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) apoenzyme
B) coenzyme
C) holoenzyme
D) prosthetic group
E) substrate
A) apoenzyme
Which one of the following is not among the six internationally accepted classes of enzymes?
A) Hydrolases
B) Ligases
C) Oxidoreductases
D) Polymerases
E) Transferases
D) Polymerases
Enzymes are potent catalysts because they:
A) are consumed in the reactions they catalyze
B) are very specific and can prevent the conversion of products back to substrates
C) drive reactions to completion while other catalysts drive reactions to equilibr
E) lower the activation energy for the reactions they catalyze
The role of an enzyme in an enzyme-catalyzed reaction is to:
A)bind to a transition state intermediate, such that it cannot be converted back to substrate
B) ensure that all of the substrate is converted to product
C) ensure that the product is more stabl
D) increase the rate at which substrate is converted into product
which one of the following statements is true of enzyme catalysts?
A) their catalytic activity is independent of pH.
B) They are generally equally active on D and L isomers of a given substrate
C) they can increase the equilibrium constant for a given rea
D) they can increase the reaction rate for a given react by a thousand-fold or more.
Which one of the following statements is true of enzyme catalysts?
A) they bind to substrates, but are never covalently attached to substrate or product
B) They increase the equilibrium constant for a reaction, thus favoring product formation.
C) they inc
D) They lower the activation energy for the conversion of substrate to product
Which of the following statements is false?
A) a reaction may not occur at a detectable rate even though it has a favorable equilibrium.
B) After a reaction, the enzyme involved becomes available to catalyze the reaction again.
C) For S-->P, a catalyst sh
C) For S-->P, a catalyst shifts the reaction equilibrium to the right.
Enzyme differ from other catalysts in that only enzymes:
A) are not consumed in the reaction
B) display specificity toward a single reactant
C) fail to influence the equilibrium point of the reaction
D) form an activated complex with the recants
E) lower
B) display specificity toward a single reactant
Compare the two reaction coordinate diagrams below and select the answer that correctly describes their relationship. In each case, the single intermediate is the ES complex.
A
Which of the following is true of the binding energy derived from enzyme-substrate interactions?
A) it cannot provide enough energy to explain the large rate accelerations brought about by enzymes.
B) It is sometimes used to hold two substrates in the opt
B) It is sometimes used to hold two substrates in the optimal orientation for reaction
The concept of "induced fit" refers to the fact that:
A) Enzyme specificity is induced by enzyme-substrate binding.
B) enzyme-substrate binding induces an increase in the reaction entropy, thereby catalyzing the retain
C) enzyme-substrate binding induces
D) substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation
In the following diagram of the first step in the reaction catalyzed by the protease chymotrypsin, the process of general base catalysis is illustrated by the number ________, and the process of covalent catalysis is illustrated by the number _________.
A
The benefit of measuring the initial rate of a reaction V0 is that at the beginning of a reaction:
A) [ES] can be measured accurately.
B) changes in [S] are negligible, so [S] can be treated as a constant.
C) changes in Km are negligible, so Km can be tre
B) changes in [S] are negligible, so [S] can be treated as a constant.
Which of the following statements about a plot of V0 vs. [S] for an enzyme that follows Michaelis-Menten kinetics is false?
A) As [S] increases, the initial velocity of reaction V0 also increases.
B) At very high [S], the velocity curve becomes a horizont
B) At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at Km.
Michaelis and Menten assumed that the overall reaction for an enzyme-catalyzed reaction could be written as
k1 k2
E +S <--? ES --> E+P
k-1
Using this reaction, the rate of breakdown of the enzyme-substrate complex can be described by the expression:
A) k1
D) k-1 [ES] + k2 [ES].
The steady state assumption, as applied to enzyme kinetics, implies:
A) Km = Ks.
B) the enzyme is regulated.
C) the ES complex is formed and broken down at equivalent rates.
D) the Km is equivalent to the cellular substrate concentration.
E) the maximum v
C) the ES complex is formed and broken down at equivalent rates.
