Galactose is derived from intestinal hydrolysis of the disaccharide lactose, the sugar found in milk. It is readily converted in the liver to glucose after being transported by GLUT5. So like fructose the majority of dietary galactose is metabolized by the liver. Galactokinase catalyzes the phosphorylation of galactose, using ATP as phosphate donor (Figure 1). Galactose-1-phosphate reacts with UDPGlc to form uridine diphosphate galactose (UDPGal) and glucose-1-phosphate, in a reaction catalyzed by galactose-1-phosphateuridyl transferase. The conversion of UDPGal to UDPGlc is catalyzed by UDPGal 4-epimerase. The reaction involves oxidation, and then reduction, at carbon 4, with NAD⁺ as a coenzyme. The UDPGlc is then incorporated into glycogen .

Fig1. Pathway of conversion of (A) galactose to glucose in the liver and (B) glucose to lactose in the lactating mammary gland.

The epimerase reaction is freely reversible, so glucose can be converted to galactose, and galactose is not a dietary essential. Galactose is required in the body not only for the formation of lactose in lactation but also as a constituent of glycolipids (cerebrosides), proteoglycans, and glycoproteins. In the synthesis of lactose in the mammary gland, UDPGal condenses with glucose to yield lactose, catalyzed by lactose synthase (see Figure 1).

Glucose Is the Precursor of Amino Sugars (Hexosamines)

Amino sugars are important components of glycoproteins , of certain glycosphingolipids (eg, gangliosides), and of glycosaminoglycans (see Chapter 50). The major amino sugars are the hexosamines glucosamine, galactosamine, and mannosamine, and the nine-carbon compound sialic acid. The principal sialic acid found in human tissues is N-acetylneuraminic acid (NeuAc).

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مواضيع ذات صلة

Ingestion of Large Quantities of Fructose has Profound Metabolic Consequences

Reactions of the Pentose Phosphate Pathway Occur in the Cytosol

Blood Glucose is Derived From the Diet, Gluconeogenesis, & Glycogenolysis

Glycolysis & gluconeogenesis share the same pathway but in opposite directions, & are reciprocally regulated

Glucose-1-Phosphate & Glycogen

Thermodynamic Barriers Prevent a Simple Reversal of Glycolysis

Gluconeogenesis & the Control of Blood Glucose: Biomedical Importance

Glycogen Metabolism is Regulated by A Balance in Activities Between Glycogen Synthase & Phosphorylase

cAMP Activates Glycogen Phosphorylase

Cyclic AMP Integrates The Regulation of Glycogenolysis & Glycogenesis

Glycogenolysis is not the Reverse of Glycogenesis, it is A Separate Pathway

Glycogenesis Occurs Mainly in Muscle & Liver