Deck 13: Enzymeskinetics and Specificity

Question 1

All are distinctive features of enzymes EXCEPT:

A) regulation.
B) catalytic activity.
C) ability to change ΔG.
D) specificity.
E) none is true.

Answer 1

C

Question 2

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.

Answer 2

E

Question 3

The free energy of activation, ΔG , is defined as:

A) The average free energy of the product formed.
B) The rate of a chemical reaction in relationship to the concentration of reactant molecules.
C) The energy required to raise the average energy of one mole of reactant to the transition state energy.
D) The amount of energy released by a spontaneous reaction.
E) The lowest point on a free energy diagram.

Answer 3

C

Question 4

Which of the following is true regarding the Briggs and Haldane steady state assumption?

A) It is defined by the equation

B) It states the rate of enzyme-substrate complex formation differs from the rate of enzyme-substrate disappearance.
C) The concentration of the enzyme-substrate complex reaches a constant value even in a dynamic system.
D) The enzyme-substrate complex will always dissociate to form E + P.
E) The total amount of enzyme is variable, depending on the amount of substrate available.

Question 5

The specific site on the enzyme where ____ binds and catalysis occurs is called the ____ site.

A) coenzyme; substrate
B) substrate; active
C) coenzyme; regulatory
D) regulatory; active
E) none of the above

Question 6

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.

Question 7

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.
E) A reaction with a rate constant in the units of M−¹s−¹.

Question 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.

Question 9

Which of the following statements is true regarding enzyme pathways?

A) the most effective way to control a pathway is to regulate every enzyme in the pathway
B) an enzyme pathway always proceeds in only one direction, never in reverse
C) a regulatory enzyme is regulated only by molecules within the given pathway
D) metabolic pathways are necessary since enzymes usually catalyze only one specific reaction
E) none of the above are true

Question 10

Enzymes work by:

A) providing an alternate reaction pathway
B) increasing the spontaneity (ΔG) of the reaction
C) lowering the activation energy of the reaction
D) altering the concentration of the reactants to achieve a favorable ΔG
E) proceeding in only one direction, from reactants to products

Question 11

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 V_max?

A) 4.7 mM/min
B) 0.67 mM/min
C) 3.19 mM/min
D) 1.5 mM/min
E) 0.32 mM/min

Question 12

Which statement is correct about the Michaelis-Menten constant, K_m, for the kinetic mechanism below?

A) It is numerically equal to the substrate concentration required to achieve one half the maximum velocity.
B) Its defined as K_m = k₁/(k−₁ + k₂).
C) It is approximately equal to the dissociation constant for the enzyme-substrate complex to E + P.
D) The value of K_m is constant for an enzyme regardless of the specific substrate molecule used to determine it.
E) Its numeric value has the units of moles−¹.

Question 13

If the rate constant for the enzyme catalyzed reaction is 2 × 10⁵/sec and the rate constant for the uncatalyzed reaction is 2 × 10−⁶/sec, the catalytic power of the enzyme is:

A) 10¹¹
B) 2 × 10−¹¹
C) 10−¹¹
D) 10−¹
E) 2 × 10−¹

Question 14

An enzyme's specificity can be due to:

A) the ratio of catalyzed rate to the uncatalyzed rate of reaction.
B) molecular recognition based on structural complementarity.
C) amount of enzyme produced by the cell.
D) amount of substrate available.
E) metabolic activators.

Question 15

The catalytically active complex of an apoenzyme and its prosthetic group is referred to a(n) ____.

A) catalytic duo
B) holoenzyme
C) prosthetic enzyme
D) dimeric enzyme
E) none of the above

Question 16

All of the following statements are true about the relationships between [S], K_m and V_max EXCEPT:

A) As the [S] is increased, v approaches the limiting value, V_max.
B) K_m = V_max/2.
C) The rate of the reaction, v, follows a first order rate equation v = K'[A] and K' = V_max/K_m.
D) The rate of product formed, v, is at V_max when [S] >> K_m.
E) K_m and V_max assist in finding the rate of the enzyme catalyzed reaction only if the reaction is irreversible.

