SwamishrijiBiochemistry- Paper No: 501 Unit :4

Enzyme Kinetics.Total Marks : 60

Name:_____________________________ Student Number___________________________

I Multiple Choice Question.1. Enzymes are potent catalysts. Because they:A) drive reactions to completion while other catalysts drive reactions to equilibrium.B) are consumed in the reactions they catalyze.C) are very specific and can prevent the conversion of products back to substrates.D) increase the equilibrium constants for the reactions they catalyze.E) lower the activation energy for the reactions they catalyze.

2. Which of the following statements is true of enzyme catalysts?A) To be effective, they must be present at the same concentration as their substrate.B) They can increase the equilibrium constant for a given reaction by a thousand-fold or more.C) They lower the activation energy for conversion of substrate to product.D) Their catalytic activity is independent of pH.E) They are generally equally active on D and L isomers of a given substrate.

3. The role of an enzyme in an enzyme-catalyzed reaction is to:A) ensure that the product is more stable than the substrate.B) make the free-energy change for the reaction more favorable.C) increase the rate at which substrate is converted into product.D) ensure that all the substrate is converted to product.E) do none of the above.

4. Which of the following statements is false?A) A reaction may not occur at a detectable rate even though it has a favorable equilibriumB) At the end of an enzyme-catalyzed reaction, the functional enzyme becomes available to catalyze the reaction againC) Substrate binds to an enzyme’s active siteD)For S→ P, a catalyst shifts the reaction equilibrium to the rightE) Lowering the temperature of a reaction will lower the reaction rate

5. The concept of "induced fit" refers to the fact that:A) when a substrate binds to an enzyme, the enzyme induces a loss of water (desolvation) from the substrate.B) substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation.C) enzyme-substrate binding induces an increase in the reaction entropy, thereby catalyzing the reaction.D) enzyme specificity is induced by enzyme-substrate binding.

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E) enzyme-substrate binding induces movement along the reaction coordinate to the transition state.

6. The benefit of measuring the initial rate of a reaction, V0, is that at the beginning of a reaction:A) changes in [S] are negligible, so [S] can be treated as a constant.B) [ES] can be measured accurately.C) V0 = Vmax.D) changes in Km are negligible, so Km can be treated as a constant.E) varying [S] has no effect on V0.

7. Which of the following statements about a plot of V0 vs. [S] for an enzyme that follows Michaelis-Menten kinetics is false?A) Km is the [S] at which V0 = 1/2 Vmax.B) The shape of the curve is a hyperbola.C) The y-axis is a rate term with units of μΜ/min.D) As [S] increases, the initial velocity of reaction, V0, also increases.E) At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at Km.

8. The steady state assumption, as applied to enzyme kinetics, implies:A) Km = KsB) The maximum velocity occurs when the enzyme is saturatedC)The ES complex is formed and broken down at equivalent ratesD) The Km is equivalent to the cellular substrate concentrationE) The enzyme is regulated

9. The following data were obtained in a study of an enzyme known to follow Michaelis-Menten kinetics:V0 Substrate added(μmol/min) (mmol/L)—————————————217 0.8325 2433 4488 6647 1,000—————————————The Km for this enzyme is approximately:A) 1 mM.B) 2 mM.C) 4 mM.D) 6 mM.E) 1,000 mM.

10. The double-reciprocal transformation of the Michaelis-Menten equation, also called the Lineweaver-Burk plot, is given by1/V0 = Km /(Vmax[S]) + 1/Vmax.

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To determine Km from a double-reciprocal plot, you would:A) take the x-axis intercept where V0 = 1/2 Vmax.B) take the reciprocal of the x-axis intercept.C) multiply the reciprocal of the x-axis intercept by −1.D) take the reciprocal of the y-axis intercept.E) multiply the reciprocal of the y-axis intercept by −1.

11. To calculate the turnover number of an enzyme you need to know the:A) initial velocity of the catalyzed reaction at low [S].B) initial velocity of the catalyzed reaction at [S] >> Km.C) Km for the substrate.D) enzyme concentration.E) both B and D.

12. In competitive inhibition, an inhibitor:A) binds at several different sites on an enzyme.B) binds reversibly at the active site.C) binds only to the ES complex.D) binds covalently to the enzyme.E) lowers the characteristic Vmax of the enzyme.

13. Vmax for an enzyme-catalyzed reaction:A) generally increases when pH increases.B) increases in the presence of a competitive inhibitor.C) is unchanged in the presence of a uncompetitive inhibitor.D) is twice the rate observed when the concentration of substrate is equal to the Km.E) is limited only by the amount of substrate supplied.

14. One of the enzymes involved in glycolysis, aldolase, requires Zn2+ for catalysis. Under conditions of zinc deficiency, when the enzyme may lack zinc, it would be referred to as the:A) holoenzyme.B) prosthetic group.C) apoenzyme.D) coenzyme.E) substrate.

15.. Enzymes differ from other catalysts in that enzymes:A) lower the activation energy of the reaction catalyzed.B) fail to influence the equilibrium point of the reaction.C) form an activated complex with the reactants.D) usually display specificity toward a single reactant.E) are not consumed in the reaction.

