How lactose is formed from monosaccahrides?

Lactose, a disaccharide, is not formed from monosaccharides but rather from the two monosaccharides glucose and galactose. The formation of lactose occurs during the process of glycosylation, which is a type of chemical reaction that links two sugars together. In the case of lactose, a glucose molecule and a galactose molecule are linked together by a specific type of glycosidic bond, known as a β-1,4-glycosidic bond. This bond forms between the first carbon atom of the glucose molecule and the fourth carbon atom of the galactose molecule.

Here's a step-by-step explanation of how lactose is formed from its constituent monosaccharides, glucose, and galactose:

Activation of Glucose:

The process begins with the activation of glucose. Glucose is phosphorylated by an enzyme called glucokinase, forming glucose-6-phosphate. This activation step prepares the glucose molecule for further reactions.

Epimerization of Glucose to Galactose:

Glucose-6-phosphate undergoes a series of enzymatic reactions, including isomerization and epimerization, to convert it into galactose-1-phosphate. Epimerization specifically involves the conversion of the hydroxyl group on the second carbon atom of glucose to a ketone group, resulting in the formation of galactose-1-phosphate.

Uridyl Transfer:

Galactose-1-phosphate reacts with uridine diphosphate glucose (UDP-glucose), another activated sugar, in a reaction catalyzed by the enzyme galactose-1-phosphate uridyltransferase. This reaction transfers the uridyl group from UDP-glucose to galactose-1-phosphate, forming UDP-galactose and glucose-1-phosphate as products.

Formation of the Lactose Disaccharide:

UDP-galactose then reacts with glucose in a condensation reaction catalyzed by the enzyme lactose synthase. During this reaction, the galactose molecule from UDP-galactose is transferred to the glucose molecule, forming a glycosidic bond between the first carbon atom of galactose and the fourth carbon atom of glucose. This results in the formation of lactose, a disaccharide composed of glucose and galactose.

Lactose is a significant carbohydrate in milk and dairy products. It contributes to the sweet taste and provides nutritional value. The process of lactose formation is essential for the synthesis and availability of lactose in these products.