Galactose - Metabolism

Metabolism

Glucose is the primary metabolic fuel for humans. It is more stable than galactose and is less susceptible to the formation of nonspecific glycoconjugates, molecules with at least one sugar attached to a protein or lipid. Many speculate that it is for this reason that a pathway for rapid conversion from galactose to glucose has been highly conserved among many species.

The main pathway of galactose metabolism is the Leloir pathway; humans and other species, however, have been noted to contain several alternate pathways, such as the De Ley Doudoroff pathway. The Leloir pathway consists of the latter stage of a two-part process that converts β-D-galactose to UDP-glucose. The initial stage is the conversion of β-D-galactose to α-D-galactose by the enzyme, mutarotase (GALM). The Leloir pathway then carries out the conversion of α-D-galactose to UDP-glucose via three principle enzymes. Galactokinase (GALK) phosphorylates α-D-galactose to galactose-1-phosphate, or Gal-1-P. Galactose-1-phosphate uridyltransferase (GALT) then transfers a UMP group from UDP-glucose to Gal-1-P to form UDP-galactose. Finally, UDP galactose-4’-epimerase (GALE) interconverts UDP-galactose and UDP-glucose, thereby completing the pathway.

However, those suffering from galactosemia cannot properly break down galactose as the result of a genetically inherited mutation in one of the enzymes in the Leloir pathway. These individuals cannot break down galactose properly, so the consumption of even small amounts of galactose is harmful to galactosemics.