The frontline
tuberculosis drug isoniazid (INH) inhibits InhA, the
NADH-dependent
fatty acid biosynthesis (FAS-II) enoyl
reductase from Mycobacterium tuberculosis (MTB), via formation of a covalent adduct with
NAD(+) (the
INH-NAD adduct). Resistance to INH can be correlated with many mutations in MTB, some of which are localized in the InhA cofactor binding site. While the InhA mutations cause a substantial decrease in the affinity of InhA for
NADH, surprisingly the same mutations result in only a small impact on binding of the
INH-NAD adduct. Based on the knowledge that InhA interacts in vivo with other components of the FAS-II pathway, we have initiated experiments to determine whether
enzyme inhibition results in structural changes that could affect
protein-
protein interactions involving InhA and how these
ligand-induced conformational changes are modulated in the InhA mutants. Significantly, while
NADH binding to wild-type InhA is hyperbolic, the InhA mutants bind the cofactor with positive cooperativity, suggesting that the mutations permit access to a second conformational state of the
protein. While cross-linking studies indicate that
enzyme inhibition causes dissociation of the InhA tetramer into dimers, analytical ultracentrifugation and size exclusion chromatography reveal that
ligand binding causes a conformational change in the
protein that prevents cross-linking across one of the dimer-dimer interfaces in the InhA tetramer. Interestingly, a similar
ligand-induced conformational change is also observed for the InhA mutants, indicating that the mutations modulate communication between the subunits without affecting the two conformational states of the
protein that are present.