Monosaccharide Metabolism

While glucose is the primary monosaccharide for ATP production, galactose and fructose can also contribute by feeding into glycolysis or other metabolic processes.
These monosaccharides are less frequently tested than glucose on the MCAT but are included for comparison.
Notice the similarities between glycolysis, galactose metabolism, and fructose metabolism.

Lactose, a disaccharide in milk, is an important source of galactose.
Lactase, a brush border enzyme of the duodenum, hydrolyzes lactose into galactose and glucose.
Galactose, along with other monosaccharides, reaches the liver through the hepatic portal vein.
Once in tissues, galactokinase phosphorylates galactose, trapping it in the cell.
The resulting galactose-1-phosphate is converted to glucose-1-phosphate by galactose-1-phosphate uridyltransferase and an epimerase.
Epimerases are enzymes that catalyze the conversion of one sugar epimer to another.
Epimers are diastereomers that differ at exactly one chiral carbon.
Key enzymes to remember: galactokinase, galactose-1-phosphate uridyltransferase, and epimerase.

Fructose is found in honey and fruit and is part of the disaccharide sucrose.
Sucrase, a duodenal brush border enzyme, hydrolyzes sucrose into glucose and fructose.
These monosaccharides are absorbed into the hepatic portal vein.
Fructokinase phosphorylates fructose in the liver to trap it in the cell.
The resulting fructose-1-phosphate is cleaved into glyceraldehyde and DHAP by aldolase B.
Smaller amounts of fructose are metabolized in renal proximal tubules.