chapter 17 biochem – Flashcards
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Glycolysis is the name given to a metabolic pathway occurring in many different cell types. It consists of 11 enzymatic steps that convert glucose to lactic acid. Glycolysis is an example of
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D) fermentation.
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The conversion of 1 mol of fructose 1,6-bisphosphate to 2 mol of pyruvate by the glycolytic pathway results in a net formation of:
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E) 2 mol of NADH and 4 mol of ATP.
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In glycolysis, fructose 1,6-bisphosphate is converted to two products with a standard free-energy change (G'°) of 23.8 kJ/mol. Under what conditions (encountered in a normal cell) will the free-energy change (G) be negative, enabling the reaction to proceed to the right?
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D) When there is a high concentration of fructose 1,6-bisphosphate relative to the concentration of products.
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Which of the following reactions in glycolysis requires ATP as a substrate?
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A) Hexokinase
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Which of the following reactions in glycolysis produces ATP as a product?
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C) Pyruvate kinase
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Which of the following reactions in glycolysis is an aldose to ketose isomerization?
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C) Phosphohexose isomerase
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Which of the following reactions in glycolysis is a ketose to aldose isomerization?
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E) Triose phosphate isomerase
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The steps of glycolysis between glyceraldehyde 3-phosphate and 3-phosphoglycerate involve all of the following except:
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C) oxidation of NADH to NAD+.
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The first reaction in glycolysis that results in the formation of an energy-rich compound is catalyzed by:
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A) glyceraldehyde 3-phosphate dehydrogenase.
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Which of the following is a cofactor in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase?
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D) NAD+
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Glycogen is converted to monosaccharide units by:
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C) glycogen phosphorylase.
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During strenuous exercise, the NADH formed in the glyceraldehyde 3-phosphate dehydrogenase reaction in skeletal muscle must be reoxidized to NAD+ if glycolysis is to continue. The most important reaction involved in the reoxidation of NADH is:
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E) pyruvate lactate.
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The anaerobic conversion of 1 mol of glucose to 2 mol of lactate by fermentation is accompanied by a net gain of:
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C) 2 mol of ATP.
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Which of these cofactors participates directly in most of the oxidation-reduction reactions in the fermentation of glucose to lactate?
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E) NAD+/NADH
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In comparison with the resting state, actively contracting human muscle tissue has a:
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B) higher rate of lactate formation.
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When a muscle is stimulated to contract aerobically, less lactic acid is formed than when it contracts anaerobically because:
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E) under aerobic conditions most of the pyruvate generated as a result of glycolysis is oxidized by the citric acid cycle rather than reduced to lactate.
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Which of the following statements is incorrect?
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E) Under anaerobic conditions pyruvate does not form because glycolysis does not occur.
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Define "fermentation" and explain, by describing relevant reactions, how it differs from glycolysis. Your explanation should include a discussion of the role of NADH in the reaction(s).
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Ans: Fermentation is the operation of the glycolytic pathway under anaerobic conditions. Under aerobic conditions, the pyruvate produced by glycolysis is oxidized to acetyl-CoA, which passes through the citric acid cycle. NADH produced in the oxidations passes electrons to O2, and is thus recycled to NAD+ allowing the continuation of the glycolytic reactions. When no O2 is available to reoxidize the NADH produced by the glyceraldehyde 3-phosphate dehydrogenase reaction, electrons from NADH must be passed to one of the products of glycolysis, such as pyruvate or acetaldehyde, forming lactate or ethanol.
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In glycolysis there are two reactions that require one ATP each and two reactions that produce one ATP each. This being the case, how can fermentation of glucose to lactate lead to the net production of two ATP molecules per glucose?
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Ans: The two reactions that produce ATP in glycolysis (those catalyzed by phosphoglycerate kinase and pyruvate kinase) involve three-carbon compounds, whereas the two reactions that consume ATP occur at the level of hexoses. In glycolysis, each hexose yields two trioses, each of which undergoes the reactions that yield ATP. The ATP yield for triose reactions therefore must be doubled for stoichiometric comparison with the ATPs consumed in hexose phosphorylation. Two ATP molecules are consumed and four are produced for each glucose that passes through the pathway, resulting in a net yield of two ATP per glucose.
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Briefly describe the possible metabolic fates of pyruvate produced by glycolysis in humans, and explain the circumstances that favor each.
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Ans: Under aerobic conditions, pyruvate is oxidized to acetyl-CoA and passes through the citric acid cycle. Under anaerobic conditions, pyruvate is reduced to lactate to recycle NADH to NAD+, allowing the continuation of glycolysis.
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Show how NADH is recycled to NAD+ under aerobic conditions and under anaerobic conditions. Why is it important to recycle NADH produced during glycolysis to NAD+?
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Ans: Cells contain a limited supply of NAD+ and NADH. The oxidation of glyceraldehyde 3-phosphate requires NAD+ as an electron acceptor—it converts NAD+ to NADH. Unless this NADH is recycled to NAD+, oxidative metabolism in this cell will cease for lack of an electron acceptor. Under aerobic conditions, NADH passes electrons to O2; under anaerobic conditions, NADH reduces pyruvate to lactate, and is thereby recycled to NAD+.
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What are the two reactions in glycolysis in which aldose to ketose isomerization is catalyzed by an enzyme? For both reactions the G'° is positive. Briefly explain how the reactions are able to proceed without the input of additional energy.
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Ans: The two reactions are those catalyzed by phosphohexose isomerase and triose phosphate isomerase:
Glucose 6-phosphate fructose 6-phosphate
(aldose) (ketose)
Dihydroxyacetone phosphate glyceraldehyde 3-phosphate
(ketose) (aldose)
Although both of these reactions have standard free-energy changes (G'°) that are positive, they can occur within cells because the products are immediately removed by the next step in the pathway. The result is a very low steady-state concentration of the products, making the actual free-energy changes (G) negative:
G = G'° + RT ln ([products]/[substrates])
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In the conversion of glucose to pyruvate via glycolysis, all of the following enzymes participate. Indicate the order in which they function by numbering them.
