Mycobacterium tuberculosis isocitrate lyase (MtbIcl): role of divalent cations in modulation of functional and structural properties.

Isocitrate lyase (Icl), an enzyme that plays an important role in the regulation of isocitrate flux and anaplerotic replenishment of pool of substrate required for biosynthetic process in Mycobacterium tuberculosis is a potential drug target for the antituberculosis drugs. Divalent cations induce differential effect of activation and inhibition of MtbIcl functional activity. The study for the first time demonstrates that interaction of cations with MtbIcl results in differential modulation of the enzyme structure which is probably the underlying mechanism for differential modulation of functional activity of enzyme by divalent cations. The Mg(2+) and Mn(2+) ions act as activators of the enzyme and in their absence no enzymatic activity was observed. These cations do not induce any significant structural alteration in the enzyme as observed by far-UV CD and solvent denaturation studies using chaotropic salts. However, the thermal denaturation studies demonstrate that they do interact with the noncatalytic alpha/beta barrel core domain of the enzyme and destabilize it. The inhibitors Zn(2+) and Cd(2+) interact directly with the catalytic domain of the enzyme and unfold it as a result of which complete loss of the enzymatic activity is observed in their presence. The results obtained from the studies provide intriguing insight into the possible mechanism of divalent cation-induced changes in structure, function, and stability of MtbIcl.