Slow-Binding Inhibition of Mycobacterium tuberculosis Shikimate Kinase by Manzamine Alkaloids.

Tuberculosis represents a significant public health crisis. There is an urgent need for novel molecular scaffolds against this pathogen. We screened a small library of marine-derived compounds against shikimate kinase from Mycobacterium tuberculosis (MtSK), a promising target for antitubercular drug development. Six manzamines previously shown to be active against M. tuberculosis were characterized as MtSK inhibitors: manzamine A (1), 8-hydroxymanzamine A (2), manzamine E (3), manzamine F (4), 6-deoxymanzamine X (5), and 6-cyclohexamidomanzamine A (6). All six showed mixed noncompetitive inhibition of MtSK. The lowest K-1 values were obtained for 6 across all MtSK substrate complexes. Time dependent analyses revealed two-step, slow-binding inhibition. The behavior of 1 was typical; initial formation of an enzyme inhibitor complex (EI) obeyed an apparent K-1 of similar to 30 mu M with forward (k(5)) and reverse (k(6)) rate constants for isomerization to an EI* complex of 0.18 and 0.08 min(-1), respectively. In contrast, 6 showed a lower K-1 for the initial encounter complex (similar to 1.5 mu M), substantially faster isomerization to EI* (k(5) = 0.91 min(-1)), and slower back conversion of EI* to EI (k(6) = 0.04 min(-1)). Thus, the overall inhibition constants, K-1*, for 1 and 6 were 10 and 0.06 mu M, respectively. These findings were consistent with docking predictions of a favorable binding mode and a second, less tightly bound pose for 6 at MtSK. Our results suggest that manzamines, in particular 6, constitute a new scaffold from which drug candidates with novel mechanisms of action could be designed for the treatment of tuberculosis by targeting MtSK.