the δ‐peptide bond between α‐aminoadipate and cysteine. The inclusion of l‐glutamate, an analogue of l‐α‐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.

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