Galactolysis

Galactolysis

Galactolysis

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Galactolysis refers to the catabolism of galactose.

In the liver, galactose is converted through the Leloir pathway to glucose 6-phosphate[1] in the following reactions:

       galacto-                uridyl                phosphogluco-
        kinase               transferase                mutase
   gal --------> gal 1 P ------------------> glc 1 P -----------> glc 6 P
                            ^           \
                           /             v
                        UDP-glc       UDP-gal
                           ^             /
                            \___________/
                              epimerase

Mutations in the enzymes involved in galactolysis result in metabolic disorders.[2]

Metabolic disorders

There are 3 types of galactosemia or galactose deficiencies:

NameEnzymeDescription
galactokinase deficiencyGalactokinaseCauses cataracts, which form due to the elevation of galactitol that accumulates when galactose is metabolized in an alternative pathway that is not the Leloir pathway.[2] These are treatable by restricting galactose from the diet.
UDPgalactose-4-epimerase deficiencyUDPgalactose-4-epimeraseIs extremely rare (only 2 reported cases). It causes nerve deafness.
Galactose-1-phosphate uridyl transferase deficiencyGalactose-1-phosphate uridyl transferaseIs the most problematic, as galactose-free diets are not effective in treating neurocognitive deficiencies (in particular language disorders such as verbal dyspraxia) and ovarian failure. If a galactose-free diet is administered, cataracts and acute symptoms such as kidney and liver failure respond immediately. Formation of cataracts is similar to that in galactokinase deficiency.[2]

References

  1. [1]
    Holden, Hazel M.; Rayment, Ivan; Thoden, James B. (7 November 2003). "Structure and Function of Enzymes of the Leloir Pathway for Galactose Metabolism". Journal of Biological Chemistry. 278 (45): 43885–43888. doi:10.1074/jbc.R300025200. PMID 12923184.
  2. [2]
    Demirbas, Didem; Coelho, Ana I.; Rubio-Gozalbo, M. Estela; Berry, Gerard T. (June 2018). "Hereditary galactosemia". Metabolism. 83: 188–196. doi:10.1016/j.metabol.2018.01.025. PMID 29409891.
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  1. [1]
    Beebe, Jane A.; Frey, Perry A. (1998-10-01). "Galactose Mutarotase: Purification, Characterization, and Investigations of Two Important Histidine Residues". Biochemistry. 37 (42): 14989–14997. doi:10.1021/bi9816047. ISSN 0006-2960.
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