Question

Briefly explain how the abundant dietary disaccharides maltose, lactose, and sucrose are metabolized by the glycolytic pathway.

Short answer

Question: Explain how the three abundant dietary disaccharides - maltose, lactose, and sucrose - are metabolized through the glycolytic pathway. Answer: Maltose, lactose, and sucrose are first broken down into their monosaccharide components: maltose into two glucose molecules, lactose into glucose and galactose, and sucrose into glucose and fructose. These monosaccharides then enter the glycolytic pathway by being converted into glycolytic intermediates: glucose directly enters as glucose-6-phosphate, galactose is converted into glucose-6-phosphate through a series of enzymatic reactions, and fructose is metabolized through the fructose-6-phosphate pathway, eventually forming glyceraldehyde-3-phosphate. This allows the three disaccharides to be metabolized in the glycolytic pathway, ultimately producing ATP as an energy source.

Step-by-step solution

Understand the disaccharide composition

Maltose, lactose, and sucrose are disaccharides, which means they are composed of two monosaccharide units. These monosaccharides are glucose, which is the main player in the glycolytic pathway. The three disaccharides are composed as follows: - Maltose: Glucose + Glucose - Lactose: Glucose + Galactose - Sucrose: Glucose + Fructose

Break down the disaccharides into monosaccharides

Each disaccharide is first broken down into its respective monosaccharides before entering the glycolytic pathway: - Maltose is broken down by the enzyme maltase into two glucose molecules. - Lactose is broken down by the enzyme lactase into one glucose and one galactose molecule. - Sucrose is broken down by the enzyme sucrase into one glucose and one fructose molecule.

Enter the glycolytic pathway

Now that the disaccharides have been broken down into their monosaccharide components, they can enter the glycolytic pathway. - Glucose, derived from all three disaccharides, enters the glycolytic pathway directly by being phosphorylated by hexokinase to glucose-6-phosphate. - Galactose (from lactose digestion) is first converted to glucose-6-phosphate by a series of enzymatic reactions involving galactokinase, galactose-1-phosphate uridylyltransferase, and phosphoglucomutase. - Fructose (from sucrose digestion) is metabolized via the fructose-6-phosphate pathway. Fructose is phosphorylated by fructokinase to fructose-1-phosphate, which is then cleaved into glyceraldehyde and dihydroxyacetone phosphate by aldolase B. Finally, glyceraldehyde is further phosphorylated by triose kinase to form glyceraldehyde-3-phosphate, which can enter the glycolytic pathway. In conclusion, maltose, lactose, and sucrose can be metabolized in the glycolytic pathway by breaking them down into their respective monosaccharide units and converting these monosaccharides into intermediates that enter glycolysis, ultimately producing ATP as an energy source.