Question 17

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 K_m?
[S] (mM)
V_o(mM/min)
1)0
2)0
4)0
2)8

A) 0.17 mM
B) 5.7 mM
C) 2.7 mM
D) 0.60 mM
E) 1.7 mM

Question 18

How do catalysts work to accelerate a chemical reaction?

A) They raise the average energy of the reactants.
B) They provide a means of acceleration by being completely consumed in the reaction.
C) They lower the energy of activation.
D) They lower the overall free energy change of the reaction.
E) They raise the overall free energy change of the reaction.

Question 19

If an enzyme has a V_max of 15 mM/min, what is the velocity if the substrate is present at 1/5 of the K_m?

A) 12 mM/min
B) 6 mM/min
C) 3 mM/min
D) 2.5 mM/min
E) cannot be determined from given information

Question 20

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; V_max
B) first; V_max
C) second; V_max/2
D) zero; V_max/2
E) first; V_max/2

Question 21

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 K_m would be altered and V_max would be decreased.
E) Competitive inhibitors usually interact with the enzyme at the binding site for a substrate.

Question 22

All are characteristics of ribozymes EXCEPT:

A) They are critical for removal of introns from mRNA
B) They are substrate specific.
C) They enhance the reaction rate.
D) On the ribosome, they catalyze the peptidyl transferase reaction
E) All are true.

Question 23

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.

Question 24

Sildenafil citrate (Viagra) was developed as an inhibitor of:

A) acid phosphatases.
B) cAMP phosphodiesterases.
C) glycogen phosphorylase.
D) cGMP phosphodiesterases.
E) testosterone reductase.

Question 25

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
B) the type of inhibition is noncompetitive
C) the type of inhibition is uncompetitive
D) the V_max with the inhibitor present has decreased
E) the K_m with the inhibitor present has decreased

Question 26

In the enzyme catalyzed reaction sequence below, can the E-PO₄− intermediate be predicted and why?

A) Yes, the mechanism is a double-displacement reaction.
B) Yes, the reaction fits the ping-pong model.
C) No, the reaction is random single-displacement.
D) No, the reaction is double-displacement.
E) None of the above.

Question 27

All are true for k_cat 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.

Question 28

Penicillin and other β-lactam antibiotics form a covalent bond to the enzyme glycoprotein transpeptidase. What type of bond is formed?

A) ester
B) thioester
C) amide
D) phosphate
E) none of the above

Question 29

Malonate inhibition of succinate dehydrogenase is an example of:

A) noncompetitive inhibition.
B) competitive inhibition.
C) mixed noncompetitive inhibition.
D) irreversible inhibition.
E) uncompetitive inhibition.

Question 30

Which of the following statements is NOT characteristic of k_cat/K_m?

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 k_cat/K_m 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.

Question 31

Catalytic antibodies, also called ____, are generated against an antigen that is:

A) abzymes; an analog of the transition-state intermediate in the reaction.
B) abzymes; the substrate of the reaction.
C) zymogens; an analog of the product of the reaction.
D) holoenzyme; an analog of the transition-state intermediate in the reaction.
E) none of the above.

Question 32

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?

A) slope = K_m/V_max
B) y-intercept is 1/V_max
C) x-intercept is 1/K_m
D) y = 1/V
E) x = 1/[S]

Question 33

Identify the type of reaction that would give the following graph:

A) double displacement bisubstrate reaction.
B) competitively inhibited reaction.
C) single displacement bisubstrate reaction.
D) mixed noncompetitively inhibited reaction.
E) pure noncompetitively inhibited reaction.

Question 34

In uncompetitive inhibition:

A) I combines only with ES.
B) I combines only with E.
C) I combines with E and ES.
D) I combines with EP.
E) none of the above.

