CHARACTERIZATION AND MUTATIONAL ANALYSIS OF TWO UDP-GALACTOSE 4-EPIMERASES IN <i>Streptococcuspneumoniae </i>TIGR4
Current clinical treatments for pneumococcal infections have many limitations and are faced with many challenges. New capsular polysaccharide structures must be explored to help prevent diseases caused by different serotypes of Streptococcus pneumoniae. UDP-galactose 4-epimerase (GalE) is an essential enzyme involved in polysaccharide synthesis. It is an important virulence factor in many bacterial pathogens. In this study, we found that two genes (galEsp1  and galEsp2) are responsible for galactose metabolism in pathogenic S. pneumoniae TIGR4. Both GalE8p1 and GalE8p2 were shown to catalyze the epimerization of UDP-glucose (UDP-Glc)/UDP-galactose (UDP-Gal), but only GalE8p2 was shown to catalyze the epimerization of UDP-N-acetylglucosamine (UDP-GlcNAc)/UDP-N-acetyl-galactosamine (UDP-GalNAc). Interestingly, GalE8p2 had 3-fold higher epimerase activity toward UDP-Glc/UDP-Gal than GalE8p1. The biochemical properties of GalE8p2 were studied. GalE8p2 was stable over a wide range of temperatures, between 30 and 70 °C, at pH 8.0. The K86G substitution caused GalE8p2 to lose its epimerase activity toward UDP-Glc and UDP-Gal; however, substitution C300Y in GalE8p2 resulted in only decreased activity toward UDP-GlcNAc and UDP-GalNAc. These results indicate that the Lys86 residue plays a critical role in the activity and substrate specificity of GalE8p2.