19. Fructose metabolism – absorption, organ specificity of degradation and association with other metabolic pathways. Defects in fructose metabolism – essential fructosuria and fructose intolerance. Nonalcoholic fatty liver disease.

Metabolism of Galactose

1. Galactose Uptake and Activation:

• Galactose is taken up into cells primarily via the sodium-dependent glucose transporter (SGLT1) or facilitated diffusion through glucose transporters.
• Inside the cell, galactose is phosphorylated to galactose-1-phosphate by the enzyme galactokinase, utilizing ATP.

Metabolism of Galactose

2. Conversion of Galactose-1-phosphate to Glucose-1-phosphate:

• Galactose-1-phosphate is converted to glucose-1-phosphate by the enzyme galactose-1-phosphate uridylyltransferase (GALT).
• GALT catalyzes the transfer of a UDP (uridine diphosphate) to galactose-1-phosphate, forming UDP-galactose and glucose-1-phosphate.

Metabolism of Galactose

3. Conversion of UDP-Galactose to UDP-Glucose:

• UDP-galactose is converted to UDP-glucose by the enzyme UDP-galactose 4-epimerase.
• This enzyme catalyzes the epimerization of the C4 hydroxyl group of UDP-galactose to a C4 hydroxyl group, forming UDP-glucose.

Metabolism of Galactose

4. Incorporation of Glucose-1-Phosphate into Glycogen or Glycolysis:

• Glucose-1-phosphate generated from galactose metabolism can be incorporated into glycogen synthesis via the enzyme glycogen synthase.
• Alternatively, glucose-1-phosphate can enter glycolysis after conversion to glucose-6-phosphate by the enzyme phosphoglucomutase.

Metabolism of Fructose:

1. Fructose Uptake and Activation:

• Fructose is transported into cells via the glucose transporter GLUT5.
• Inside the cell, fructose is phosphorylated to fructose-1-phosphate by fructokinase, utilizing ATP.