Galactose Metabolism

1. 1

   Lactose + H2O → glucose + galactose

   Brush-border lactase-phlorizin hydrolase (beta-galactosidase) 3.2.1.108 ST-0053 Irreversible/directional H2O
   In Lactose H2O

   Out Glucose Galactose
   › Notes

   Brush-border lactase-phlorizin hydrolase (beta-galactosidase) hydrolyzes dietary lactose in the small-intestinal lumen/brush border. This digestive precursor reaction requires water; lactase deficiency impairs galactose generation from lactose but is distinct from inherited Leloir-pathway disorders.

2. 2

   Alpha-D-galactose ↔ beta-D-galactose

   Galactose mutarotase (GALM; aldose 1-epimerase) 5.1.3.3 ST-0054 Reversible
   In Alpha-D-galactose

   Out beta-D-galactose
   › Notes

   Galactose mutarotase (aldose 1-epimerase; GALM) interconverts galactose anomers so that alpha-D-galactose is available for galactokinase. No ATP or redox cofactor is required.

3. 3

   Galactose + ATP → galactose-1-phosphate + ADP

   Galactokinase (GALK1) 2.7.1.6 ST-0055 Irreversible ATP Mg2+
   In Galactose ATP

   Out Galactose-1-phosphate ADP
   › Notes

   Galactokinase (GALK1) requires ATP and Mg2+ and phosphorylates galactose at carbon 1. This is effectively irreversible and traps galactose intracellularly.

4. 4

   Galactose-1-phosphate + UDP-glucose → glucose-1-phosphate + UDP-galactose

   Galactose-1-phosphate uridylyltransferase (GALT) 2.7.7.12 ST-0056 Reversible
   In Galactose-1-phosphate UDP-glucose

   Out Glucose-1-phosphate UDP-galactose
   › Notes

   Galactose-1-phosphate uridylyltransferase (GALT) exchanges uridyl groups. This central Leloir reaction requires the activated-sugar substrate UDP-glucose; it is reversible, but physiologic flux is directed toward glucose-1-phosphate formation. It is the critical/committed processing step for galactose-1-phosphate.

5. 5

   UDP-galactose ↔ UDP-glucose

   UDP-galactose 4-epimerase (GALE) ST-0057 Reversible NAD+
   In UDP-galactose

   Out UDP-glucose
   › Notes

   UDP-galactose 4-epimerase (GALE) requires tightly bound NAD+ as a catalytic cofactor and reversibly epimerizes carbon 4. It regenerates UDP-glucose for step 4 and also supports UDP-galactose production for glycosylation reactions.

6. 6

   Glucose-1-phosphate ↔ glucose-6-phosphate

   Phosphoglucomutase ST-0058 Reversible
   In Glucose-1-phosphate

   Out Glucose-6-phosphate
   › Notes

   Phosphoglucomutase reversibly forms glucose-6-phosphate through a glucose-1,6-bisphosphate intermediate. The product can enter glycolysis, glycogenesis, or, in liver, be converted to free glucose by glucose-6-phosphatase.

7. 7

   Galactose + NADPH + H+ → galactitol + NADP+

   Aldose reductase (AKR1B1) 1.1.1.21 ST-0059 Irreversible/directional NADP+ NADPH
   In Galactose NADPH H+

   Out Galactitol NADP+
   › Notes

   Aldose reductase reduces excess galactose to galactitol using NADPH. This is a side pathway rather than a normal energy-yielding Leloir step; galactitol is poorly metabolized and can accumulate, particularly in the lens.

Showing all 7 steps.

Overview

Galactose metabolism converts dietary galactose, principally derived from lactose, into glucose-1-phosphate through the Leloir pathway. This permits galactose carbon to enter glycogen metabolism, glycolysis, or hepatic glucose production. Galactose is also required in activated forms for synthesis of glycoproteins, glycolipids, and proteoglycans.

Cellular location

The Leloir pathway occurs in the cytosol, particularly in liver, but is present in many tissues. Intestinal lactase first hydrolyzes lactose at the brush border to glucose and galactose; absorbed galactose travels through portal blood to liver. The lens is clinically important because aldose reductase can reduce excess galactose to galactitol, which accumulates in this osmotically sensitive tissue.

Net energetics

Conversion of galactose to glucose-6-phosphate through the Leloir pathway consumes 1 ATP at galactokinase and has no direct net production of NADH, NADPH, FADH2, or GTP; UDP-glucose is regenerated by GALE during catalytic cycling. When the resulting glucose-6-phosphate enters glycolysis, it bypasses the hexokinase ATP cost, so conversion to two pyruvate yields net 3 ATP and 2 NADH per galactose (assuming no additional diversion). Reduction of galactose to galactitol consumes NADPH and does not generate ATP.

Clinical significance

The Leloir pathway makes galactose nutritionally usable while maintaining UDP-galactose for glycosylation, so defects can affect both energy metabolism and protein/lipid glycosylation. Classic galactosemia due to GALT deficiency causes accumulation of galactose-1-phosphate and galactitol, with neonatal vomiting, jaundice, hepatomegaly, failure to thrive, *Escherichia coli* sepsis risk, cataracts, and later neurodevelopmental or ovarian complications without early dietary restriction. Galactokinase deficiency more selectively causes infantile cataracts from galactitol accumulation, whereas GALE deficiency ranges from benign peripheral forms to severe generalized disease. Lipid metabolism governs the synthesis, storage, breakdown, and transport of fatty acids, triacylglycerols, cholesterol, and lipoproteins, providing the body’s most energy-dense fuel reserve and the structural and signaling lipids required by every cell membrane. These pathways interconvert with carbohydrate and amino acid metabolism through shared intermediates such as acetyl-CoA, and their dysregulation contributes to obesity, atherosclerosis, and metabolic syndrome. Understanding lipid handling is therefore essential to both basic physiology and cardiovascular medicine.