Kinetic analysis of the effects of monovalent cations and divalent metals on the activity of Mycobacterium tuberculosis α-isopropylmalate synthase

Mycobacterium tuberculosis α-isopropylmalate synthase (MtIPMS) is a member of the family of enzymes that catalyze a Claisen-type condensation. In this work we characterized the monovalent and divalent specificity of MtIPMS using steady-state kinetics. The monovalent cation dependence of the kinetic parameters of substrates and divalent metals indicates that K⁺ is the likely physiological activator. K⁺ acts most likely as an allosteric activator, and exerts part of its effect through the catalytic divalent metal. The divalent metal specificity of MtIPMS is broad, and Mg²⁺ and Mn²⁺ are the metals that cause the highest activation. Interestingly, Zn²⁺, first assigned as the catalytic metal, inhibits the enzyme with submicromolar affinity. The features of monovalent cation and divalent metal activation, as well as the inhibition by Zn²⁺ and Cd²⁺, are discussed in light of the kinetic and structural information available for MtIPMS and other relevant enzymes.