Sugar nucleotide recognition by Klebsiella pneumoniae UDP-D-galactopyranose mutase: fluorinated substrates, kinetics and equilibria.

A series of selectively fluorinated and other substituted UDP-D-galactose derivatives have been evaluated as substrates for Klebsiella pneumoniae UDP-D-galactopyranose mutase. This enzyme, which catalyses the interconversion of the pyranose and furanose forms of galactose as its UDP adduct, is a prospective drug target for a variety of microbial infections. We show that none of the 2''-, 3''- or 6''-hydroxyl groups of UDP-D-galactopyranose are essential for substrate binding and turnover. However, steric factors appear to play an important role in limiting the range of substitutions that can be accommodated at C-2'' and C-6'' of the sugar nucleotide substrate. Attempts to invert the C-2'' stereochemistry from equatorial to axial, changing D-galacto- to D-talo-configuration, in an attempt to exploit the higher percentage of furanose at equilibrium in the talo-series, met with no turnover of substrate.