BIOCHEM

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) describes a strict "lock and key" model, whereas (b) describes a transition-state complementarity model.

The role of the metal ion (Mg2+) in catalysis by enolase is to:

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)

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:

apoenzyme.

The benefit of measuring the initial rate of a reaction V0 is that, at the beginning of a reaction:

changes in [S] are negligible, so [S] can be treated as a constant.

Enzymes differ from other catalysts in that only enzymes:

display specificity toward a single reactant.

Penicillin and related drugs inhibit the enzyme _____; this enzyme is produced by _____.

enzyme is produced by _____. A) β-lacamase; bacteria B) transpeptidase; human cells C) transpeptidase; bacteria D) lysozyme; human cells E) aldolase; bacteria C)

The role of an enzyme in an enzyme-catalyzed reaction is to:

increase the rate at which substrate is converted into product.

Michaelis and Menten assumed that the overall reaction for an enzyme-catalyzed reaction could be written as: Using this reaction, the rate of breakdown of the enzyme-substrate complex can be described by the expression:

k-1 [ES] + k2 [ES].

Enzymes are potent catalysts because they::

lower the activation energy for the reactions they catalyze.

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 intercept by -1. B) multiply the reciprocal of the y-axis intercept by -1. C) take the reciprocal of the x-axis intercept. D) take the reciprocal of the y-axis intercept. E) take the x-axis intercept, where V0 = 1/2 Vmax.

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

Which enzymes are NOT among the six internationally accepted classes of enzymes?

polymerases

The concept of "induced fit" refers to the fact that:

substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation.

The steady state assumption, as applied to enzyme kinetics, implies:

the ES complex is formed and broken down at equivalent rates.

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 _____.

1; 2

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 reaction mixture was 12 μmol. If, in a separate experiment, one-third as much enzyme and twice as much substrate had been combined, how long would it take for the same amount (12 μmol) of product to be formed?

27 min

How is trypsinogen converted to trypsin?

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 an allosteric modulator, cAMP, causing dissociation into active trypsin monomers. E) Two inactive trypsin C)

Which statement about allosteric control of enzymatic activity is FALSE?

A) Allosteric effectors give rise to sigmoidal V0 versus [S] kinetic plots. B) Allosteric proteins are generally composed of several subunits. C) An effector may either inhibit or activate an enzyme. D) Binding of the effector changes the conformation of the enzyme molecule. E) Heterotropic allosteric effectors compete with substrate for binding sites. E)

Which statement BEST describes why niacin is required in the mammalian diet? A) Niacin is a precursor of nicotinamide adenine dinucleotide (NAD), which is a coenzyme required by many oxidoreductase enzymes. B) Niacin is a precursor of flavin adenine dinucleotide (FAD), which is a coenzyme required by many ligases. C) Niacin is a precursor of thiamine pyrophosphate, which is a coenzyme required by many transferases. D) Niacin is a component of coenzyme A, which is involved in many transferase reactions. E) Niacin is not actually required in the mammalian diet.

A) Niacin is a precursor of nicotinamide adenine dinucleotide (NAD), which is a coenzyme required by many oxidoreductase enzymes.

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 His residue on the enzyme is involved in the reaction. C) the enzyme has a metallic cofactor. D) the enzyme is found in gastric secretions. E) the reaction relies on specific acid-base catalysis. B)

f you were attempting to design a new drug for the treatment of a disease by interfering with enzyme activity in the disease-causing organism, which type of inhibitor would likely be the MOST effective? A) a transition-state analog that is an irreversible inhibitor B) a substrate analog that is a competitive inhibitor C) a transition-state analog that is a competitive inhibitor D) a product analog that is an uncompetitive inhibitor E) a substrate analog that is a mixed inhibitor

A) a transition-state analog that is an irreversible inhibitor

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 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 strict Michaelis-Menten kinetics. C)

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)

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)

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)

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) solve, graphically, for the rate of an enzymatic reaction at infinite substrate concentration. E) solve, graphically, for the ratio of products to reactants for any starting substrate concentration. D.

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) turnover number. E)

Which factor 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 structure E) residues near the phosphorylation site A)

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 substrate is equal to the Km. E) is unchanged in the presence of a uncompetitive inhibitor. D)

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:

A) glucose has more —OH groups per molecule than does water. 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. C) the —OH group of water is attached to an inhibitory H atom, while the glucose —OH group is attached to C. D) water and the second substrate, ATP, compete for the active site resulting in a competitive inhibition of the enzyme. E) water normally will not reach the active site because it is hydrophobic. B)

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) irreversible B) competitive C) noncompetitive D) mixed E) pH inhibition A)

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 the transition state for the normal enzyme-substrate complex. E) typically reacts more rapidly with an enzyme than the normal substrate. C)

For the simplified representation of an enzyme-catalyzed reaction shown below, the statement "ES is in steady-state" means that:

A) k2 is very slow. B) k1 = k2. C) k1 = k-1. D) k1[E][S] = k-1[ES] + k2[ES]. E) k1[E][S] = k-1[ES]. D.

