Isothermal Titration Microcalorimetry Reveals the Cooperative and Noncompetitive Nature of Inhibition of Sinorhizobium meliloti L5-30 Dihydrodipicolinate Synthase by (S)-Lysine†

2008 
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 105 times more tightly than the first.
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