Galactose mutarotase catalyzes the conversion of beta-D-galactose to alpha-D-galactose. Beta-D-galactose is produced by the degradation of lactose, a disaccharide composed of beta-D-glucose and beta-D-galactose. This epimerization reaction is the first step in the four-step Leloir pathway, which converts galactose into metabolically important glucose. This epimerization step is followed by the phosophorylation of alpha-D-galactose by galactokinase, an enzyme which can only act on the alpha anomer. A glutamate and a histidine residue of the galactose mutarotase have been shown to be critical for catalysis, the glutamate serves as the active site base to initiate the reaction by removing the proton from the C-1 hydroxyl group of the sugar substrate, and the histidine as the active site acid to protonate the C-5 ring oxygen. Galactose mutarotase is a member of the aldose-1-epimerase superfamily.
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