Galactose Is Needed For The Synthesis Of Lactose Glycolipids Proteoglycans Glycoproteins

Galactose is derived from intestinal hydrolysis of the disaccharide lactose, the sugar of milk. It is readily converted in the liver to glucose. Galactokinase catalyzes the phosphorylation of galactose, using ATP as phosphate donor (Figure 20-6A). Galactose 1-phosphate reacts with uridine diphosphate glucose (UDPGlc) to form uridine diphosphate galactose (UDPGal) and glucose 1-phosphate, in a reaction catalyzed by galactose 1-phosphate uridyl transferase. The conversion of UDPGal to UDPGlc is catalyzed by UDPGal 4-epim-erase. Epimerization involves an oxidation and reduction at carbon 4 with NAD+ as coenzyme. Finally, glucose is liberated from UDPGlc after conversion to glucose 1-phosphate, probably via incorporation into glycogen followed by phosphorolysis (Chapter 18).

Since the epimerase reaction is freely reversible, glucose can be converted to galactose, so that galactose is not a dietary essential. Galactose is required in the body not only in the formation of lactose 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 (Figure 20-6B).

PHOSPHO-GLUCOMUTASE

CH2-O-(i a-D-Glucose 6-phosphate

CH2OH

*CH2OH L-Xylulose

NADP+

BLOCK IN PENTOSURIA

*ch2oh

CH2OH

Xylitol

UDPGIC PYRO-PHOSPHORYLASE

HO C

CH2OH

Glucose 1-phosphate

HO C

Uridine diphosphate glucose (UDPGlc)

Uridine diphosphate glucose (UDPGlc)

Uridine diphosphate glucuronate

Glycolate

Glycolaldehyde

NADH

NAD+

Oxalate

*ch2oh L-Gulonate

L-Gulonolactone

NADP+

II O

D-Glucuronate

BLOCK IN PRIMATES AND GUINEA PIGS

BLOCK IN HUMANS

2-Keto-L-gulonolactone

D-Xylulose 1-phosphate

NAD+

*ch2oh

D-XYLULOSE REDUCTASE

CH2OH

Diet

D-Xylulose 5-phosphate

*ch2oh L-Ascorbate

HO-C-H Oxalate *ch2oh L-Dehydroascorbate

Pentose phosphate pathway Figure 20-4. Uronic acid pathway. (Asterisk indicates the fate of carbon 1 of glucose; — PO32-.)

Glycogen

Glucose 6-phosphate

Glycogen

ALDOSE REDUCTASE

PHOSPHOHEXOSE ISOMERASE

D-Sorbitol

GLUCOSE-6-PHOSPHATASE

SORBITOL DEHYDROGENASE

HEXOKINASE

NAD+

NADH

Fructose 6-phosphate

FRUCTOSE-1,6-BISPHOSPHATASE

ATM PHOSPHOFRUCTOKINASE

FRUCTOKINASE

BLOCK IN ESSENTIAL FRUCTOSURIA

Fructose 1,6-bisphosphate

Fructose 1-phosphate

ALDOLASE A

ALDOLASE B

PHOSPHO-

TRIOSE ISOMERASE

Glyceraldehyde 3-phosphate

2-Phosphoglycerate

BLOCK IN HEREDITARY FRUCTOSE INTOLERANCE

Dihydroxyacetone-phosphate

Fatty acid esterification

TRIOKINASE

ALDOLASE B

D-Glyceraldehyde

Pyruvate

Fatty acid synthesis

Figure 20-5. Metabolism of fructose. Aldolase A is found in all tissues, whereas aldolase B is the predominant form in liver. (*, not found in liver.)

Glucose Is the Precursor of All Amino Sugars (Hexosamines)

Amino sugars are important components of glycoproteins (Chapter 47), of certain glycosphingolipids (eg, gangliosides) (Chapter 14), and of glycosaminoglycans (Chapter 48). The major amino sugars are glucosa-mine, galactosamine, and mannosamine and the nine-carbon compound sialic acid. The principal sialic acid found in human tissues is A-acetylneuraminic acid (NeuAc). A summary of the metabolic interrelationships among the amino sugars is shown in Figure 20-7.

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