43 terms

Biochem Chapter 13

Enzymes - Kinetics and Specificity
1. All are distinctive features of enzymes EXCEPT:
a. regulation.
b. catalytic activity.
c. ability to change ΔG.
d. specificity.
e. none is true.
2. Enzymes work by:
c. lowering the activation energy of the reaction
3. Which of the following statements is true regarding enzyme pathways? Acting in sequence, enzymes form ____________ and situated at key junctions are specialized ___________ enzymes capable of sensing the momentary metabolic needs and adjusting their ___________ rates accordingly.
d. metabolic pathways are necessary since enzymes usually catalyze only one specific reaction
4. If the rate constant for the enzyme catalyzed reaction is 2 X 10^5/sec and the rate constant for the uncatalyzed reaction is 2 X 10^-6/sec, the catalytic power of the enzyme is:
5. An enzyme's specificity can be due to:
b. molecular recognition based on structural complementarity.
6. The specific site on the enzyme where __________ binds and catalysis occurs is called the _____________ site.
b. substrate; active
7. The trivial term for an enzyme that is an ATP-dependent phosphotransferase is a(n):
d. kinase
8. All of the following are properties of a coenzyme EXCEPT:
a. They are usually actively involved in the catalytic reaction of the enzyme.
b. They tend to be stable to heat.
c. They can serve as intermediate carriers of functional groups.
d. They are protein components.
e. They may contain vitamins as part of their structure.
9. The catalytically active complex of an apoenzyme and its prosthetic group is referred to a(n) _______________.
b. holoenzyme
10. What reaction would NOT proceed via bimolecular elementary steps?
a. C + D → T + U
b. A reaction with a rate constant in the units of s-1.
c. 2A → D + E
d. A reaction with a molecularity of 2.
11. The free energy of activation, ΔG‡, is defined as:
c. The energy required to raise the average energy of one mole of reactant to the transition state energy.
12. All of the following are true statements about the transition state of a reaction EXCEPT:
a. The transition state is not an appropriate indication of the rate of a reaction.
b. The transition state is located at the height of a free energy diagram.
c. The energy required to raise the average energy of one mole of reactant to the transition state is the free energy of activation.
d. Reaching the transition state indicates that there is a high probability that the reaction will occur.
e. The transition state energy level is the sum of the energy levels of the reactants and products.
13. How do catalysts work to accelerate a chemical reaction?
c. They lower the energy of activation.
14. All are true for catalysts EXCEPT:
a. They work by lowering the energy of activation.
b. The average energy of the reaction is unchanged.
c. They combine transiently with the reactants promoting a reactive transition state condition.
d. They are regenerated after each reaction cycle.
e. All are true.
15. When every enzyme molecule in the reaction mixture has its substrate-binding site occupied by substrate, the kinetics become _________-order, and the velocity is ______________.
a. zero; Vmax
16. Which of the following is true regarding the Briggs and Haldane steady state assumption?
c. The concentration of the enzyme-substrate complex reaches a constant value even in a dynamic system.
17. Which statement is correct about the Michaelis-Menten constant, Km, for the kinetic mechanism below?

k1 k2
E + S ↔ ES → E + P
a. It is numerically equal to the substrate concentration required to achieve one half the maximum velocity.
18. All of the following statements are true about the relationships between [S], Km and Vmax EXCEPT:
a. As the [S] is increased, v approaches the limiting value, Vmax.
b. Km = Vmax/2.
c. The rate of the reaction, v, follows a first order rate equation
v = K'[A] and K' = Vmax/Km.
d. The rate of product formed, v, is at Vmax when [S] >> Km.
e. Km and Vmax assist in finding the rate of the enzyme catalyzed reaction only if the reaction is irreversible.
19. If an enzyme has a Vmax of 15 mM/min, what is the velocity if the substrate is present at ¼ of the Km?
d. 3 mM/min
20. For an enzyme-catalyzed reaction, the initial velocity was determined at two different concentrations of the substrate. Which of the following would be closest to the value of Km?

[S] (mM) Vo(mM/min)
1.0 2.0
4.0 2.8
d. 0.60 mM
21. For an enzyme-catalyzed reaction, the initial velocity was determined at two different concentrations of the substrate. Which of the following would be closest to the value of Vmax?

