Human UDP-galactose 4-epimerase - Accommodation of UDP-N-acetylglucosaminewithin the active site

UDP-galactose 4-epimerase catalyzes the interconversion of UDP-galactose an d UDP-glucose during normal galactose metabolism. One of the key structural features in the proposed reaction mechanism for the enzyme is the rotation of a 4'-ketopyranose intermediate within the active site pocket. Recently, the three-dimensional structure of the human enzyme with bound NADH and UD P-glucose was determined, Unlike that observed for the protein isolated fro m Escherichia coil, the human enzyme can also turn over UDP-GlcNAc to UDP-G alNAc and vice versa, Here we describe the three-dimensional structure of h uman epimerase complexed with NADH and UDP-GlcNAc. To accommodate the addit ional N-acetyl group at the C2 position of the sugar, the side chain of Asn -207 rotates toward the interior of the protein and interacts with Glu-199, Strikingly, in the human enzyme, the structural equivalent of Tyr-299 in t he E, coli protein is replaced with a cysteine residue (Cys-307) and the ac tive site volume for the human protein is calculated to be similar to 15% l arger than that observed for the bacterial epimerase, This combination of a larger active site cavity and amino acid residue replacement most likely a ccounts for the inability of the E, coil enzyme to interconvert UDP-GlcNAc and UDP-GalNAc.