What is the Difference Between Lactose and Allolactose

The main difference between lactose and allolactose is that lactose is a disaccharide formed by the condensation of glucose and galactose, whereas allolactose is a product of transglycosylation of lactose by β-galactosidase that serves as the inducer for the Lac operon. 

Lactose and allolactose are two disaccharides. Also, both are sugars found in milk.  

Key Areas Covered

1. What is Lactose
– Definition, Features, Importance
2. What is Allolactose
– Definition, Features, Importance
3. Similarities Between Lactose and Allolactose
– Outline of Common Features
4. Difference Between Lactose and Allolactose
– Comparison of Key Differences

Key Terms 

Allolactose, Lactose

What is Lactose

Lactose is the disaccharide sugar found in milk and milk products. The condensation of galactose and glucose makes up lactose. It forms a β-1→4 glycosidic linkage. Hence, the systematic name of lactose is β-D-galactopyranosyl-(1→4)-D-glucose. Glucose and galactose are major hexose sugars that are important in many metabolic reactions, including energy production. The dairy industry produces tons of lactose. Moreover, whey consists of 4/8% of lactose. When you consider the uses, lactase or β-D-galactopyranosyl-(1→4)-D-glucose is the enzyme responsible for the digestion of lactose inside the small intestine. In addition, digestion turns lactose into two simple sugars: glucose and galactose.

Figure 1: Lactose

Furthermore, lactose is important in absorbing calcium, copper, zinc, and phosphorus. Further to these, undigested lactose is used by intestinal microbiota. Lactose also promotes the growth of bifidobacteria in the gastrointestinal tract. Besides, lactose is one of the main sources of galactose, which plays a role in developing the brain and nervous system. However, it’s important to note that 65% of the human population cannot digest lactose without the enzyme lactase. This condition is known as lactose intolerance. 

What is Allolactose

Allolactose is another disaccharide sugar found in milk. It is composed of glucose and galactose. However, it forms β1-6 glycosidic linkage between glucose and galactose. Instead, a β1-4 glycosidic linkage occurs between glucose and galactose in lactose. Moreover, the formation of allolactose occurs by the occasional transglycosylation of lactose by β-galactosidase. Function-wise, allolactose is a repressor of the Lac operon present in E. coli and other enteric bacteria. The tetrameric Lac repressor binds to the allolactose, inducing a conformational change that prevents the binding of Lac repressor to the Lac operator. It induces the transcription of the Lac operon.

Figure 2: Allolactose

Moreover, a non-hydrolyzable analog of allolactose, IPTG (isopropyl β-D-1-thiogalactopyranoside), is used for the induction of the Lac operon in molecular biology. An inducer can regulate gene expression in two ways: removing the repressor or binding the activators. Notably, the inducer binds to the repressor, removing it from the operator site. It allows the binding of RNA polymerase with the promoter. The inducer of the Lac operon is allolactose. Or else, activators can bind into the activator DNA sequences when the inducer is not present. 

Similarities Between Lactose and Allolactose

• Lactose and allolactose are two disaccharides found in milk.
• They provide galactose for the synthesis of glycogen in infants. 

Difference Between Lactose and Allolactose

Definition

Lactose refers to a disaccharide, sugar C₁₂H₂₂O₁₁, which is present in milk and yields glucose and galactose upon hydrolysis, especially lactic acid upon fermentation. In contrast, allolactose refers to a disaccharide sugar found in milk similar to lactose.

Formation

Glucose and galactose form lactose, while transglycosylation of lactose by β-galactosidase forms allolactose.

Linkage

Glucose and galactose form β1-4 glycosidic linkage in lactose, while glucose and galactose form β1-6 glycosidic linkage in allolactose. 

Importance

Lactose provides galactose for synthesizing liver glycogen, while allolactose induces Lac operon.

Conclusion

In brief, lactose and allolactose are two disaccharide sugars found in milk. They provide galactose for the synthesis of glycogen in infants. Lactose is composed of glucose and galactose. In comparison, allolactose forms by the transglycosylation of lactose by β-galactosidase. Also, it can induce Lac operon. Therefore, the main difference between lactose and allolactose is their formation. 

References:

1. Lactose. Lactose – an overview | ScienceDirect Topics. (n.d.).
2. Allolactose. Allolactose – an overview | ScienceDirect Topics. (n.d.).

Image Courtesy:

1. “Lactose” By Laboratório ICB UFMG – Own work (CC-BY SA 4.0) via Commons Wikimedia
2. “Allolactose” By Edgar181 – Own Work (Public Domain) via Commons Wikimedia