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Chemistry LibreTexts


Galactose is more commonly found in the disaccharide, lactose or milk sugar. It is found as the monosaccharide in peas. Galactose is classified as a monosaccharide, an aldose, a hexose, and is a reducing sugar.

Galactosemia - Genetic Enzyme Deficiency

One baby out of every 18,000 is born with a genetic defect of not being able to utilize galactose. Since galactose is in milk as part of lactose, it will build up in the blood and urine. Undiagnosed it may lead to mental retardation, failure to grow, formation of cataracts, and in sever cases death by liver damage. The disorder is caused by a deficiency in one or more enzymes required to metabolize galactose. The treatment for the disorder is to use a formula based upon the sugar sucrose rather than milk with lactose. The galactose free diet is critical only in infancy, since with maturation another enzyme is developed that can metabolize galactose.


Ring Structure for Galactose

The chair form of galactose follows the same pattern as that for glucose. The anomeric carbon is the center of a hemiacetal functional group. A carbon that has both an ether oxygen and an alcohol group is a hemiacetal.

Figure : Compare Alpha and Beta Galactose in the Chair form below.


The Beta position is defined as the -OH being on the same side of the ring as the C # 6. In the chair structure this results in a horizontal projection (Haworth - an upwards projection). The Alpha position is defined as the -OH being on the opposite side of the ring as the C # 6. In the chair and Haworth structure this results in a downward projection.

Compare Glucose and Galactose in the Chair Structures

The position of the -OH group on the carbon (#4) is the only distinction between glucose and galactose. Glucose is defined as the -OH on C # 4 in a horizontal projection in the chair form, (down in the Haworth structure). Galactose is defined as the -OH on C # 4 in a upward projection in the chair form,(also upward in the Haworth structure). Both glucose and galactose may be either alpha or beta on the anomeric carbon, so this is not distinctive between them.




Which carbon in the structure on the left is the anomeric carbon on galactose?