Question 35

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 V_max 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.

Question 36

The International Units of an enzyme are based on the:

A) ratio of enzyme to other proteins.
B) micromoles of product formed per minute.
C) moles of substrate reacted.
D) micromoles of product produced at V_max/2.
E) none of the above.

Question 37

Penicillin is an example of a mechanism-based enzyme inactivator and is a(n):

A) competitive inhibitor.
B) noncompetitive inhibitor.
C) suicide substrate.
D) uncompetitive inhibitor.
E) none of the above.

Question 38

In the glutamate:aspartate aminotransferase catalyzed reaction mechanism, glutamate would react with ____ to form α-ketoglutarate.

A) E-pyridoxal phosphate complex
B) E-pyridoxamine phosphate complex
C) aspartate
D) oxaloacetate
E) none of the above

Question 39

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.

Question 40

In the reaction mechanism below, ____ are competitive for binding to free enzyme E.

A) P and A
B) A and B
C) B and Q
D) Q and A
E) All of the above

Question 41

Explain bimolecular reactions.​

Question 42

Discuss the effect of substrate concentration, [S], on the rate of a reaction, v.​

Question 43

Explain the mechanism of action of penicillin as an example of irreversible inhibition.​

Question 44

Discuss the induced fit model that explains enzyme specificity.​

Question 45

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.

Question 46

Which of the following statements regarding inhibitors is correct?

A) a transition state analog binds covalently to the enzyme
B) a non-competitive inhibitor causes an increase in the V_max
C) the presence of a competitive inhibitor can be overcome by addition of more inhibitor
D) a competitive inhibitor results in a higher apparent K_m value
E) none of the above are correct

Question 47

How is a Lineweaver-Burk plot obtained?​

Question 48

Carbonic anhydrase has two substrates, carbon dioxide and bicarbonate, which are both converted to carbonic acid. Kinetic data for each is given below. While determining the kinetics of HCO₃^- as a substrate, how would the addition of CO₂ effect the reaction if the rate were measured by the disappearance of bicarbonate?
Substrate
K_m (mM)
K_cat (sec^-1)
K_cat/K_m (m M^-1sec^-1)
CO₂
HCO₃^-
12
26
1×10⁶
4×10⁵
8)3×10⁴
1.5×10⁴

A) CO₂ would increase the activity of the enzyme
B) CO₂ would cause an apparent decrease in the K_m for HCO₃^-
C) CO₂ would act as a noncompetitive inhibitor
D) CO₂ would act as a competitive inhibitor
E) none of the above

Question 49

An enzymatic reaction has a V_max of 30 μM/min and a K_m of 50 μM. If the concentration of the substrate is 25 μM, which of the following is true?

A) the velocity will be 7.5 μM/min
B) the velocity will be 20 μM/min
C) the velocity will be 10 μM/min
D) the velocity will be 25 μM/min
E) the velocity will be 15 μM/min

Question 50

Carbonic anhydrase has two substrates, carbon dioxide and bicarbonate, which are both converted to carbonic acid. Kinetic data for each is given below. Typical values for K_cat/K_m range from less than 1 mM^-1sec^-1 to as high as 2.8×10⁵ mM^-1sec^-1. What can be said about the K_cat/K_m for CO₂?
Substrate
K_m (mM)
K_cat (sec^-1)
K_cat/K_m (m M^-1sec^-1)
CO₂
HCO₃^-
12
26
1×10⁶
4×10⁵
8)3×10⁴
1.5×10⁴

A) carbonic anhydrase binds CO₂ well but poorly converts it to carbonic acid
B) carbonic anhydrase binds CO₂ poorly but effectively converts it to carbonic acid
C) carbonic anhydrase binds CO₂ well and converts it to carbonic acid very well
D) carbonic anhydrase binds CO₂ poorly and poorly converts it to carbonic acid
E) the data above is useless for this question