B. Insulin stimulates the uptake of glucose.
C. Most enzymes that are regulated by covalent modification are in the phosphorylated state.
D. The oxidation of acetyl coenzyme A is increased.
E. The synthesis of glucokinase is repressed.
Correct answer = A. The increased insulin and decreased glucagon levels characteristic of the absorptive state promote the synthesis of fructose 2,6bisphosphate, which allosterically activates phosphofructokinase-1 of glycolysis. Most covalently modified enzymes are in the dephosphorylated state and are active. Acetyl coenzyme A is not oxidized in the well-fed state because it is being used in fatty acid synthesis. Uptake of glucose (by glucose transporter-2) into the liver is insulin independent. Synthesis of glucokinase is induced by insulin in the well-fed state.
4.3. Which one of the following enzymes is phosphorylated and active in an individual who has been fasting for 12 hours?
A. Arginase
B. Carnitine palmitoyltransferase-I
C. Fatty acid synthase
D. Glycogen synthase
E. Hormone-sensitive lipase F. Phosphofructokinase-1
G. Pyruvate dehydrogenase
Correct answer = E. Hormone-sensitive lipase of adipocytes is phosphorylated and activated by protein kinase A in response to epinephrine. Choices A, B, C, and F are not regulated covalently. Choices D and G are regulated covalently but are inactive if phosphorylated.
4.4. For a 70-kg man, in which one of the periods listed below do ketone bodies supply the major portion of the caloric needs of brain?
A. Absorptive period
B. Overnight fast
C. Three-day fast
D. Four-week fast
E. Five-month fast
Correct answer = D. Ketone bodies, made from the acetyl coenzyme A product of fatty acid oxidation, increase in the blood in fasting but must reach a critical level to cross the blood–brain barrier. Typically, this occurs in the second to third week of a fast. Fat stores in a 70-kg (~154-lb) man would not be able to supply his energy needs for 5 months.
24.5. The diagram below shows inputs to and outputs from pyruvate, a central molecule in energy metabolism.
Which letter on the diagram represents a reaction that requires biotin and is activated by acetyl coenzyme A?
Correct answer = C. Pyruvate carboxylase, a mitochondrial enzyme of gluconeogenesis, requires biotin (and ATP) and is allosterically activated by acetyl coenzyme A from fatty acid oxidation. None of the other choices meets these criteria. A = pyruvate kinase; B = pyruvate dehydrogenase complex; D = aspartate aminotransferase; E = alanine aminotransferase; F = lactate dehydrogenase.
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I. OVERVIEW