Glactose and glucose are epimers that differ only in their configuration about C4. The enzymes of glycolysis are specific and do not recognize the galactose configuration. An epimerization reaction must therefore be carried out before galactose enters the glycolytic pathway.

This reaction takes place afetr the conversion of galactose to its uridine diphosphate derivative.

Galactose is converted into glucose-6-P in four steps:

1-Galactokinase catalyzes galactose phosphorylation at C1 by ATP.

2-Galactose-1-P then acquires an uridyl group from uridine diphosphate glucose (UDP-galactose), an intermediate in the synthesis of glycosidic linkages. The products of this reaction, which is catayzed by galactose-1-P uridyl transferase, are UDP-galactose and glucose-1-P.

3-UDP-galactose-4-epimerase converts UDP-galactose back to UDP-glucose. Note that UDP-glucose is not consumed in the conversion of galactose to glucose because its regenerated from UDP-galactose by the epimerase. This reaction is reversible, and the product of the reverse direction is also important (it's essential for the synthesis of galactosyl residues in complex polysaccharides and glycoproteins if the amount of galactose in the diet is inadequate to meet these needs.

4-G1P is converted to the glycolytic intermediate G6P by the action of phosphoglucomutase.