Past Years Questions Chpater 6

Transcription

1 Past Years Questions Chpater 6 **************************************** 1) Which of the following about enzymes is Incorrect? A) Most enzymes are proteins. B) Enzymes are biological catalysts. C) Enzymes increase the rate of reactions by lowering the free energy of activation. D) Enzymes alter the equilibrium constant of the reaction they catalyze. Solution: the answer c is correct because the free energy of activation is the same as activation energy (same meaning). *************************************************** 2) Which of the following statements is NOT CORRECT for an enzyme-catalyzed reaction? A) The substrate binds to the enzyme B) both the substrate and product molecules are bound to the enzyme molecule. C) the product is formed at the enzyme molecule then released D) product-enzyme complex is formed during the reaction. Note: the product molecules aren t bound to the enzyme, it is formed at the enzyme then released. *************************************************** 3) To catalyze a reaction an enzyme must: A) not bind to its substrate. B) increase the activation energy of the reaction. C) increase the rate of the reaction. 4) The rate constant for the dissociation of the ES complex to free enzyme and substrate is: A) k 1 B) k 2 Page 1 of 11

2 C) kcat D) k-1 5) At the beginning of an enzyme-catalyzed reaction the is negligible. A) Formation of ES B) Formation of P C) Conversion of ES to E + S D) Disappearance of ES 6) The turnover numbers of the following enzymes are: Chymortypsin, 1.9x10 2 sec -1 Carbonic anhydrase, 1x10 6 sec -1 Acetylcholinesterase, 1.4x10 4 sec -1 Lysozyme 0.5 sec -1. now The most efficient enzyme of these is: A) Chymotrypsin B) Acetylcholinesterase C) Lysozyme D) Carbonic anhydrase. Solution: The most efficient enzyme is the highest turnover number. 7) In a Lineweaver-Burk plot, one plots 1/ [S] on the X-axis versus on the Y-axis: A) [S] B) 1/V C) 1/V max D) 1/ Km Page 2 of 11

3 8) To catalyze a reaction an enzyme must : A) increase the equilibrium constant of the reaction. B) bind to its substrate. C) increase the activation energy of the reaction. D) decrease the rate of the reaction. 9) The turnover number of an enzyme, a quantity equal to which of the following constants? A) k 1 B) k 2 C) k cat D) k -1 10) The rate constant for the formation of E + S from the enzymesubstrate complex is: A) k 1 B) k 2 C) k cat D) k -1 11) Which of the following relations is correct when [S] is much greater than K m? A) the reaction is zero order. B) V initial is proportional to [S]. C) V initial = ½ V max. D) V initial = V max. note: when the reactants are so high the reaction is always zero order. Page 3 of 11

4 12) Increasing temperature (T) has this effect on enzyme reactions: A) Temperature has little effect on enzyme reactions. B) Increasing T increases the rate of enzyme reactions over wide temperature ranges. C) Increasing T increases the rate of enzyme reactions until the heat denatures the enzyme. D) Enzymes always work fastest at the normal T of the organism in which they are found. 13) Given the rate law, rate = k[a][b], the overall reaction order is A) Zero B) One C) Two D) can t be determined. solution: the overall reaction order = sum of the all exponents of the reactants concentrations mean [A] 1 [B] 1 = +1+1= +2 14) Which of the following are related for a given enzyme? A) V max, K m, and percentage of alpha helix. B) V max, k cat, and percentage of ßeta sheet. C) V max, k cat, and turnover number. D) V max, K m, and molecular weight. 15) In a Lineweaver-Burk plot, the y intercept equals: A) [S] B) K m C) 1/V max D) 1/K m Page 4 of 11

5 16) Which of the following statements about enzymes is NOT CORRECT? A) Most enzymes are globular proteins. B) Enzymes are biological catalysts. C) Enzymes decrease the rate of the reactions they catalyze. D) Enzymes do not affect the equilibrium constant of the reaction they catalyze. 17) An enzyme-catalyzed reaction has zero order at : A) [S] = 2Km B) V max C) [S] << Km D) [S] = Km note: the zero order reaction is characterized by V max and high concentration of substrate ((more more than K m )). 18) To catalyze a reaction an enzyme must: A) not alter the equilibrium constant of the reaction. B) not bind to its substrate. C) decrease the activation energy of the reaction. D) decrease the rate of the reaction. 19) The Michaelis constant (K m ) of an enzyme catalyzed a single substrate reaction is: A) The equilibrium constant for the reaction between substrate and enzyme. B) It is a rate constant for the forward reaction. C) An index of the catalytic power of the enzyme. D) substrate concentration giving ½ maximum reaction velocity. Page 5 of 11

