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FINALANSW%20FA07 - METABOLIC BIOCHEMISTRY Immo E Scheffler...

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METABOLIC BIOCHEMISTRY Fall 2007 Immo E. Scheffler FINAL EXAM ANSWERS All answers are to be written into the Blue Book. Leave the first inside page blank for scoring. There are 10 questions. Make sure that each answer is clearly identified with the question number at the top or left side of the page. Consider the statement on the back of the Blue Book; fill it out and sign it if you want to have your exam returned in the hallway outside of 3234 Bonner Hall. QUESTION 1 a) [3 min] Gluconeogenesis must use “bypass reactions” to circumvent three reactions in the glycolytic pathway that are highly exergonic and essentially irreversible. Reactions carried out by which three of the enzymes listed must be bypassed in the gluconeogenic pathway? 1) Hexokinase 2) Phosphoglycerate kinase 3) Phosphofructokinase-1 4) Pyruvate kinase 5) Triosephosphate isomerase A) 1, 2, 3 B) 1, 2, 4 C) 1, 4, 5 D) 1, 3, 4 E) 2, 3, 4 b) [2 min] Which of the following statements about gluconeogenesis in animal cells is true? A) A rise in the cellular level of fructose-2,6-bisphosphate stimulates the rate of gluconeogenesis. B) An animal fed a large excess of fat in the diet will convert any fat not needed for energy production into glycogen to be stored for later use. C) The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate is not catalyzed by phosphofructokinase-1, the enzyme involved in glycolysis. D) The conversion of glucose 6-phosphate to glucose is catalyzed by hexokinase, the same enzyme involved in glycolysis. E) The conversion of phosphoenol pyruvate to 2-phosphoglycerate occurs in two steps, including a carboxylation. c) [6 min] Under what circumstances does the bifunctional protein phosphofructokinase-2/fructose 2,6- bisphosphatase (PFK-2/FBPase-2) become phosphorylated, and what are the consequences of its phosphorylation to the glycolytic and gluconeogenic pathways? Ans: Glucagon, signaling low blood sugar, stimulates cAMP synthesis, which activates protein kinase A (PKA) to phosphorylate PFK-2/FBPase-2 (among other proteins). This phosphorylation enhances FBPase-2 activity and inhibits PFK-2 activity of the enzyme, resulting in lower levels of fructose 2,6-bisphosphate
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(F26BP). In the absence of F26BP as an allosteric effector, the activity of PFK-1 is reduced (inhibiting glycolysis) and the activity of FBPase-1 is enhanced (stimulating gluconeogenesis), thus enabling the liver to replenish blood glucose. See Figs. 15-22 and 15-23. QUESTION 2 a) [4 min] Acetyl-CoA labeled with 14 C in both of its acetate carbon atoms is incubated with unlabeled oxaloacetate and a crude tissue preparation capable of carrying out the reactions of the citric acid cycle.
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