Biochemistry researchers are investigating the speed at which various carbohydrates are metabolized within the liver. They hypothesize that different monosaccharides delivered to the liver have different rates of intracellular metabolism. Which of the following substances is most likely to have the fastest rate of metabolism in the glycolytic pathway?

Question

Biochemistry researchers are investigating the speed at which various carbohydrates are metabolized within the liver.  They hypothesize that different monosaccharides delivered to the liver have different rates of intracellular metabolism.  Which of the following substances is most likely to have the fastest rate of metabolism in the glycolytic pathway?

Quizplus · Accepted Answer

11ec160b_36d3_47d6_9249_4384f22df69f_MD0001_00 Non-glucose monosaccharides (eg, galactose, mannose, fructose) enter the glycolytic pathway at different points as intermediates of glycolysis. Of these, fructose is the only one whose metabolites bypass phosphofructokinase, one of the key enzymes involved in regulating the rate of glycolysis. As a result, fructose is metabolized by the liver faster than the other monosaccharides and is rapidly cleared from the bloodstream following dietary absorption. Metabolism of fructose in the liver begins with phosphorylation by fructokinase to fructose-1-phosphate (F1P). Aldolase B can use both fructose-1,6-bisphosphate and F1P as substrates; it converts F1P into dihydroxy acetone phosphate (DHAP) and glyceraldehyde. Glyceraldehyde can be either phosphorylated to glyceraldehyde-3-phosphate by triokinase or converted to DHAP. DHAP is converted by triose phosphate isomerase to glyceraldehyde-3-phosphate, which continues down the glycolytic pathway. (Choices B, C, D, and E) Galactose-1-phosphate, glucose-1-phosphate, glucose-6-phosphate, and mannose-6-phosphate enter glycolysis upstream of phosphofructokinase, a major rate-limiting enzyme of glycolysis. This slows down the rate of their metabolism relative to fructose and its metabolites (eg, F1P). Educational objective: Dietary fructose is phosphorylated in the liver to F1P and is rapidly metabolized because it bypasses PFK-1, the major rate-limiting enzyme of glycolysis. Other sugars (eg, glucose, galactose, mannose) enter glycolysis prior to PFK-1 and as a result are metabolized more slowly.

Biochemistry Researchers Are Investigating the Speed at Which Various Carbohydrates