Question

1. A patient presents in your surgery with an inability to maintain blood sugar levels in long periods (20 hours) between meals. Tests show that glucose is converted to lactate normally in liver in the well-fed state, and that pyruvate carboxylase and phosphoenolpyruvate carboxykinase have normal activities in liver in periods between meals. Your colleague reminds you that glucagon stimulates glycogen degradation, so you administer glucagon to the patient (who was well fed prior to the test) and find that blood sugar rises in response as normal. What pathway is defective in this patient, and what enzyme do you suspect has a reduced activity? Explain your reasoning. 2· The conversion of glucose to malate is responsible for the tart flavor of some wines. This transformation occurs in yeast cultures growing anaerobically in the presence of dissolved carbon dioxide. Based on the metabolic pathways you are now familiar with, pose a metabolic route for the fermentative production of L-malate from glucose. Your pathway should be redox balanced (with respect to NAD and NADH), should involve no net depletion of TCA cycle intermediates, but does not necessarily need to yield a net gain of ATP. Fats are usually degraded to acetyl CoA, which enters the TCA cycle. Glucose can be synthesized from oxaloacetate, a TCA cycle intermediate, by gluconeogenesis. After 3. exercise, glycogen stores are depleted and must be restored by eating carbohydrate. Why can we not replace glycogen by converting fat into glucose via oxaloacetate?

Answer 1

1 Answer:

After 20 hours of fasting, supplies of glycogen will be exhausted, and in this situation, gluconeogenesis will play an important role. Since problems appeared only after glycogen was depleted, it must be a problem in gluconeogenesis, and not glycogen utilization. The glucose to lactate conversion was normal; therefore, it must be one of the four reactions peculiar to gluconeogenesis that can be defective. Tests for the PEPCK and pyruvate carboxylase show normal. From the glucogaon test, we come to know that glycogen degradation is normal; therefore, glucose-6-phosphatase must be normal (and this must be normal for glycogen utilization to happen during first 20 hours). From these observations, we can conclude that the Fructose-1,6-bis-phosphatase is the most likely defective enzyme.