Galactolysis

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:

Name	Enzyme	Description
galactokinase deficiency	Galactokinase	Causes 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 deficiency	UDPgalactose-4-epimerase	Is extremely rare (only 2 reported cases). It causes nerve deafness.
Galactose-1-phosphate uridyl transferase deficiency	Galactose-1-phosphate uridyl transferase	Is 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

^ 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.
^ ^a ^b ^c 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.

Metabolism, catabolism, anabolism

General

Metabolic pathway
Metabolic network
Primary nutritional groups

Energy
metabolism

Aerobic respiration	Glycolysis → Pyruvate decarboxylation → Citric acid cycle → Oxidative phosphorylation (electron transport chain + ATP synthase)
Anaerobic respiration	Electron acceptors other than oxygen
Fermentation	Glycolysis → Substrate-level phosphorylation ABE Ethanol Lactic acid

Specific
paths

Protein metabolism

Protein synthesis
Catabolism (protein→peptide→amino acid)

Amino acid	Amino acid synthesis Amino acid degradation (amino acid→pyruvate, acetyl CoA, or TCA intermediate) Urea cycle
Nucleotide metabolism	Purine metabolism Nucleotide salvage Pyrimidine metabolism Purine nucleotide cycle

Carbohydrate metabolism
(carbohydrate catabolism
and anabolism)

Human

Glycolysis ⇄ Gluconeogenesis
Glycogenolysis ⇄ Glycogenesis
Pentose phosphate pathway Fructolysis Polyol pathway Galactolysis Leloir pathway
Glycosylation N-linked O-linked

Nonhuman

Photosynthesis Anoxygenic photosynthesis Chemosynthesis Carbon fixation DeLey-Doudoroff pathway Entner-Doudoroff pathway
Xylose metabolism Radiotrophism

Lipid metabolism
(lipolysis, lipogenesis)

Fatty acid metabolism	Fatty acid degradation (Beta oxidation) Fatty acid synthesis
Other	Steroid metabolism Sphingolipid metabolism Eicosanoid metabolism Ketosis Reverse cholesterol transport

Other

v t e Metabolism: carbohydrate metabolism fructose and galactose enzymes
Fructose / Fructolysis	Hepatic fructokinase Aldolase B Triokinase
Sorbitol	Sorbitol dehydrogenase Aldose reductase
Galactose / Galactolysis	Galactokinase Galactose-1-phosphate uridylyltransferase/UDP-glucose 4-epimerase Aldose reductase Galactose mutarotase^[1]
Lactose	Lactose synthase Lactase
Mannose	Mannose phosphate isomerase

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^ 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.