chapter 14 Glycolysis, Gluconeogenesis, and the .Chapter 14 Glycolysis, Gluconeogenesis, and the

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Text of chapter 14 Glycolysis, Gluconeogenesis, and the .Chapter 14 Glycolysis, Gluconeogenesis, and the

  • Glycolysis,Gluconeogenesis, andthe Pentose PhosphatePathway

    S-160

    1. Equation for the Preparatory Phase of Glycolysis Write balanced biochemical equations for allthe reactions in the catabolism of glucose to two molecules of glyceraldehyde 3-phosphate (the prepara-tory phase of glycolysis), including the standard free-energy change for each reaction. Then write theoverall or net equation for the preparatory phase of glycolysis, with the net standard free-energy change.

    Answer The initial phase of glycolysis requires ATP; it is endergonic. There are five reactionsin this phase:1. Glucose ATP 88n glucose 6-phosphate ADP G 16.7 kJ/mol2. Glucose 6-phosphate 88n fructose 6-phosphate G 1.7 kJ/mol3. Fructose 6-phosphate ATP 88n fructose 1,6-bisphosphate G 14.2 kJ/mol4. Fructose 1,6-bisphosphate 88n

    dihydroxyacetone phosphate glyceraldehyde 3-phosphate G 23.8 kJ/mol5. Dihydroxyacetone phosphate 88n glyceraldehyde 3-phosphate G 7.5 kJ/mol

    The net equation for this phase is

    Glucose 2ATP 88n 2 glyceraldehyde 3-phosphate 2ADP 2H

    The overall standard free-energy change can be calculated by summing the individual reac-tions: G 2.1 kJ/mol (endergonic).

    2. The Payoff Phase of Glycolysis in Skeletal Muscle In working skeletal muscle under anaerobicconditions, glyceraldehyde 3-phosphate is converted to pyruvate (the payoff phase of glycolysis), andthe pyruvate is reduced to lactate. Write balanced biochemical equations for all the reactions in thisprocess, with the standard free-energy change for each reaction. Then write the overall or net equa-tion for the payoff phase of glycolysis (with lactate as the end product), including the net standardfree-energy change.

    Answer The payoff phase of glycolysis produces ATP, and thus is exergonic. This phase con-sists of five reactions, designated 6 to 10 in the text:6. Glyceraldehyde 3-phosphate Pi NAD

    88n 1,3-bisphosphoglycerate NADH H

    G 6.3 kJ/mol7. 1,3-Bisphosphoglycerate ADP 88n 3-phosphoglycerate ATP G 185 kJ/mol8. 3-Phosphoglycerate 88n 2-phosphoglycerate G 4.4 kJ/mol9. 2-Phosphoglycerate 88n phosphoenolpyruvate G 7.5 kJ/mol

    10. Phosphoenolpyruvate ADP 88n pyruvate ATP G 31.4 kJ/mol

    The pyruvate is then converted to lactate:

    Pyruvate NADH H 88n lactate NAD G 25.1 kJ/mol

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  • Chapter 14 Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway S-161

    The net equation is

    Glyceraldehyde 3-phosphate 2ADP Pi 88n lactate NAD G 57 kJ/mol

    Because the payoff phase uses two glyceraldehyde 3-phosphate molecules from each glucoseentering glycolysis, the net equation is

    2 Glyceraldehyde 3-phosphate 4ADP 2Pi 88n 2 lactate 2NAD

    and the energetic payoff for the net reaction is G 114 kJ/mol.

    3. GLUT Transporters Compare the localization of GLUT4 with that of GLUT2 and GLUT3, andexplain why these localizations are important in the response of muscle, adipose tissue, brain, andliver to insulin.

    Answer GLUT2 (and GLUT1) is found in liver and is always present in the plasma membraneof hepatocytes. GLUT3 is always present in the plasma membrane of certain brain cells.GLUT4 is normally sequestered in vesicles in cells of muscle and adipose tissue and enters theplasma membrane only in response to insulin. Thus, liver and brain can take up glucose fromblood regardless of insulin level, but muscle and adipose tissue take up glucose only when in-sulin levels are elevated in response to high blood glucose.

    4. Ethanol Production in Yeast When grown anaerobically on glucose, yeast (S. cerevisiae) convertspyruvate to acetaldehyde, then reduces acetaldehyde to ethanol using electrons from NADH. Write theequation for the second reaction, and calculate its equilibrium constant at 25 C, given the standardreduction potentials in Table 137.

    Answer CH3CHO NADH H CH3CH2OH NAD

    Acetaldehyde Ethanol

    Solve for Keq using the E values in Table 137 and Equations 133 and 137.

    G RT lnKeq

    G n E

    RT lnKeq n E

    lnKeq

    In this reaction, n 2, and E 0.123 V (calculated from values in Table 137 as shown inWorked Example 133). Substitute the standard values for the faraday and R, and 298 K forthe temperature:

    lnKeq 9.58

    Keq e9.58 = 1.45 104

    5. Energetics of the Aldolase Reaction Aldolase catalyzes the glycolytic reaction

    Fructose 1,6-bisphosphate 88n glyceraldehyde 3-phosphate dihydroxyacetone phosphate

    The standard free-energy change for this reaction in the direction written is 23.8 kJ/mol. The con-centrations of the three intermediates in the hepatocyte of a mammal are: fructose 1,6-bisphosphate,1.4 105 M; glyceraldehyde 3-phosphate, 3 106 M; and dihydroxyacetone phosphate, 1.6 105 M.At body temperature (37 C), what is the actual free-energy change for the reaction?

