Structural and Functional Studies of Mycothiol Biosynthesis Precursor Enzyme in Mycobacterium tuberculosis

MshA is a glycosyltransferase that synthesizes the precursor of mycothiol, a low-molecular-weight thiol found exclusively in Actinomycetes, including the virulent pathogen Mycobacterium tuberculosis (Mtb). The structure of MshA from Mtb (herein coined as TbMshA) and its complex with uridine diphosphate N-acetyl-glucosamine (UDP-GlcNAc) have been solved to resolutions of 2.32 A and 2.89 A respectively. Both structures form two monomers in the asymmetric unit cell and exhibit typical beta/alpha/beta Rossmann folds. Upon binding of UDP-GlcNAc, the C-terminal domain of TbMshA undergoes conformational changes in order to interact with UDP-GlcNAc at the binding site. In addition, ligand-bound TbMshA structure enables the identification of critical residues for enzymatic interactions, especially the residue Glu-353 (E353) at the active site that is believed to serve as a nucleophile in the sugar transfer of TbMshA. In order to verify this, a mutant of TbMshA with a single amino acid mutation from glutamate to glutamine at residue 353 is generated. The mutant (E353Q) has shown reduced enzyme activity by more than four-fold compared to the wild-type TbMshA (Vmax for wild-type is 0.17 plus/minus 0.02 microM sec^-1, whereas Vmax for E353Q is 0.04 plus/minus 0.01 microM sec-1). The kcat/Km for wild-type TbMshA (3.5 plus/minus 1.1 * 10^3 M^-1 sec^-1) is an order of magnitude higher than that of the mutant (0.3 plus/minus 0.1 * 10^3 M^-1 sec^-1), indicating the catalytic efficiency is greatly suppressed by the mutation. Mass spectrometry data also reveals that E353Q is unable to form the product of the reaction catalyzed by the wild-type TbMshA. These findings suggest the important role of Glu-353 in the structure and activity of TbMshA.