[S] (mM) Vo(mM/min)
1.0 2.0
4.0 2.8
c. 3.19 mM/min
22. All are true for kcat EXCEPT :
a. referred to as the molecular activity of the enzyme.
b. called the turnover number of the enzyme.
c. measures the maximal catalytic activity or kinetic efficiency of an enzyme.
d. defines the number of substrate molecules converted into product/enzyme molecule/unit of time when the enzyme is saturated with substrate.
e. all are true
23. Which of the following statements is NOT characteristic of kcat/Km?
a. It corresponds to a second-order rate constant.
b. It provides an excellent parameter for comparison of the catalytic efficiency of enzymes.
c. It reflects the property of the enzyme when substrate concentration is at saturation.
d. The upper limit for the kcat/Km value is fixed by the diffusion-controlled limit for reactions, which is 109 M-1 s-1.
e. It is also referred to as the turnover number.
24. The International Units of an enzyme are based on the:
b. micromoles of product formed per minute.
25. In transforming the Michaelis-Menten equation into a straight line equation, y = mx + b, the Lineweaver-Burk double reciprocal plot, which of the following is NOT a true representation?
c. x-intercept is 1/Km
26. The pH optimum of an enzyme is:
d. occurs when there is optimum secondary and tertiary structure in the active site of the enzyme.
27. All are true for inhibitor I if it is a competitive inhibitor EXCEPT:
a. It binds a site other than the active site.
b. It is structurally similar to the substrate.
c. EI does not give rise to E + P.
d. For a given [I], v decreases.
e. At some point, S can displace all of I on E.
28. All of the following statements about competitive inhibition are correct EXCEPT:
a. Competitive inhibitors are often chemical analogs of the substrate.
b. For a group-specific enzyme, one substrate would be a competitive inhibitor of reactions of the other possible substrate.
c. Sometimes a product of an enzyme-catalyzed reaction is a competitive inhibitor of its own production.
d. In the presence of a competitive inhibitor, the apparent Km would be altered and Vmax would be decreased.
e. Competitive inhibitors usually interact with the enzyme at the binding site for a substrate.
29. Malonate inhibition of succinate dehydrogenase is an example of:
b. competitive inhibition.
30. All of the following statements about noncompetitive inhibition are true EXCEPT:
a. They interact with the enzyme as well as the enzyme-substrate complex.
b. Increasing the concentration of [S] can overcome the inhibition.
c. The Vmax value does not remain the same as for a reaction that is not inhibited.
d. The inhibitor can cause a conformational change in the enzyme.
e. The inhibitor binds to a different site than does the substrate.
31. Identify the type of reaction that would give the following graph
d. mixed noncompetitively inhibited reaction.
32. A plot of 1/V vs. 1/[S] for an enzyme catalyzed reaction gave a line with an equation of y = 0.5x + 0.2. The same enzyme with an inhibitor present gave a line with an equation of y = 1.1x + 0.2. Which of the following statements is true?
a. the type of inhibition is competitive
33. In uncompetitive inhibition:
a. I combines only with ES.
34. Penicillin is an example of a mechanism-based enzyme inactivator and is a(n):
c. suicide substrate.
35. An antibiotic that acts by forming a covalent derivative of the enzyme glycoprotein peptidase is:
d. penicillin.
36. Sildenafil citrate (Viagra) was developed as an inhibitor of:
d. cGMP phosphodiesterases.
37. In the enzyme catalyzed reaction sequence below, can the E-PO4- intermediate be predicted and why?
c. No, the reaction is random single-displacement.
38. In the reaction mechanism below, _________ are competitive for binding to free enzyme E.
a. P and A
39. In the glutamate:aspartate aminotransferase catalyzed reaction mechanism, glutamate would react with ____________ to form α-ketoglutarate.
a. E-pyridoxal phosphate complex
40. All are characteristics of ordered single-displacement reactions EXCEPT:
a. Lineweaver-Burk plots with lines that intersect to the left of the 1/v axis.
b. a chemically modified enzyme-intermediate.
c. the lack of any exchange reaction activity.
d. the lack of competitive substrate effects.
e. single substrate initial binding activity.
41. All are characteristics of ribozymes EXCEPT:
a. They emerge from the reaction unchanged.
b. They are substrate specific.
c. They enhance the reaction rate.
d. They are RNA molecules.
e. All are true.
42. Catalytic antibodies, also called ____________, are generated against an antigen that is:
a. abzymes; an analog of the transition-state intermediate in the reaction.
43. All are characteristics of the enzyme hexokinase EXCEPT:
a. It is not highly specific and will catalyze phosphorylation of a number of hexoses at the six position.
b. Hexoses bind the active site and induce a solvent inaccessible fit to the hexose.
c. Glycerol may fit into the active site, but it does not bind to induce a conformational change necessary for catalysis.
d. When a hexose binds the active site, the active site is modified to promote changes in the substrate to a transitional-state intermediate.
e. All are true.