Isothermal titration microcalorimetry reveals the cooperative and noncompetitive nature of inhibition of Sinorhizobium meliloti L5-30 dihydrodipicolinate synthase by (S)-lysine.

MosA, a dihydrodipicolinate synthase (DHDPS) from Sinorhizobium meliloti L5-30, catalyzes a class I aldolase reaction that is allosterically inhibited by (S)-lysine. The thermodynamics of (S)-lysine binding to apoenzyme, and to enzyme saturated with pyruvate or with 2-oxobutyrate, are evaluated here using isothermal titration microcalorimetry. Results unambiguously support a noncompetitive mechanism, with substrate-dependent differences in the energetics of inhibitor binding. Inhibition is strikingly cooperative: a second molecule of (S)-lysine binds 10(5) times more tightly than the first.