Dihydrodipicolinate synthase (DHDPS, E.C. 4.2.1.52) mediates the first unique reaction of (S)-lysine biosynthesis in plants and microbes-the condensation of (S)-aspartate-beta-semialdehyde ((S)-ASA) and pyruvate. It has been shown that DHDPS is partially feedback inhibited by (S)-lysine; it is suggested that this mechanism regulates flux through the DAP biosynthetic pathway. Others have characterised DHDPS from Escherichia coli with respect to (S)-lysine inhibition. They have concluded that, with respect to pyruvate, the first substrate of the reaction, DHDPS shows uncompetitive inhibition: as such, they further suggest that (S)-lysine inhibits DHDPS via interaction with the binding site for the second substrate, (S)-ASA. Yet, this finding is based on the assumption that (S)-lysine is a fully uncompetitive inhibitor. In light of crystallographic studies, which lead to the proposal that (S)-lysine affects the putative proton-relay of DHDPS, we re-evaluated the inhibition mechanism of DHDPS with respect to (S)-lysine by incorporating the observed hyperbolic inhibition. Our data showed that lysine is not an uncompetitive inhibitor, but a mixed inhibitor when pyruvate and (S)-lysine concentrations were varied. Thus, consistent with the crystallographic data, (S)-lysine must have an effect on the initial steps of the DHDPS reaction, including the binding of pyruvate and Schiff base formation.