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Galactosemia is a rare genetic metabolic disorder which affects an individual’s ability to properly metabolize the sugar galactose.
Goppert first described the disease in 1917,[1] with its cause as a defect in galactose metabolism being identified by a group led by Herman Kalckar in 1956.[2]
Its incidence is about 1 per 55,000 births (classic type). It is much rarer in Japan and much more common in Italy, specifically the traveler region. Galactosemia is also very common within the Irish Traveller population. This is attributed to consanguinity within a relatively small gene pool.[citation needed]
Lactose in food (such as dairy products) is broken down by the enzyme lactase into glucose and galactose. In individuals with galactosemia, the enzymes needed for further metabolism of galactose are severely diminished or missing entirely, leading to toxic levels of galactose to build up in the blood, resulting in hepatomegaly (an enlarged liver), cirrhosis, renal failure, cataracts, brain damage, and ovarian failure. Without treatment, mortality in infants with galactosemia is about 75%.
The 4th carbon on Galactose has an axial hydroxyl (-OH) group. This causes Galactose to favor the open form as it is more stable than the closed form. This leaves an aldehyde (O=CH-) group available to react with nucleophiles, particularly proteins which contain amino (-NH2) groups, in the body. This uncontrolled reactivity gives way to glycolation. Glycolation causes disease by altering the structure of proteins in ways that were not intended for biochemical processes.
Galactose is converted into glucose by the action of three enzymes, known as the Leloir pathway. Accordingly, there are 3 known types of Galactosemia; type 1, 2 and 3:
The order of these three types is not the same as the order that the enzymes are encountered by galactose on its metabolic path (which is closer to GALK, GALT, and then GALE, though many variations can occur.)
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