16.Briggs and Haldane assumed that the overall scheme for an enzyme-catalyzed reaction could be written as

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k1 k2 E + S → ES → P k−1Using this reaction, the rate of formation of the enzyme-substrate complex can bedescribed by the expression:A) k1([Et] − [ES])[S].B) k−1[ES] + k2[ES].C) k2[ES].D) k−1[ES].E) k1([Et] − [ES]).

17. To determine Km from the Hanes-Woolf transformation of the Michaelis-Mentenequation you would:A) take the x-axis intercept where V0 = 1/2 Vmax.B) take the reciprocal of the x-axis intercept.C) multiply the x-axis intercept by −1.D) take the reciprocal of the y-axis intercept.E) multiply the y-axis intercept by −1.

18. The term, molecular activity, refers to:A) the number of moles of substrate transformed into product per unit time per mole ofenzyme active siteB) the maximum velocity of a reaction over the number of moles of substrateC) the number of moles of substrate transformed into product per unit time per mole ofenzymeD) the initial velocity of the catalyzed reaction at low [S].E) none of the above.

19. The specificity constant, kcat/KM is a useful parameter for comparing the catalyticefficiencies of enzymes with substrates because:A) it is a measure of how rapidly an enzyme can work at low [S]B) it is a first order rate constant that relates [S] to reaction velocityC) it is a measure of how rapidly an enzyme can work at high [S]D) it indicates how rapidly an enzyme collides with its substrateE) none of the above

20. A noncompetitive inhibitor is:A) a substance that binds either to the free enzyme or to the E-S complexB) a substance that binds to the E-S complex onlyC) a substance that binds to the free enzyme onlyD) a substance that causes a change in Vmax and KM for the reactionE) none of the above

21) In a plot of l/V against 1/[S] for an enzyme-catalyzed reaction, the presence of a competitive inhibitor will alter the:

A) curvature of the plot.

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B) intercept on the l/[S] axis. C) intercept on the l/V axis. D) pK of the plot. E) Vmax.

22. How do the intermediate steps in multi-substrate enzyme mechanisms (sequential displacement vs double displacement) differ? [2]

23. How are competitive, uncompetitive, and non-competitive inhibitions kinetically distinguishable? Explanation must include information about (1) the extent of inhibitor as it relates to [S], (2) substrate/enzyme/inhibitor complex, and (3) changes in Km and/or Vmax. [3]

24.What is the Gibbs free energy of activation? Explain how enzymes increase reaction rates as related to Gibbs free energy of activation. [2]

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25. For a reaction that can take place with or without catalysis by an enzyme, what would be the effect of the enzyme on the each of the followings: [4]

(a) standard free energy change of the reaction?__________________________________

(b) activation energy of the reaction?___________________________________________

(c) initial velocity of the reaction?______________________________________________

(d) equilibrium constant of the reaction? ________________________________________

26. You measure the initial rate of an enzyme reaction as a function of substrate concentration in the presence and in the absence of 10 mM inhibitor. The following data are obtained:

[4]

[S] mM V0 (mM/sec)

Inhibitor +Inhibitor

0.1 33 17

0.2 50 36

0.5 71 59

1.0 83 67

2.0 91 80

5.0 96 91

10.0 98 95

a) What is the Vmax in the absence of inhibitor?

b) What is the Km in the presence of inhibitor?

c) What is the binding constant of this inhibitor?

d) What kind of inhibitor is it likely to be?

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appM M iK K (1 [I]/K )

Problem 18

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

-6 -4 -2 0 2 4 6 8 10 12

1/[S]

1/V

No Inh

Inhibitor

Linear (NoInh)

Linear(Inhibitor)

27.A chemical reaction has a DGo = -60 kJ/mol. If this were an enzyme-catalyzed reactionwhat can you predict about the kinetics?A. It will exhibit very rapid kinetics.B. It will exhibit very slow kinetics.C. The kinetics of the reaction can not be predicted.D. The kinetics depend on the nature of the reactants.E. The kinetics depend on the nature of the products.

28. Suppose that the standard free energy change for an uncatalyzed reaction (S « P) is +60kJ/mol, and a specific enzyme enhances the rate of the reaction by 10 7- fold. Understandard conditions in the presence of the enzyme, in which direction will the reaction go?

29. If the activation energy for an uncatalyzed reaction is + 25 kJ/mol, then what can you sayabout the activation energy for the enzyme catalyzed reaction?

30. An approximate "rule of thumb" is that the rate of a chemical reaction will double (reactionwill go twice as fast) for every 10o increase in temperature. This observation holds forenzyme-catalyzed reactions up to a certain temperature range (different for each enzyme),at which the rate stops increasing and then decreases sharply as the temperature increases.Explain.

31. Assuming they have equal affinity for the enzyme, why would a noncompetitive inhibitorbe a more effective drug than a competitive inhibitor?

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32. Why does Vmax but not Km depend on the amount of enzyme used in an enzyme reaction?

33.Derive Michaelis Menten Equation for enzyme catalysed reaction [9]

34.Giving example explain ping pong mechanism [4]

35.Explain reversible inhibition. [4]

36.Define specific activity of enzyme. [1]

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