6 20) In the induced-fit model of substrate binding to enzymes : A) the substrate changes its conformation to fit the active site B) the active site changes its conformation to fit the substrate C) there is a conformational change in the enzyme when the substrate binds. D) there is aggregation of several enzyme molecules when the substrate binds. 21) A Lineweaver-Burk plot is useful in the analysis of enzymatic reactions because: A) it is easier to see whether points deviate from a straight line than from a curve. B) it is not affected by the presence of inhibitors C) it can be used whether or not the enzyme displays Michaelis- Menten kinetics D) It is faster in establishing a conclusion about the reaction. 22) The K m of hexokinase for glucose = 0.15 mm and for fructose K m = 1.5 mm, Which is the preferred substrate? A) Glucose. B) Fructose. C) Neither substrate is preferred over the other. D) You cannot tell from the data given. Solution: the Lower Km is the higher affinity and preferred subdstrate. 23) The rate constant for the decomposition of the enzyme-substrate complex into enzyme and product is: A) k1 B) k-2 C) k2 Page 6 of 11

7 24) The Michaelis constant (K m ) of an enzyme-catalyzed reaction represents the dissociation constant of ES when: A) k-1 = k2 B) k-1 = k1 C) k-1 >> k2 D) k1 >>k2 solution: because K m = (k -1 + k 2 ) / k 1 so when K -1 >>> k 2 The k 2 become negligible and the K m = k -1 /k 1 (k-1 = dissociation constant of ES). 25) At which of the following substrate concentration values will the reaction velocity (V) be equal to ½ of V max? A) [S] = K m B) [S] = 10 K m C) [S] = ½ K m D) [S] = 2 K m 26) The order of enzymatic reaction at a substrate concentration much smaller than K m is: A) Zero-order. B) First-order. C) Second-order. D) Third-order. 27) In the double reciprocal plot of data from enzyme-catalyzed reactions the slope equals: A) 1/V max B) -1/K m C) K m /V max D) V max /K m Page 7 of 11

8 28) Which of the following statements about most enzymes is INCORRECT? A) Are proteins. B) Are highly specific. C) Increase the rate of reaction. D) Increase the activation energy. 29) The reason to rewrite the Michaelis-Menten equation (such as the Lineweaver-Burk plot) is to : A) visualize reactions better. B) form enzyme kinetic data as a hyperbolic curve. C) calculate catalytic proficiency. D) calculate V max and K m. 30) The Michaelis constant, K m, is equal to the. A) maximum velocity that any given enzyme reaction can achieve B) substrate concentration which gives the best enzyme assay for an enzyme reaction C) substrate concentration when the rate is equal to half its maximal value. D) maximum velocity divided by two. 31) Which enzyme below is fastest? A) kinase, kcat = 10 3 B) papain, kcat = 10 C) carboxypeptidase, kcat = 10 2 D) catalase, kcat = 10 7 Solution: The highest K cat (turnover number) is the fastest enzyme. END OF CHAPTER 6 Page 8 of 11

9 Past Years Questions Chapter 7 **************************************** 1) Allosteric effectors: A) induce a conformational change in the protein to alter its activity. B) convert the enzyme either to the R or the T state C) can be very different in structure than the substrates of the enzyme D) All of the above are correct. 2) The reactive center of coenzyme A is : A) ADP. B) pantothenate. C) beta-alanine. D) 2-mercaptoethylamine. 3) Allosteric enzymes have all the following properties EXCEPT A) can be affected by feedback inhibition. B) can be activated and inhibited by effectors. C) can be described by Michaelis-Menten kinetics. D) their kinetics follow a sigmoidal curve. 4) Which of the following is true about the mechanism of chymotrypsin action? A) The two amino acid residues Ser195, and His57,are involved. B) The oxygen atom of Ser195 acts as a nucleophile. C) Water acts as a nucleophile. D) All of these are correct. Page 9 of 11

10 5) Which of the following statements is False about effects of binding inhibitors with the concerted model for allosteric behavior? A) There is an increase in the number of T-conformers. B) An inhibitor inhibits association of S and A with R. C) An inhibitor decreases cooperativity of substrate saturation curve. D) An inhibitor raises the apparent value of L. 6) Which of the following statements is NOT CORRECT for glycogen phosphorylase? A) the phosphorylated form of the T state is more active than the phosphorylated form of the R state. B) its activity can be affected by allosteric regulation. C) glucose-6-phosphate is an inhibitor of the enzyme. D) its phosphorylation requires the enzyme phosphoprylas kinase. 7) Which of the following statements is CORRECT about the enzyme carboxypeptidase A? A) its action involves metal-ion catalysis. B) its action requires Mg (II) ion. C) its activated by phosphorylation. D) it catalyzes the hydrolysis of amylase. 8) Which is incorrect : - while changing the K system, the V max change. 9) Which is incorrect : A) L initially is low. B) T initially is high. Page 10 of 11

11 10) Which is correct : - Glycogen phosphorylase is under allosteric modification. 11) The 1 st step to prepare chymotripsin is : - cleavage by trypsin. 12) The coenzyme that remove one Carbon is : -Tetrahydrofolate. 13)Which is correct about Co-enzyme A : A) It contains ATP B) The reaction is endorgonic. C) Contain pantothenic vitamin. 14) Which of the following is incorrect about ATcase? A) inhibtiory by CTP. B) inhibitory by ATP. END OF CHAPTER 7 Good Luck in your exam ^^ Your brother : Ammar Madkhana Page 11 of 11