UDPgalactose 4-epimerase (epimerase) catalyzes the reversible conversion between UDPgalactose and UDPglucose and is an important enzyme of the galactose metabolic pathway. The Saccharomyces cerevisiae epimerase encoded by the GAL10 gene is about twice the size of either the bacterial or human protein. Sequence analysis indicates that the yeast epimerase has an N-terminal domain (residues 1-377) that shows significant similarity with Escherichia coli and human UDPgalactose 4-epimerase, and a C-terminal domain (residues 378-699), which shows extensive identity to either the bacterial or human aldose 1-epimerase (mutarotase). The S. cerevisiae epimerase was purified to > 95% homogeneity by sequential chromatography on DEAE-Sephacel and Resource-Q columns. Purified epimerase preparations showed mutarotase activity and could convert either alpha-d-glucose or alpha-d-galactose to their beta-anomers. Induction of cells with galactose led to simultaneous enhancement of both epimerase and mutarotase activities. Size exclusion chromatography experiments confirmed that the mutarotase activity is an intrinsic property of the yeast epimerase and not due to a copurifying endogenous mutarotase. When the purified protein was treated with 5'-UMP and l-arabinose, epimerase activity was completely lost but the mutarotase activity remained unaffected. These results demonstrate that the S. cerevisiae UDPgalactose 4-epimerase is a bifunctional enzyme with aldose 1-epimerase activity. The active sites for these two enzymatic activities are located in different regions of the epimerase holoenzyme.
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