An enzyme-catalyzed reaction was carried out with the substrate concentration initially a thousand times greater than the Km for that substrate. After 9 minutes, 1% of the substrate had been converted to product, and the amount of product formed in the re
C) 27 min
Which of these statements about enzyme-catalyzed reactions is false?
A) At saturating levels of substrate, the rate of an enzyme-catalyzed reaction is proportional to the enzyme concentration.
B) If enough substrate is added, the normal Vmax of a reaction
D) The activation energy for the catalyzed reaction is the same as for the uncatalyzed reaction, but the equilibrium constant is more favorable in the enzyme-catalyzed reaction.
The following data were obtained in a study of an enzyme known to follow Michaelis-Menten kinetics:
V0 Substrate added
(?mol/min) (mmol/L)
�������������
217 0.8
325 2
433 4
488 6
647 1,000
�������������
The Km for this enzyme is approximately:
A) 1 mM.
B)
C) 2 mM.
For enzymes in which the slowest (rate-limiting) step is the reaction
k2
ES ? P
Km becomes equivalent to:
A) kcat.
B) the [S] where V0 = Vmax.
C) the dissociation constant, Kd, for the ES complex.
D) the maximal velocity.
E) the turnover number
C) the dissociation constant, Kd, for the ES complex.
The Lineweaver-Burk plot is used to:
A) determine the equilibrium constant for an enzymatic reaction.
B) extrapolate for the value of reaction rate at infinite enzyme concentration.
C) illustrate the effect of temperature on an enzymatic reaction.
D) solv
D) solve, graphically, for the rate of an enzymatic reaction at infinite substrate concentration.
The double-reciprocal transformation of the Michaelis-Menten equation, also called the Lineweaver-Burk plot, is given by
1/V0 = Km /(Vmax[S]) + 1/Vmax.
To determine Km from a double-reciprocal plot, you would:
A) multiply the reciprocal of the x-axis inte
A) multiply the reciprocal of the x-axis intercept by ?1.
To calculate the turnover number of an enzyme, you need to know:
A) the enzyme concentration.
B) the initial velocity of the catalyzed reaction at [S] >> Km.
C) the initial velocity of the catalyzed reaction at low [S].
D) the Km for the substrate.
E) bot
E) both A and B.
The number of substrate molecules converted to product in a given unit of time by a single enzyme molecule at saturation is referred to as the:
A) dissociation constant.
B) half-saturation constant.
C) maximum velocity.
D) Michaelis-Menten number.
E) turn
E) turnover number.
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.
B) intercept on the l/[S] axis.
C) intercept on the l/V axis.
D) pK of the plot.
E) Vmax.
B) intercept on the l/[S] axis.
In competitive inhibition, an inhibitor:
A) binds at several different sites on an enzyme.
B) binds covalently to the enzyme.
C) binds only to the ES complex.
D) binds reversibly at the active site.
E) lowers the characteristic Vmax of the enzyme.
D) binds reversibly at the active site.
Vmax for an enzyme-catalyzed reaction:
A) generally increases when pH increases.
B) increases in the presence of a competitive inhibitor.
C) is limited only by the amount of substrate supplied.
D) is twice the rate observed when the concentration of subst
D) is twice the rate observed when the concentration of substrate is equal to the Km.
Enzyme X exhibits maximum activity at pH = 6.9. X shows a fairly sharp decrease in its activity when the pH goes much lower than 6.4. One likely interpretation of this pH activity is that:
A) a Glu residue on the enzyme is involved in the reaction.
B) a H
B) a His residue on the enzyme is involved in the reaction.
Phenyl-methane-sulfonyl-fluoride (PMSF) inactivates serine proteases by binding covalently to the catalytic serine residue at the active site; this enzyme-inhibitor bond is not cleaved by the enzyme. This is an example of what kind of inhibition?
A) irrev
A) irreversible
Both water and glucose share an �OH that can serve as a substrate for a reaction with the terminal phosphate of ATP catalyzed by hexokinase. Glucose, however, is about a million times more reactive as a substrate than water. The best explanation is that:
B) the larger glucose binds better to the enzyme; it induces a conformational change in hexokinase that brings active-site amino acids into position for catalysis.
