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Transcript of Galactose metabolism
The main source of galactose in the human diet is lactose (which is obtained from milk and milk products).
The milk sugar, lactose, is hydrolyzed in the intestine to its constituent monosaccharides, glucose and galactose.
Galactose metabolism Galactose is metabolized primarily in the liver, in a sequence of three reactions:
First reaction: galactose is phosphorylated to yield galactose 1-phosphate.
Second reaction: galactose 1-phosphate and UDP-glucose react to form UDP-galactose and glucose 1-phosphate.
Third reaction: UDP-galactose undergoes epimerization to form UDP-glucose.
Galactose in the liver First reaction:
Phosphorylation of galactose. Before galactose can be metabolized, it must be phosphorylated.
To phosphorylate galactose, a specific enzyme is used. This specific enzyme is called galactokinase, and is present in most tissues.
Galactokinase produces galactose 1-phosphate.
During this reaction, ATP serves as a phosphate donor.
Galactose is a monosaccharide sugar that is less sweet than glucose. It is a C-4 epimer of glucose. Phosporylation of Galactose Second reaction:
Formation of UDP-galactose. In order for galactose 1-phosphate to enter the glycolytic pathway, it has to convert to a UDP-galactose. Second reaction:
Formation of UDP-galactose.
This conversion occurs in an exchange reaction. In the exchange reaction, UDP-glucose reacts with galactose 1-phosphate and produces UDP-galactose and glucose 1-phosphate.
The enzyme that catalyzes this conversion reaction is called galactose 1-phoshpate uridyltransferase (GALT).
UDP-galactose usage as a carbon source for glycolysis or gluconeogenesis. In order for a UDP-galactose to enter the mainstream of glucose metabolism, it needs to convert into a UDP-glucose.
The conversion of UDP-galactose into UDP-glucose is catalyzed by UDP-hexose 4-epimerase.
This produced UDP-glucose has the ability to take part in many biosynthetic reactions. It can also participate in the GALT reaction.
Third reaction Role of UDP-galactose in biosynthetic reactions. UDP-galactose works as a donor of galactose units in many synthetic pathways. It’s a galactose unit donor in the synthesis of lactose, glycoproteins, glycolipids and glycosaminoglycans.
In people who are lactose-intolerant, galactose is not provided by the diet. Because of this, galactose cannot be released from lactose. This is a result of the lack of -galactosidase.
In order to produce UDP-galactose in such case, all the tissue requirements for UDP-galactose can be met by the action of an enzyme on a UDP-glucose.
This enzyme is the UDP-hexose 4-epimerase, which is produced from glucose 1-phosphate. Disorders of galactose metabolism. In individuals with classic galactosemia, the amount of GALT is less than required. In this disorder, galactose 1-phosphate and galactose accumulate in the cells.
The accumulated galactose is expelled into side pathways such as galactitol production pathway.
This reaction is catalyzed by aldose reductase, which is the same enzyme that converts glucose into sorbitol.
Disorders of galactose metabolism. Galactose-1-phosphate uridylyltransferase deficiency, also called classic galactosemia or GALT deficiency, is the most common type of galactosemia.
It's a genetic metabolic disorder that affects an individual's ability to metabolize the galactose properly.
It's caused by a deficiency of the enzyme galactose-1-phosphate uridylyltransferase.
It is an autosomal recessive metabolic disorder that can cause liver disease and death if untreated. Difference between normal and abnormal galactose metabolism Normal metabolic pathway for galactose in humans. If the activity of the enzyme galactose 1-phosphate uridyltransferase is decreased, there is a buildup of the precursors in the pathway, mainly galactose 1-phosphate and galactose. Thank you for your attention! Sources Lippincotts's Illustrated Reviews
Biochemistry; 5th Edition; by
Richard Harvey and Denise Ferrier