Figure 1. The Leloir pathway of galactose metabolism. Human UDP-galactose 4-epimerase catalyzes step 4 of the pathway (7).
Figure 2. The three-step mechanism occuring in the active site of hGALE. First, Tyr-157 abstracts the 4’-hydroxyl hydrogen from UDP-galactose and transfers it to the C4 of the NAD+ cofactor. This leads to a 4’-ketopyranose intermediate and NADH. Finally, the 4’-ketopyranose rotates 180° in the active site allowing the return of the hydride from NADH to the opposite face of UDP-galactose, generating UDP-glucose (7).
Figure 3. Ramachandran plot for hGALE (9).