For enzymes in which the slowest (rate-limiting) step is the reaction 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.

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) both the enzyme concentration and the initial velocity of the catalyzed reaction at [S] >> Km. E)

Which statement about a plot of V0 versus [S] for an enzyme that follows Michaelis-Menten kinetics is FALSE?

At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at Km.

If chemical reactions will eventually reach an equilibrium state, what is the purpose of enzymes in a biological system? A) Enzymes are consumed to speed up chemical reactions. B) Enzymes speed up chemical reactions without being used up in the process. C) Enzymes slow down chemical reactions. D) Enzymes alter the equilibrium state between reactants and products. E) Enzymes prevent the formation of unstable reaction intermediates.

B) Enzymes speed up chemical reactions without being used up in the process.

A double-reciprocal plot of 1/V0 versus 1/[S] for an enzyme in the presence of increasing concentrations of an uncompetitive inhibitor will have lines corresponding to the differentinhibitor concentrations that are BEST described by which statement? A) The lines will intersect at the x-axis to the left of the y-axis. B) The lines will cross the y-axis and will be parallel to each other. C) The lines will intersect to the left of the y-axis but above the x-axis. D) The lines will intersect below the x-axis, to the right of the y-axis. E) The lines will be parallel to each other but will not cross the y-axis.

B) The lines will cross the y-axis and will be parallel to each other.

The Mg2+ metal ions involved in the enolase reaction mechanism are required for which reason? A) They form covalent bonds with a porphoryin ring to coordinate the substrate. B) They enhance the electron-withdrawing potential of the carbonyl group of 2-phosphoglycerate. C) They stabilize the phosphate group of 2-phosphoglycerate. D) They allow a Lys in the active site to donate a proton to the 2-phosphoglycerate substrate. E) They prevent the reverse catalysis of 3-phosphoglycerate to 2-phosphoglycerate from taking place.

B) They enhance the electron-withdrawing potential of the carbonyl group of 2-phosphoglycerate.

A reaction that has a ΔG′° of 25 kJ/mol is likely to have which property? A) a large equilibrium constant B) a small equilibrium constant C) a slow reaction rate D) both a large equilibrium constant and a slow reaction rate E) both a small equilibrium constant and a slow reaction rate

B) a small equilibrium constant

Which protein polymerizes into a gel-like matrix after it is activated to assist in blood clotting? A) trypsinogen B) fibrinogen C) prothrombin D) thrombin E) warfarin

B) fibrinogen

Which enzyme uses general acid-base catalysis and/or covalent catalysis mechanisms to hydrolyze peptidoglycan? A) enolase B) lysozyme C) chymotrypsin D) pepsin E) β lactamase

B) lysozyme

The allosteric enzyme ATCase is regulated by CTP, which binds to the T-state of ATCase. CTP is a: A) positive regulator. B) negative regulator. C) cofactor. D) competitive inhibitor. E) coenzyme.

B) negative regulator.

The enzyme pepsin is produced in the stomach lining initially as a _____, which requires _____ for activation in the stomach. A) kinase; phosphorylation B) zymogen; irreversible proteolytic cleavage C) proprotein; reversible proteolytic cleavage D) zymogen; ubiquitination E) phosphorylase; irreversible proteolytic cleavage

B) zymogen; irreversible proteolytic cleavage

In what way does an uncompetitive inhibitor bind to an enzyme? A) It reversibly binds to the enzyme active site. B) It irreversibly binds to the enzyme active site. C) It reversibly binds to the enzyme-substrate complex but does not bind to the free enzyme. D) It reversibly binds to both the free enzyme and the enzyme-substrate complex. E) It irreversibly binds to an enzyme allosteric site.

C) It reversibly binds to the enzyme-substrate complex but does not bind to the free enzyme.

Which type of enzyme is responsible for attaching phosphate groups to specific amino acids? A) a protein phosphatase B) a ribosylase C) a protein kinase D) a protein glycosylase E) an ATPase

C) a protein kinase

What would be the systematic name for an enzyme that catalyzes the following reaction? alanine + 2-oxoglutarate → pyruvate + glutamate A) alanine:2-oxoglutarate hydrolyase B) alanine:2-oxoglutarate ligase C) alanine:2-oxoglutarate aminotransferase D) glutamate oxidoreductase E) pyruvate:glutamate phosphotransferase