A good transition-state analog:
A) binds covalently to the enzyme.
B) binds to the enzyme more tightly than the substrate.
C) binds very weakly to the enzyme.
D) is too unstable to isolate.
E) must be almost identical to the substrate.
B) binds to the enzyme more tightly than the substrate.
A transition-state analog:
A) is less stable when binding to an enzyme than the normal substrate.
B) resembles the active site of general acid-base enzymes.
C) resembles the transition-state structure of the normal enzyme-substrate complex.
D) stabilizes
C) resembles the transition-state structure of the normal enzyme-substrate complex.
The role of the metal ion (Mg2+) in catalysis by enolase is to
A) act as a general acid catalyst.
B) act as a general base catalyst.
C) facilitate general acid catalysis.
D) facilitate general base catalysis.
E) stabilize protein conformation
D) facilitate general base catalysis.
Penicillin and related drugs inhibit the enzyme ; this enzyme is produced by .
A) ?-lacamase; bacteria
B) transpeptidase; human cells
C) transpeptidase; bacteria
D) lysozyme; human cells
E) aldolase; bacteria
C) transpeptidase; bacteria
Which of the following statements about allosteric control of enzymatic activity is false?
A) Allosteric effectors give rise to sigmoidal V0 vs. [S] kinetic plots.
B) Allosteric proteins are generally composed of several subunits.
C) An effector may eithe
E) Heterotropic allosteric effectors compete with substrate for binding sites.
A small molecule that decreases the activity of an enzyme by binding to a site other than the catalytic site is termed a(n):
A) allosteric inhibitor.
B) alternative inhibitor.
C) competitive inhibitor.
D) stereospecific agent.
E) transition-state analog.
A) allosteric inhibitor.
Allosteric enzymes:
A) are regulated primarily by covalent modification.
B) usually catalyze several different reactions within a metabolic pathway.
C) usually have more than one polypeptide chain.
D) usually have only one active site.
E) usually show str
C) usually have more than one polypeptide chain.
A metabolic pathway proceeds according to the scheme R ? S ? T ? U ? V ? W. A regulatory enzyme, X, catalyzes the first reaction in the pathway. Which of the following is most likely correct for this pathway?
A) Either metabolite U or V is likely to be a
C) The last product, W, is likely to be a negative modulator of X, leading to feedback inhibition.
Which of the following has not been shown to play a role in determining the specificity of protein kinases?
A) Disulfide bonds near the phosphorylation site
B) Primary sequence at phosphorylation site
C) Protein quaternary structure
D) Protein tertiary st
A) Disulfide bonds near the phosphorylation site
How is trypsinogen converted to trypsin?
A) A protein kinase-catalyzed phosphorylation converts trypsinogen to trypsin.
B) An increase in Ca2+ concentration promotes the conversion.
C) Proteolysis of trypsinogen forms trypsin.
D) Trypsinogen dimers bind a
C) Proteolysis of trypsinogen forms trypsin.
Define the terms "cofactor" and "coenzyme.
A cofactor is any chemical component required for enzyme activity; it includes both organic molecules, called "coenzymes," and inorganic ions.
Draw and label a reaction coordinate diagram for an uncatalyzed reaction, S ? P, and the same reaction catalyzed by an enzyme, E.
See Fig. 6-3, p. 187
The difference in (standard) free energy content, ?G'�, between substrate S and product P may vary considerably among different reactions. What is the significance of these differences?
The difference in free energy content between substrate (or reactant) and product for each reaction reflects the relative amounts of each compound present at equilibrium. The greater the difference in free energy, the greater the difference in amounts of
For a reaction that can take place with or without catalysis by an enzyme, what would be the effect of the enzyme on the:
(a) standard free energy change of the reaction?
(b) activation energy of the reaction?
(c) initial velocity of the reaction?
(d) equ
(a) no change; (b) decrease; (c) increase; (d) no change
Sometimes the difference in (standard) free-energy content, ?G'�, between a substrate S and a product P is very large, yet the rate of chemical conversion, S ? P, is quite slow. Why?
