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recap 8 (8.1 recap (1.What is the difference between endergonic and…
recap 8
8.1 recap
1.What is the difference between endergonic and exergonic reactions, and what is the importance of positive and negative ΔG?
Exergonic reactions release free energy because the energy of the reactants is greater than that of the products. The reverse is true for endergonic reactions, which require an input of energy. △G is the free energy change of a reaction—products minus reactants. A positive △G means that the reaction is endergonic and requires energy, while a negative △G indicates that a reaction is exergonic and releases energy.
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8.2 recap
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2.The following is a biochemical reaction that involves ATP: Phosphoenolpyruvate → pyruvate (∆G = +10 kcal/mol)
a.Is the reaction exergonic or endergonic?
b.Is ATP hydrolyzed or formed during the reaction?
a.The reaction is endergonic (positive △G).
b.ATP hydrolysis can drive the reaction (△G = -7.3 kcal/mol).
8.3 recap
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3.When potatoes are peeled, the enzyme polyphenol oxidase causes discoloration by catalyzing the oxidation of certain molecules, using O2 as a substrate. Explain these observations:
a.Browning is reduced in potatoes that are peeled under water and kept there.
b.Potatoes that have been boiled at 100°C and then cooled do not brown when they are sliced.
a.The presence of water may prevent O2 from reaching the enzyme.
b.Boiling denatures proteins, so polyphenol oxidase is irreversibly altered by boiling, and its active site is destroyed.
8.4 recap
1.The chemical composition of an enzyme before and after a catalyzed reaction is the same, but during the reaction an enzyme can change. How and why does this happen?
As part of the catalysis, an enzyme can add or remove H+ in a substrate, which may facilitate reaction with the substrate. The course of these H+ reactions involves acidic or basic R groups on amino acids at the active site. Metal ions of enzymes may gain or lose electrons during the reaction, donating them temporarily to the substrate.
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3.Plot reaction rate versus substrate concentration, and explain the shape of the curve.
At the beginning, the reaction rate will increase linearly with added enzyme since the enzyme will be saturated with substrate even after many additions. Thus the reaction rate will increase proportionally with the amount of enzyme added, but only until the enzyme is no longer saturated with substrate. At this point the enzyme will be present in excess with respect to substrate, and additional enzyme will have no additional effect. The curve will level off to a constant maximum rate when this point is reached.
8.5 recap
1.What is feedback inhibition? How might the reactions shown in Figure 8.18A fit into a systems diagram like the one shown in Figure 8.14?
Feedback inhibition occurs in a biochemical pathway when that pathway’s end product can act as an inhibitor of the enzyme that catalyzes the first step in the pathway. In a systems diagram, each node represents an enzymatic transformation. Feedback inhibition can cross multiple pathways, thereby allowing the changing concentration of one molecule to affect several pathways that lead to its synthesis.
2.Consider an enzyme that is subject to allosteric regulation. If a competitive inhibitor (not an allosteric inhibitor) is added to a solution containing such an enzyme, the ratio of enzyme molecules in the active form to those in the inactive form increases. Explain this observation.
See Figure 8.17. A competitive inhibitor binds to the active site of the enzyme and shifts the equilibrium to enzyme molecules in the active form.
3.In humans, hydrogen peroxide (H2O2) is a dangerous toxin produced as a by-product of several metabolic pathways.The accumulation of H2O2 is prevented by its conversion to harmless H2O, a reaction catalyzed by the appropriately named enzyme catalase.Air pollutants can inhibit this enzyme and leave individuals susceptible to tissue damage by H2O2. How would you investigate whether catalase has an allosteric or a nonallosteric mechanism and whether the pollutants are acting as competitive or noncompetitive inhibitors?
To determine whether catalase has an allosteric or a nonallosteric mechanism, perform an experiment with varying amounts of substrate and plot the rate of cat-alase-versus-substrate concentration. An S-shaped curve will indicate an allosteric mechanism. A hyperbolic curve will indicate a nonallosteric enzyme. To determine if a pollutant is a competitive or a noncompetitive inhibitor, add the pollutant to the catalase to lower the rate of reaction, then add increasing amounts of substrate. A competitive inhibitor will be removed from the active site and the rate of reaction will increase. A noncompetitive inhibitor will not allow the rate to increase as more substrate is added.