Galactose 1-Phosphatase Uridyltransferase (GALT) the rate-limiting enzyme of galactose metabolism is deficient in galactosemia. Galactose undergoes an exchange reaction with UDP-glucose to produce glucose 1-phosphate and UDP-galactose, using the rate-limiting enzyme galactose 1-phosphate uridyltransferase (GALT). Galactose metabolism: Galactose and its derivatives play a central role in the biosynthesis of complex carbohydrates, glycoproteins and glycolipids. In many organisms galactose also can serve as an impoant source of energy. No direct catabolic pathways exist for galactose metabolism. Galactose is therefore preferentially conveed into Glucose-1Phosphate, which may be shunted into glycolysis or into UDP galactose which can be used in synthetic pathways, including synthesis of lactose, glycoproteins, glycolipids, and glycosaminoglycans. Leloir pathway for Galactose metabolism: Galactose metabolism involves conversion of Galactose into Glucose-1-Phoshate, which may be shunted into Glycolysisor into UDP Galactose which can be used in synthetic pathways, including synthesis of Lactose, glycoproteins, glycolipids and glycosaminoglycans. This is carried out by the three principal enzymes in a mechanism known as Leloir pathways. Three major Enzymes of Galactose Metabolism and the metabolic consequence of result of their Deficiency: GALT: Galactose-1-Phosphate-Uridyl-Transferase (Classical Galactosemia; Most common: rate limiting Step) GALK: Galactokinase (Non classical Galactosemia; Less common) GALE: UDP-Galactose-4-Epimerase (Rare) These are diseases associated with deficiencies of each of these three enzymes the Leloir pathway Type 1 Type 2 Type 3 GALT GALK1 GALE Classic Galactosemia Galactokinase Deficient Galactosemia Galactose Epimerase Deficient Galactosemia Ref: Harper