The rate of conversion from substrate to product (or the reverse reaction, from product to substrate) does not depend on the free-energy difference between them. The rate of the reaction depends upon the activation energy of the reaction ?G'�, which is th
Write an equilibrium expression for the reaction S ? P and briefly explain the relationship between the value of the equilibrium constant and free energy.
Keq' = [P]/[S]. The value of Keq' reflects the difference between the free energy content of S and P. Free energy and equilibrium constant are related by the expression:
?G'� = -RT ln Keq'
For each change in Keq' by one order of magnitude, ?G'� changes by
What is the difference between general acid-base catalysis and specific acid-base catalysis? (Assume that the solvent is water.)
Specific acid-base catalysis refers to catalysis by the constituents of water, in other words, the donation of a proton by the hydronium ion, H3O+, or the acceptance of a proton by the hydroxyl ion OH-. General acid-base catalysis refers to the donation o
Michaelis-Menten kinetics is sometimes referred to as "saturation" kinetics. Why?
According to the Michaelis-Menten model of enzyme-substrate interaction, when [S] becomes very high, an enzyme molecule's active site will become occupied with a new substrate molecule as soon as it releases a product. Therefore, at very high [S], V0 does
Two different enzymes are able to catalyze the same reaction, A ? B. They both have the same Vmax, but differ their Km the substrate A. For enzyme 1, the Km is 1.0 mM; for enzyme 2, the Km is 10 mM. When enzyme 1 was incubated with 0.1 mM A, it was observ
a) 0.022 mmol/min; b) 0.0022 mmol/min; c) 0.022 mmol/min
An enzyme can catalyze a reaction with either of two substrates, S1 or S2. The Km for S1 was found to be 2.0 mM, and the Km, for S2 was found to be 20 mM. A student determined that the Vmax was the same for the two substrates. Unfortunately, he lost the p
Vmax = 101
Write out the equation that describes the mechanism for enzyme action used as a model by Michaelis and Menten. List the important assumptions used by Michaelis and Menten to derive a rate equation for this reaction.
The two equations are
k1 k2
E + ES E + P
k-1 k-2
One assumption is that [P] = 0, so that the rate of the reaction depends exclusively on the breakdown of ES and is not influenced by the reverse reaction; that is, k-2 can be ignored and V0 = k2 [ES]. This
For the reaction E + S ? ES ? P, the Michaelis-Menten constant, Km, is actually a summary of three terms. What are they? How is Km determined graphically?
Km = (k2 + k-1)/ k1, where k-1 and k1 are the rate constants for the breakdown and association,
respectively, of the ES complex and k2 is the rate constant for the breakdown of ES to form E + P. Km can be determined graphically on a plot of V0 vs. [S] by
An enzyme catalyzes a reaction at a velocity of 20 ?mol/min when the concentration of substrate (S) is 0.01 M. The Km for this substrate is 1 � 10-5 M. Assuming that Michaelis-Menten kinetics are followed, what will be the reaction velocity when the conce
The velocity of 20 ?mol/min is essentially Vmax because it is measured at [S] >> Km. (a) When [S] = 10-5 M = Km, V = 1/2 Vmax, or 10 ?mol/min. (b) When [S] is 10-6 M, velocity can be calculated from the Michaelis-Menten equation:
V0 = Vmax [S]/( Km + [S])
Give the Michaelis-Menten equation and define each term in it. Does this equation apply to all enzymes? If not, to which kind does it not apply?
The Michaelis-Menten equation is: V0 = Vmax [S]/( Km + [S]), in which V0 is the initial velocity at any given concentration of S, Vmax is the velocity when all enzyme molecules are saturated with S, [S] is the concentration of S, and Km is a constant char
A biochemist obtains the following set of data for an enzyme that is known to follow Michaelis-Menten kinetics.
Substrate Initial
concentration velocity
(?M) (?mol/min)
�������������
1 49
2 96
8 349
50 621
100 676
1,000 698
5,000 699
�������������
(a) Vma
(a) Vmax is about 700. In a plot of V vs. [S], the asymptote is Vmax. Simple inspection of the data shows the approach to Vmax�the rate increases by only 1 unit when [S] increases five-fold.
