DHDPS (dihydrodipicolinate synthase; EC is the enzyme that catalyses the first unique step of lysine biosynthesis in plants and micro-organisms. As such, it has attracted much attention as a target for herbicide and anti-microbial action. DHDPS has two substrates: pyruvate and (S)-aspartate β-semialdehyde [(S)-ASA]. There are various literature reports that suggest that high levels of (S)-ASA inhibit the enzyme [Karsten (1997) Biochemistry 36, 1730–1739; Stahly (1969) Biochim. Biophys. Acta 191, 439–451], whereas others have not observed this phenomenon. We have resolved this long-running literature debate and shown unequivocally that this difference in reported behaviour can be attributed to differences in the preparation of (S)-ASA used by each researcher. DHDPS is not inhibited by its substrate; rather, the inhibition is due to an, as yet, unidentified inhibitor in preparations of the substrate generated by ozonolysis. Furthermore, we demonstrate that (R)-ASA is neither an inhibitor nor a substrate of DHDPS from Escherichia coli.

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