Gene Expression Patterns And Catalytic Properties Of Udp-D-Glucose 4-Epimerases From Barley (Hordeum Vulgare L.)

UGE (UDP-Glc 4-epimerase or UDP-Gal 4-epimerase; EC 5.1.3.2) catalyses the interconversion of UDP-Gal and UDPGlc. Both nucleotide Sugars act as activated sugar donors for the biosynthesis of cell wall polysaccharides such as cellulose, xyloglucans, (1,3;1,4)-beta-D-glucan and pectins, together with other biologically significant compounds including glycoproteins and glycolipids. Three members of the HvUGE (barley UGE) gene family, designated HvUGE1, HvUGE2 and HvUGE3, have been characterized. Q-PCR (quantitative real-time PCR) showed that MUM mRNA was most abundant in leaf tips and mature roots, but its expression levels were relatively low in basal leaves and root tips. The HvUGE2 gene wits transcribed at significant levels in all organs examined, while HvUGE3 mRNA levels were very low in till the organs. Heterologous expression of it near full-length cDNA confirmed that HvUGE1 encodes a functional UGE. A non-covalently bounds NAD(+) was released from the enzyme after denaturing with aqueous ethanol and was identified by its spectrophotometric properties and by electrospray ionization MS. The K. values were 40 mu M for UDP-Gal and 55 mu M for UDP-Glc. HvUGE also catalyses the interconversion of UDP-GalNAc and UDP-GlcNAc, although it is not known if this has arty biological significance. A three-dimensional model of the HvUGE revealed that its overall structural fold is highly conserved compared with the human UGE and provides a structural rationale for its ability to bind UDP-GlcNAc.