C) alanine:2-oxoglutarate aminotransferase

The HIV protease enzyme uses a general acid-base catalysis mechanism to cleave viral polypeptides but does not use a covalent catalysis. This enzyme functions optimally in the pH range of 4-6. Due to the specific amino acids involved in this catalysis, HIV protease is a member of which subclass of proteases? A) metalloproteases B) serine proteases C) aspartyl proteases D) cysteine proteases E) lysine proteases

C) aspartyl proteases

Treatment of methanol poisoning by using ethanol is an example of what type of enzyme inhibition? A) mixed inhibition B) uncompetitive inhibition C) competitive inhibition D) noncompetitive inhibition E) suicide inhibition

C) competitive inhibition

The combination of amoxicillin and clavulanic acid into the drug known as Augmentin is widely used as an antibacterial agent. The purpose of amoxicillin in this drug is to target _____, which catalyzes crosslinking of peptidoglycan, and the purpose of clavulanic acid in this drug is to target _____, which normally would inactivate the amoxicillin. Both of these compounds act as _____ inhibitors of enzyme function. A) transpeptidase; hexokinase; competitive B) peptidoglycanase; β lactamase; uncompetitive C) transpeptidase; β lactamase; suicide D) glycanprotease; penicillinase; competitive E) β lactamase; lysozyme; suicide

C) transpeptidase; β lactamase; suicide

Which amino acid is NOT one that may be targeted for phosphorylation to modulate the activity of an enzyme? A) Tyr B) His C) Ser D) Ala E) Thr

D) Ala

Which amino acid is NOT capable using its side chain (R group) to participate in general acid-base catalysis? A) Asp B) His C) Ser D) Val E) Lys

D) Val

43. A complex organic molecule that is necessary for enzyme function but is NOT permanently associated with the enzyme is a: A) prosthetic group. B) cofactor. C) holofactor. D) coenzyme. E) metal ion.

D) coenzyme.

An enzyme's specificity constant (kcat/Km) has a maximum upper limit due to: A) substrate concentration. B) enzyme concentration. C) the enzyme turnover number. D) diffusion of the enzyme and substrate together. E) the Michaelis constant.

D) diffusion of the enzyme and substrate together.

Enzymes that cleave carbon-carbon bonds without the use of water in the chemical reaction are members of which class of enzymes? A) oxidoreductases B) transferases C) hydrolases D) lyases E) isomerases

D) lyases

Which molecule would NOT be effective to administer as an anticoagulant? A) aspirin B) warfarin C) heparin D) vitamin K E) antithrombin

D) vitamin K

Which statement regarding enzyme activity is CORRECT? A) Enzymes bind their substrates better than the transition state. B) Enzymes bind the transition state better than the reaction products. C) Enzymes reduce the activation energy required for the reaction to take place. D) Enzymes bind their substrates better than the transition state and the transition state better than the reaction products. E) Enzymes bind the transition state better than the reaction products and reduce the activation energy required for the reaction to take place.

E) Enzymes bind the transition state better than the reaction products and reduce the activation energy required for the reaction to take place.

Chymotrypsin catalyzes the cleavage of proteins at peptide bonds adjacent to aromatic amino acid residues by using which catalytic mechanism? A) general acid-base catalysis B) metal ion catalysis C) covalent catalysis D) both general acid-base catalysis and metal ion catalysis E) both general acid-base catalysis and covalent catalysis

E) both general acid-base catalysis and covalent catalysis

Blood coagulation involves: A) a kinase cascade. B) zymogen activation. C) serine proteases. D) both a kinase cascade and zymogen activation. E) both zymogen activation and serine proteases.

E) both zymogen activation and serine proteases.

An enzyme that can convert glucose into fructose is a member of which class of enzymes? A) oxidoreductases B) transferases C) hydrolases D) lyases E) isomerases

E) isomerases

Binding energy between an enzyme and a substrate contributes to catalysis in which way? A) Binding energy provides the enzyme specificity for the substrate. B) Binding energy contributions allow for entropy reduction in the substrate-enzyme complex. C) Binding energy compensates for energy changes as a result of desolvation of the substrate. D) Binding energy contributes to the process of induced fit between the substrate and the enzyme. E) All of the answers are correct.

E) All of the answers are correct.

Which statement is FALSE?

For S → P, a catalyst shifts the reaction equilibrium to the right.

Which statement is TRUE of the binding energy derived from enzyme-substrate interactions?

It is sometimes used to hold two substrates in the optimal orientation for reaction.

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 0.8 325 2 433 4 488 6 647 1,000

The Km for this enzyme is approximately: A) 1 mM. B) 1000 mM. C) 2 mM. D) 4 mM. E) 6 mM. C.

Which statement about enzyme-catalyzed reactions is FALSE?

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.

Which statement is TRUE of enzyme catalysts?

They can increase the reaction rate for a given reaction by a thousand-fold or more.

Which statement is TRUE of enzyme catalysts?

They lower the activation energy for the conversion of substrate to product.