Biochemical and structural studies of malate synthase from Mycobacterium tuberculosis

Establishment or maintenance of a persistent infection by Mycobacterium tuberculosis requires the glyoxylate pathway. This is a bypass of the tricarboxylic acid cycle in which isocitrate lyase and malate synthase (GlcB) catalyze the net incorporation of carbon during growth of microorganisms on acetate or fatty acids as the primary carbon source. The glcB gene from M. tuberculosis, which encodes malate synthase, was cloned, and GlcB was expressed in Escherichia coli. The influence of media conditions on expression in M. tuberculosis indicated that this enzyme is regulated differentially to isocitrate lyase. Purified GlcB had K-m values of 57 and 30 muM for its substrates glyoxylate and acetyl coenzyme A, respectively, and was inhibited by bromopyruvate, oxalate, and phosphoenolpyruvate. The GlcB structure was solved to 2.1-Angstrom resolution in the presence of glyoxylate and magnesium. We also report the structure of Glcbeta in complex with the products of the reaction, coenzyme A and malate, solved to 2.7-Angstrom resolution. Coenzyme A binds in a bent conformation, and the details of its interactions are described, together with implications on the enzyme mechanism.