___ hexokinase
___ triose phosphate isomerase
___ phosphohexose isomerase
___ enolase
___ glyceraldehyde 3-phosphate dehydrogenase
___ pyruvate kinase
___ phosphofructokinase-1
Which of the enzymes represents a major regulation point in glycolysis?
Which catalyzes a reaction in which ATP is produced?
Which catalyzes a reaction in which NADH is produced?
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Ans: 1; 4; 2; 6; 5; 7; 3; phosphofructokinase-1; pyruvate kinase; glyceraldehyde 3-phosphate dehydrogenase
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At which point in glycolysis do C-3 and C-4 of glucose become chemically equivalent?
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Ans: When dihydroxyacetone phosphate is converted into glyceraldehyde 3-phosphate by triose phosphate isomerase, C-3 and C-4 of glucose become equivalent; they are both C-1 of glyceraldehyde 3-phosphate. (See Fig. 14-7.)
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Explain why Pi (inorganic phosphate) is absolutely required for glycolysis to proceed.
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Ans: Inorganic phosphate (Pi) is an essential substrate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
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When glucose labeled with a 14C at C-1 (the aldehyde carbon) passes through glycolysis, the glyceraldehyde 3-phosphate that is produced from it still contains the radioactive carbon atom. Draw the structure of glyceraldehyde 3-phosphate, and circle the atom(s) that would be radioactive.
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Ans: The labeled carbon is C-3.
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Yeast can metabolize D-mannose to ethanol and CO2. In addition to the glycolytic enzymes, the only other enzyme needed is phosphomannose isomerase, which converts mannose 6-phosphate to fructose 6-phosphate. If mannose is converted to ethanol and CO2 by the most direct pathway, which of the compounds and cofactors in this list are involved?
A. Lactate
B. Acetaldehyde
C. Acetyl-CoA
D. FAD
E. Glucose 6-phosphate
F. Fructose 1-phosphate
G. Pyruvate
H. Lipoic acid
I. Thiamine pyrophosphate
J. Dihydroxyacetone phosphate
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Ans: B, G, I, J
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Explain why the phosphorolysis of glycogen is more efficient than the hydrolysis of glycogen in mobilizing glucose for the glycolytic pathway.
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Ans: Phosphorolysis yields glucose 1-phosphate, which can be converted into glucose 6-phosphate without the investment of energy from ATP. Hydrolysis of glycogen yields free glucose, which must be converted into glucose 6-phosphate (at the expense of ATP) before it can enter glycolysis.
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Explain the biochemical basis of the human metabolic disorder called lactose intolerance.
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Ans: In lactose intolerance, the enzyme lactase, found in the surface of intestinal epithelial cells in children, has been lost in adulthood. Without this enzyme, the individual cannot hydrolyze lactose in the small intestine and take up the resulting monosaccharides; instead, lactose passes into the large intestine, where it is metabolized by bacteria, producing gastric distress.
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The yeast used in brewing the alcoholic beverage beer can break down glucose either aerobically or anaerobically using alcoholic fermentation. Explain why beer is brewed under anaerobic conditions.
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Ans: Since aerobic breakdown of glucose yields more energy than fermentation, in the presence of oxygen, yeast cells would not carry out fermentation and no alcohol would be produced. Under anaerobic conditions, the yeast ferment the glucose to carbon dioxide and ethanol, both of which are key ingredients in beer.
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Explain with words, diagrams, or structures why lactate accumulates in the blood during bursts of very vigorous exercise (such as a 100-meter dash).
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Ans: During vigorous exercise, the cardiovascular system cannot deliver O2 to the muscle tissue fast enough to maintain aerobic conditions. As glycolysis proceeds under anaerobic conditions, NAD+ is converted to NADH (during the glyceraldehyde 3-phosphate dehydrogenase reaction), but the muscle tissue has no O2 to which NADH can pass electrons. To recycle NADH to NAD+, which is essential for continuing glycolysis, electrons from NADH are used to reduce pyruvate to lactate.
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Describe the fate of pyruvate, formed by glycolysis in animal skeletal muscle, under two conditions: (a) at rest, and (b) during an all-out sprint. Show enough detail in your answer to explain why pyruvate metabolism is different in these two cases.
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Ans: (a) At rest, plenty of O2 is being delivered to the muscle, and pyruvate formed during glycolysis is oxidized to acetyl-CoA by the pyruvate dehydrogenase complex. Acetyl groups then enter the citric acid cycle and are oxidized to CO2. (b) Under the conditions of all-out exertion, skeletal muscle cannot be supplied with enough O2 to keep metabolism completely aerobic; under these conditions, muscle tissue must function anaerobically. Pyruvate is reduced to lactate to recycle NADH, formed by glycolysis, to NAD+ so that glycolysis can continue.
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During strenuous activity, muscle tissue demands large quantities of ATP, compared with resting muscle. In white skeletal muscle (in contrast with red muscle), ATP is produced almost exclusively by fermentation of glucose to lactate. If a person had white muscle tissue devoid of the enzyme lactate dehydrogenase, how would this affect his or her metabolism at rest and during strenuous exercise?
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Ans: Lactate dehydrogenase allows cells to pass electrons from NADH to pyruvate, thus regenerating NAD+ for continued glycolysis under anaerobic conditions. The lack of this enzyme would cause no significant problems at rest because aerobic red muscle tissue would function well. During strenuous exercise, however, the absence of lactate dehydrogenase would severely reduce the ability of muscle to perform anaerobically.
