BY: Ria Fazulbhoy (MSIWM031)
Galactose is a sweet tasting monosaccharide sugar which is a carbohydrate, which is an aldohexose molecule. Galactose is a very important molecule in the body as it is needed for the synthesis of lactose, glycolipids, glycoproteins as well as proteoglycans. It is also known as “brain sugar” as it is useful in formation of the glycolipids which occur in the brain and the myelin sheaths of the nerve cells. Galactose can be hydrolysed from interstitial disaccharide lactose, which is known as the ‘sugar of milk’. (lactose = glucose + galactose). Glucose and galactose are stereoisomers of each other, differing in stereochemistry at carbon 4.
Conversion of galactose to glucose
Galactose can be readily converted to glucose in the liver. This is done in the following steps:
- The first step is the conversion of galactose to galactose-1-phosphate on the action of galactokinase. It is a phosphorylation reaction, in which ATP acts as the donor of the phosphate group.
- Galactose-1-phosphate reacts with UDP glucose i.e. uridine diphosphate glucose (UDPGlc) to form uridine diphosphate galactose (UDPGla). Glucose-1-phosphate is also formed and given out in this step.
This reaction is catalysed by an enzyme called galactose-1-phosphate uridyl transfer
- After this, UDP galactose which is formed is converted to UDP glucose by an epimerization reaction.
This is catalysed by UDP-4 glucose epimerase.
- The epimerization reaction will involve the oxidation and reduction of carbon 4, using NAD+ as coenzyme.
- Finally, from the so formed UDP glucose, glucose is released in the form of glucose 1 phosphate
NOTE: The epimerization reaction is freely reversible, thus glucose can convert back to galactose as and when required. This is the reason that galactose is not a dietary essential.
To yield lactose in the mammary gland, UDPGal condenses with glucose. This reaction is catalysed by lactose synthase
Galactosemia is a disease related to this pathway:
- Galactosemia is the disruption of galactose metabolism.
- Classic galactosemia is the most common type, in which a deficiency of galactose 1 phospahte uridyl transferase activity is inherited.
- Due to this, galactose 1 phosphate is accumalated and as a consequence, the inorganic phosphate of the liver is depleted.
- This results in the ulitimate liver failure and mental detoriation.
- Infants who are afflicted seriously fail to thrive. They go through symptoms of vomitting and diarhhea after consuming dairy products, enlargement of the liver, and jaundice (which can progress to cirrhosis)
- Cataract, lethargic developement and retarded mental developement is also very common amongst children. Cataract is observed due to deposition of a galactose metabolite, galactitol, in the lenses of the eyes. Galactitol is osmotically active, due to which water diffuses in the eyes and instigates cataract formation.
- Other side effects include increase in level of blood galactose level, which is then found in urine.
- Females can also display ovarian failure.
- A stirct limitation of galactose consumption in diet can greatly reduce these symptoms.
- An important diagnostic criterion for this the absence of transferae in the red blood cells.
Thus, galactose is a very important carbohydrate present in the body which can be easily converted to glucose ( in the form of glucose-1-phosphate) for energy. This is a reversible reaction, as glucose and galactose are stereoisomers. This is why galactose is not an essential need for the body.
Deficiency in the enzymes that carry out this conversion may result in a vast range of symptoms under galactosemia.