The product complex ofM. tuberculosismalate synthase revisited

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Enzymes of the glyoxylate shunt have been implicated as virulence factors in several pathogenic organisms, notably Mycobacterium tuberculosis and Candida albicans. Malate synthase has thus emerged as a promising target for design of anti-microbial agents. For this effort, it is essential to have reliable models for enzyme:substrate complexes. A 2.7 Å resolution crystal structure for M. tuberculosis malate synthase in the ternary complex with magnesium, malate, and coenzyme A has been previously described. However, some unusual aspects of malate and Mg++ binding prompted an independent determination of the structure at 2.3 Å resolution, in the presence of saturating concentrations of malate. The electron density map of the complex reveals the position and conformation of coenzyme A to be unchanged from that found in the previous study. However, the coordination of Mg++ and orientation of bound malate within the active site are different. The revised position of bound malate is consistent with a reaction mechanism that does not require reorientation of the electrophilic substrate during the catalytic cycle, while the revised Mg++ coordination is octahedral, as expected. The results should be useful in the design of malate synthase inhibitors.

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