Chapter 13: Carbohydrates: Gluconeogenesis, the Synthesis of New Glucose
Acetyl-CoA enhances the rate of gluconeogenesis by acting as an obligate activator of which of the following enzymes?
A. phosphoenolpyruvate carboxykinase
B. phosphoglycerate kinase
C. phosphoglycerate mutase
Answer D: Unlike allosteric enzymes that function at some level in the absence on an allosteric effector, pyruvate carboxylase (PC) has no activity in the absence of acetyl-CoA. This property defines acetyl-CoA as an obligate activator of PC.
The purpose of the Cori cycle is to shift the metabolic burden in which of the following directions?
B. cardiac to skeletal muscle
Answer E: The Cori cycle represents a link between metabolic processes that occur in skeletal muscle and erythrocytes with those occurring in the liver. During anaerobic glycolysis in muscle the pyruvate is reduced to lactate. This pathway functions exclusively within erythrocytes because they lack mitochondria necessary for oxidation of NADH to NAD+. The lactate leaves these tissues, enters the blood, and is transported to the liver. Within the liver the lactate is oxidized to pyruvate and the pyruvate then serves as a substrate for glucose synthesis. The glucose leaves the liver and can be used by muscle and erythrocytes again.
The maximum capacity of the liver to carry out gluconeogenesis during starvation depends upon which of the following?
A. activation of glucose 6-phosphatase by insulin
B. activation of pyruvate kinase by insulin
C. induction of PEP carboxykinase synthesis by glucagon
D. inhibition of fructose 1,6-bisphosphatase by F1,6BP
E. inhibition of glucokinase by AMP
Answer C: When blood glucose levels fall, the pancreas releases glucagon to stimulate the liver to carry out gluconeogenesis. The effects of glucagon exerted on hepatic metabolism are both short term and long term. Prolonged fasting and starvation lead to long-term glucagon effects which are exerted at the level of gene expression. A major hepatic target of these long-term effects is the transcription of the PEPCK gene which is induced by glucagon.