    2(96,480 J/V mol)(0.123 V)

    (8.315 J/mol K)(298 K)

    n E

    RT

    zy

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  • S-162 Chapter 14 Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway

    Answer For this reaction, G G RT ln [glyceraldehyde 3-phosphate][dihydroxyace-tone phosphate]/[fructose 1,6-bisphosphate]:

    G 23.8 kJ/mol (8.315 103 kJ/mol K) (310 K) ln [(3 106)(1.6 105)/(1.4 105)]

    23.8 kJ/mol (2.578 kJ/mol) ln (3.43 106)

    23.8 kJ/mol (2.578 kJ/mol) (12.58)

    23.8 kJ/mol (32.4 kJ/mol) 8.6 kJ/mol

    6. Pathway of Atoms in Fermentation A pulse-chase experiment using 14C-labeled carbon sourcesis carried out on a yeast extract maintained under strictly anaerobic conditions to produce ethanol.The experiment consists of incubating a small amount of 14C-labeled substrate (the pulse) with theyeast extract just long enough for each intermediate in the fermentation pathway to become labeled.The label is then chased through the pathway by the addition of excess unlabeled glucose. The chaseeffectively prevents any further entry of labeled glucose into the pathway.(a) If [1-14C]glucose (glucose labeled at C-1 with 14C) is used as a substrate, what is the location of

    14C in the product ethanol? Explain.(b) Where would 14C have to be located in the starting glucose to ensure that all the 14C activity is

    liberated as 14CO2 during fermentation to ethanol? Explain.

    Answer Anaerobiosis requires the regeneration of NAD from NADH in order to allow glycol-ysis to continue.(a) Figure 146 illustrates the fate of the carbon atoms of glucose. C-1 (or C-6) becomes C-3 of

    glyceraldehyde 3-phosphate and subsequently pyruvate. When pyruvate is decarboxylatedand reduced to ethanol, C-3 of pyruvate becomes the C-2 of ethanol (14CH3CH2OH).

    (b) If all the labeled carbon from glucose is converted to 14CO2 during ethanol fermentation,the original label must have been on C-3 and/or C-4 of glucose, because these are con-verted to the carboxyl group of pyruvate.

    7. Heat from Fermentations Large-scale industrial fermenters generally require constant, vigorous cool-ing. Why?

    Answer Fermentation releases energy, some conserved in the form of ATP but much of it dis-sipated as heat. Unless the fermenter contents are cooled to counterbalance this heat produc-tion, the temperature would become high enough to kill the microorganisms.

    8. Fermentation to Produce Soy Sauce Soy sauce is prepared by fermenting a salted mixture of soy-beans and wheat with several microorganisms, including yeast, over a period of 8 to 12 months. Theresulting sauce (after solids are removed) is rich in lactate and ethanol. How are these two compoundsproduced? To prevent the soy sauce from having a strong vinegar taste (vinegar is dilute acetic acid),oxygen must be kept out of the fermentation tank. Why?

    Answer Soybeans and wheat contain starch, a polymer of glucose, which is broken down toglucose by the microorganisms. The glucose is then degraded to pyruvate via glycolysis. Be-cause the process is carried out in the absence of oxygen (i.e., it is a fermentation), pyruvateis reduced to lactic acid and ethanol. If oxygen were present, pyruvate would be oxidized toacetyl-CoA and then to CO2 and H2O. Some of the acetyl-CoA, however, would also be hy-drolyzed to acetic acid (vinegar) in the presence of oxygen.

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  • 9. Equivalence of Triose Phosphates 14C-Labeled glyceraldehyde 3-phosphate was added to a yeastextract. After a short time, fructose 1,6-bisphosphate labeled with 14C at C-3 and C-4 was isolated.What was the location of the 14C label in the starting glyceraldehyde 3-phosphate? Where did the sec-ond 14C label in fructose 1,6-bisphosphate come from? Explain.

    Answer Problem 1 outlines the steps in glycolysis involving fructose 1,6-bisphosphate, glyc-eraldehyde 3-phosphate, and dihydroxyacetone phosphate. Keep in mind that the aldolase re-action is readily reversible and the triose phosphate isomerase reaction catalyzes extremelyrapid interconversion of its substrates. Thus, the label at C-1 of glyceraldehyde 3-phosphatewould equilibrate with C-1 of dihydroxyacetone phosphate (G 7.5 kJ/mol). Because thealdolase reaction has G 23.8 kJ/mol in the direction of hexose formation, fructose 1,6-bisphosphate would be readily formed, and labeled in C-3 and C-4 (see Fig. 146).

    10. Glycolysis Shortcut Suppose you discovered a mutant yeast whose glycolytic pathway was shorterbecause of the presence of a new enzyme catalyzing the reaction

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