(b) Km is about 8 ?M, the [S] at which the velocity is half-maxi
Why is the Lineweaver-Burk (double reciprocal) plot (see Box 6-1, p. 197) more useful than the standard V vs. [S] plot in determining kinetic constants for an enzyme? (Your answer should probably show typical plots.)
The plot of V vs. [S] is hyperbolic; maximum velocity is never achieved experimentally, because it is impossible to do experiments at infinitely high [S]. The Lineweaver-Burk transformation of the Michaelis-Menten equation produces a linear plot that can
An enzyme catalyzes the reaction A ? B. The initial rate of the reaction was measured as a function of the concentration of A. The following data were obtained:
[A], micromolar V0,
nmoles/min
0.05 0.08
0.1 0.16
0.5 0.79
1 1.6
5 7.3
10 13
50 40
100 53
500
a) 50 micromolar; b) 37 nmoles/min; c) 0.0125 (nmole/min)-1; d) ?0.02 micromolar
The turnover number for an enzyme is known to be 5,000 min-1. From the following set of data, calculate the Km and the total amount of enzyme present in these experiments.
Substrate Initial
concentration velocity
(mM) (?mol/min)
1 167
2 250
4 334
6 376
10
Km = about 2 mM (the concentration of S needed to achieve one-half of Vmax, which is about 500). The total enzyme present is producing about 500 ?mol of product per minute. Because the turnover number is 5,000/min, the amount of enzyme present must be 0.1
When 10 ?g of an enzyme of Mr 50,000 is added to a solution containing its substrate at a concentration one hundred times the Km, it catalyzes the conversion of 75 ?mol of substrate into product in 3 min. What is the enzyme's turnover number?
Because the velocity measured occurs far above Km, it represents Vmax. Ten ?g of the enzyme represents 10 � 10-6 g/(5 � 104 g/mol), or 2 � 10-10 mol of enzyme. In 3 minutes, this amount of enzyme produced 75 ?mol of product, equivalent to 25 � 10-6 mol of
Fifteen ?g of an enzyme of Mr 30,000 working at Vmax catalyzes the conversion of 60 ?mol of substrate into product in 3 min. What is the enzyme's turnover number?
The amount of enzyme present is 15 � 10-6 g, which is (15 � 10-6 g)/(3 � 104 g/mol) = 5 � 10-10 mol of enzyme. The rate of product formation is 60 � 10-6 mol/3 min, or 20 � 10-6 mol of product per minute. The turnover number is therefore (20 � 10-6 mol/mi
How does the total enzyme concentration affect turnover number and Vmax?
Ans: The turnover number, kcat, is the number of substrate molecules converted to product in a given time by a single enzyme molecule, so turnover number is not affected by the total enzyme concentration, [Et]. For any given reaction, however, Vmax can ch
Enzymes with a kcat / Km ratio of about 108 M-1s-1 are considered to show optimal catalytic efficiency. Fumarase, which catalyzes the reversible-dehydration reaction
fumarate + H2O--> malate
has a ratio of turnover number to the Michaelis-Menten constant
If the turnover number is nearly identical for both substrates, then the Km for malate must be much larger than for fumarate. Similar turnover numbers suggest no significant differences in rate of conversion of substrate to product, but the different Km v
Methanol (wood alcohol) is highly toxic because it is converted to formaldehyde in a reaction catalyzed by the enzyme alcohol dehydrogenase:
NAD+ + methanol ? NADH + H+ + formaldehyde
Part of the medical treatment for methanol poisoning is to administer e
Ans: Ethanol is a structural analog of methanol, and competes with methanol for the binding site of alcohol dehydrogenase, slowing the conversion of methanol to formaldehyde, and allowing its clearance by the kidneys. The effect of ethanol is that of a co
You measure the initial rate of an enzyme reaction as a function of substrate concentration in the presence and absence of an inhibitor. The following data are obtained:
[S] V0
?Inhibitor +Inhibitor
0.0001 33 17
0.0002 50 29
0.0005 71 50
0.001 83 67
0.002
a) 100; b) 0.0002; c) 66.7; d) 40; e) competitive
An enzyme follows Michaelis-Menten kinetics. Indicate (with an "x") which of the kinetic parameters at the left would be altered by the following factors. Give only one answer for each.
