Practice Exam For Carbohydrate Metabolism PDF

Practice Exam for Carbohydrate Metabolism 1. Which of the following reactions is considered the committed step in glycolysis? a. Glucose → Glucose-6-phosphate b. Glucose-6-phosphate → Fructose-6-phosphate c. Fructose-6-phosphate → Fructose-1,6-bisphosphate d. Fructose-1,6-bisphosphate → Dihydroxya...

Practice Exam for Carbohydrate Metabolism 1. Which of the following reactions is considered the committed step in glycolysis? a. Glucose → Glucose-6-phosphate b. Glucose-6-phosphate → Fructose-6-phosphate c. Fructose-6-phosphate → Fructose-1,6-bisphosphate d. Fructose-1,6-bisphosphate → Dihydroxyacetone phosphate + Glyceraldehyde 3-phosphate Answer: c The conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, catalyzed by phosphofructokinase-1 (PFK-1), is the committed step in glycolysis because after this reaction, the molecule is committed to proceeding through the glycolytic pathway. 2. Which enzyme catalyzes the only oxidation-reduction reaction in glycolysis? a. Glyceraldehyde 3-phosphate dehydrogenase b. Phosphoglycerate kinase c. Enolase d. Pyruvate kinase Answer: a Glyceraldehyde 3-phosphate dehydrogenase catalyzes the conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate, which involves the reduction of NAD+ to NADH. 3. Describe the function of a mutase in the context of glycolysis. A mutase catalyzes the intramolecular shift of a functional group, such as a phosphate, within a molecule. In glycolysis, phosphoglycerate mutase catalyzes the conversion of 3-phosphoglycerate to 2-phosphoglycerate by shifting the phosphate group from the 3rd to the 2nd carbon atom of the glycerate molecule. 4. Explain why the aldolase reaction in glycolysis, despite having a large positive standard free energy change (ΔG°'), can proceed in the forward direction under physiological conditions. The aldolase reaction, which cleaves fructose-1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, has a large positive standard free energy change (ΔG°'), suggesting that it is highly unfavorable under standard conditions. However, under physiological conditions, the actual free energy change (ΔG) is small and negative, allowing the reaction to proceed. This discrepancy is due to the concentrations of the reactants and products in the cell. The rapid removal of the products of the aldolase reaction by subsequent steps in glycolysis drives the reaction forward, making it thermodynamically feasible despite its unfavorable standard free energy change. 5. What are the three irreversible reactions of glycolysis that necessitate bypass reactions in gluconeogenesis?  1. Hexokinase (Glucose → Glucose-6-phosphate) 2. Phosphofructokinase-1 (Fructose-6-phosphate → Fructose-1,6-bisphosphate) 3. Pyruvate kinase (Phosphoenolpyruvate → Pyruvate) 6. Name two gluconeogenic precursors. 1. Lactate 2. Amino Acids 7. Why is glucose-6-phosphatase only expressed in the liver and kidney? Glucose-6-phosphatase catalyzes the final step of gluconeogenesis, removing the phosphate group from glucose-6-phosphate to produce free glucose. This free glucose can then be released into the bloodstream to maintain blood glucose levels. The expression of glucose-6-phosphatase is restricted to the liver and kidney because these organs are responsible for maintaining glucose homeostasis. 8. Explain the difference in regulation of hexokinase I-III and glucokinase (hexokinase IV) in response to glucose-6-phosphate. Hexokinase I, II, and III are feedback inhibited by their product, glucose-6-phosphate. This inhibition prevents the accumulation of glucose-6-phosphate when glycolysis is downregulated. Glucokinase (hexokinase IV) is not directly inhibited by glucose-6-phosphate. Instead, its regulation is primarily controlled by fructose-6-phosphate and a nuclear regulatory protein. 9. How does fructose-2,6-bisphosphate (F2,6BP) act as a coordinate regulator of glycolysis and gluconeogenesis? F2,6BP allosterically activates PFK-1, promoting glycolysis. Conversely, F2,6BP allosterically inhibits FBPase-1, suppressing gluconeogenesis. The concentration of F2,6BP is regulated by the bifunctional enzyme PFK-2/FBPase-2, which is itself hormonally regulated by insulin and glucagon. 10. What is the Warburg effect, and how does the high rate of glycolysis benefit rapidly dividing cancer cells? The Warburg effect is the observation that cancer cells tend to rely heavily on glycolysis for ATP production, even in the presence of oxygen. This high rate of glycolysis may benefit rapidly dividing cancer cells by: 1. Producing ATP more rapidly than oxidative phosphorylation. 2. Providing intermediates for biosynthetic pathways.  Please note that the potential benefits of the Warburg effect for cancer cells are not explicitly stated in the sources provided. This information should be independently verified. 11. How does the constitutive activation of the committed step in fatty acid biosynthesis lead to the inhibition of β-oxidation? The committed step in fatty acid biosynthesis is the formation of malonyl-CoA from acetyl-CoA, catalyzed by acetyl-CoA carboxylase. Malonyl-CoA is a potent inhibitor of carnitine acyltransferase I, the enzyme responsible for transporting fatty acids into mitochondria for β-oxidation. Therefore, if the enzyme catalyzing malonyl-CoA synthesis is constantly active, malonyl-CoA levels will remain high, inhibiting β-oxidation. 12. What is the primary difference between dietary polysaccharides and disaccharides? Dietary polysaccharides, like starch, are complex carbohydrates composed of many monosaccharide units linked together. Disaccharides, such as sucrose and lactose, are simpler carbohydrates consisting of only two monosaccharide units linked together. 13. Name two enzymes involved in the digestion of dietary carbohydrates in the small intestine. 1. Maltase 2. Lactase 14. What is the role of the small intestine in fructose metabolism, and what happens when fructose consumption is excessive? The small intestine converts dietary fructose into glucose and organic acids, effectively shielding the liver from fructose exposure. However, when fructose consumption is excessive, the small intestine's capacity to metabolize fructose is overwhelmed. This leads to fructose spilling over to the liver, potentially causing toxicity. The exact mechanisms of fructose toxicity are not fully understood. 15. How does the regulation of pyruvate kinase in the liver differ from that in muscle? Liver pyruvate kinase (L-type) is subject to allosteric regulation by fructose-1,6-bisphosphate (activator) and ATP (inhibitor). It is also hormonally regulated by insulin and glucagon, which modulate its phosphorylation state. Muscle pyruvate kinase (M-type) is not subject to the same allosteric or hormonal regulation as the liver isozyme.