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 Angstroms 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 Angstroms 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.