(a) Km; (b) both; (c) both; (d) neither
The enzymatic activity of lysozyme is optimal at pH 5.2 and decreases above and below this pH value. Lysozyme contains two amino acid residues in the active site essential for catalysis: Glu35 and Asp52. The pK values for the carboxyl side chains of these
Ans: For the enzyme to be active, it is likely that Asp52 is unprotonated and Glu35 is protonated. When the pH is below 4.5, Asp52 becomes protonated, and when it is above 5.9, Glu35 is deprotonated, either of which decreases the activity of the enzyme. (
Why does pH affect the activity of an enzyme?
Ans: The state of ionization of several amino acid side chains is affected by pH, and the activity of many enzymes requires that certain of the amino acid residue side chains be in a specific ionization state. (See Fig. 6-20, p. 207.)
Chymotrypsin belongs to a group of proteolytic enzymes called the "serine proteases," many of which have an Asp, His, and Ser residue that is crucial to the catalytic mechanism. The serine hydroxyl functions as a nucleophile. What do the other two amino a
Ans: In chymotrypsin, histidine functions as a general base, accepting a proton from the serine hydroxyl, thereby increasing serine's reactivity as a nucleophile. The negatively charged Asp stabilizes the positive charge that develops on the His.
For serine to work effectively as a nucleophile in covalent catalysis in chymotrypsin, a nearby amino acid, histidine, must serve as general base catalyst. Briefly describe, in words, how these two amino acids work together.
Ans: The serine is a polar hydroxyl, with the oxygen functioning as an electronegative nucleophile. A nearby histidine residue, with pKa ? 6.0, however, functions as a base to abstract the proton from the serine hydroxyl group. The result is to substantia
Penicillin and related antibiotics contain a 4-membered ?-lactam ring. Explain why this feature is important to the mechanism of action of these drugs.
Ans: The strained 4-membered ring is easily opened (?G << 0); this energy drives the reaction that covalently inactivates the transpeptidase.
On the enzyme hexokinase, ATP reacts with glucose to produce glucose 6-phosphate and ADP five orders of magnitude faster than ATP reacts with H2O to form phosphate and ADP. The intrinsic chemical reactivity of the �OH group in water is about the same as t
Ans: The binding of glucose to hexokinase induces a conformation change that brings the amino acid residues that facilitate the phosphoryl transfer into position in the active site. Binding of water alone does not induce this conformational change.
Why is a transition-state analog not necessarily the same as a competitive inhibitor?
Ans: The structure of a competitive inhibitor may be similar to the structure of the free substrate. Similar structure will mean that the competitive inhibitor can associate with the enzyme at the active site, effectively blocking the normal substrate fro
The scheme S ? T ? U ? V ? W ? X ? Y represents a hypothetical pathway for the metabolic synthesis of compound Y. The pathway is regulated by feedback inhibition. Indicate where the inhibition is most likely to occur and what the likely inhibitor is:
S ? T ? U ? V ? W ? X ? Y (most likely inhibitor)
? ?
- - - - - - - ? - - - - - - - -
(most likely inhibited step)
Explain how a biochemist might discover that a certain enzyme is allosterically regulated.
Ans: The enzyme would show kinetics that do not fit the Michaelis-Menten equation; the plot of V vs. [S] would be sigmoidal, not hyperbolic. The enzyme kinetics would be affected by molecules other than the substrate(s).
What is a zymogen (proenzyme)? Explain briefly with an example.
Ans: A zymogen is an inactive form of an enzyme that is activated by one or more proteolytic cleavages in its sequence. Chymotrypsinogen, trypsinogen, and proelastase are all zymogens, becoming chymotrypsin, trypsin, and elastase, respectively, after prop