Question 1

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.

Accepted Answer

Allosteric enzymes usually have more than one polypeptide chain, with each chain having its own active site. The binding of a regulatory molecule at one of these sites can affect the enzyme's activity at the other active site(s).

Question 2

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.

Accepted Answer

The metal ion (Mg²⁺) in enolase catalysis functions to facilitate general base catalysis. It stabilizes the enolate intermediate, which is formed upon abstraction of the proton from the C2 carbon of 2-phosphoglycerate by a nearby base residue (e.g. Lys 345), by chelating the negative charge generated. This enhances the nucleophilicity of the enolate intermediate, which increases the rate of the subsequent reactions.

Question 3

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)V_max.

Accepted Answer

The presence of a competitive inhibitor will increase the apparent Km of the enzyme, which results in a higher intercept on the 1/[S] axis. The curvature of the plot remains the same and the intercept on the l/V axis, pK of the plot, and V_max are not affected by the presence of a competitive inhibitor.

Question 4

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.

Accepted Answer

The turnover number, also known as kcat, is defined as the maximum number of substrate molecules that can be converted to product by a single enzyme molecule per unit time when the enzyme is fully saturated with substrate. This is the best choice because the other options do not accurately describe the concept of turnover number. The dissociation constant (Kd) is the concentration of substrate at which half of the enzyme active sites are occupied by substrate, and is related to the affinity of the enzyme for its substrate. The half-saturation constant, also known as Km, is the concentration of substrate at which the enzyme is working at half of its maximum velocity. Maximum velocity (Vmax) is the maximum rate of product formation that can be achieved by the enzyme under saturated conditions. The Michaelis-Menten number is not a commonly used term in biochemistry.

Question 5

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.

Accepted Answer

A good transition-state analog should bind to the enzyme more tightly than the substrate in order to effectively mimic the transition state and inhibit the enzyme's activity. Covalent binding (A) may not accurately mimic the transition state and can also permanently inactivate the enzyme. Weak binding (C) would not effectively inhibit the enzyme. Being too unstable to isolate (D) would make the analog ineffective for use. While being almost identical to the substrate (E) is important for specificity, the most critical aspect for a good transition-state analog is tight binding.

Question 6

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)Non-competitive
D)Mixed
E)pH inhibition

Accepted Answer

Phenyl-methane-sulfonyl-fluoride (PMSF) forms a covalent bond with the catalytic serine residue at the active site of serine proteases, which cannot be cleaved by the enzyme. This irreversible inhibition is a characteristic feature of covalent or irreversible inhibitors.

Question 7

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.

Accepted Answer

The sharp decrease in activity below pH 6.4 suggests involvement of a histidine (His) residue, as its side chain has a pKa around 6, making it sensitive to changes in pH around this value. Histidine can act as a proton donor or acceptor near its pKa, playing a crucial role in enzyme catalysis at this pH range.

Question 8

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)co-factor.
D)competitive inhibitor.
E)coenzyme.

Accepted Answer

CTP is a negative regulator because it binds to the T-state of ATCase, stabilizing it and making it more difficult for the enzyme to convert to its active R-state. Thus overall activity of the enzyme is reduced or inhibited by the presence of CTP.

Question 9

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 V_max of the enzyme.

Accepted Answer

In competitive inhibition, the inhibitor binds reversibly at the active site of the enzyme. This prevents the substrate from binding and reduces the rate of the catalyzed reaction. The binding of the inhibitor is reversible because it does not involve covalent bonds. The inhibitor does not bind at several different sites on the enzyme, it does not bind only to the ES complex, and it does not lower the characteristic V_max of the enzyme.

Question 10

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] >> K_m. C)the initial velocity of the catalyzed reaction at low [S]. D)the K_m for the substrate. E)both A and B.

Accepted Answer

The turnover number (k_cat) is calculated using the maximum reaction velocity (V_max) and the enzyme concentration. V_max is determined when substrate concentration [S] is much greater than K_m, which ensures the enzyme is saturated with substrate.

Question 11

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

Accepted Answer

Blood coagulation involves zymogen activation (such as prothrombin to thrombin) and the involvement of serine proteases (such as thrombin, factor Xa, and factor IXa) that participate in the coagulation cascade. Therefore, options B and C are correct, making option E the best choice.

Question 12

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.

Accepted Answer

An allosteric inhibitor binds to a site other than the catalytic site of an enzyme, thereby decreasing its activity. This is different from a competitive inhibitor, which competitively binds to the active site, and a transition-state analog, which mimics the transition state of a reaction and binds specifically to the active site. Alternative inhibitor and stereospecific agent are not commonly used terms in the context of enzyme inhibition.

Question 13

V_max 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 K_m. E)is unchanged in the presence of a uncompetitive inhibitor.

Accepted Answer

The answer of V_max for an enzyme-catalyzed reaction: A)generally increases...

Question 14

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 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.

Accepted Answer

Heterotropic allosteric effectors do not compete with substrate for binding sites. Homotropic allosteric effectors do compete with substrate for binding sites.

Question 15

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 structure
E)Residues near the phosphorylation site

Accepted Answer

Disulfide bonds are not typically located near the phosphorylation site and therefore do not have a direct impact on specificity of protein kinases. The primary sequence, protein quaternary and tertiary structure, and residues near the phosphorylation site have all been shown to contribute to specificity of protein kinase activity.

Question 16

Penicillin and related drugs inhibit the enzyme ;this enzyme is produced by .
A)($\beta$-lacamase;bacteria)
B)transpeptidase;human cells
C)transpeptidase;bacteria
D)lysozyme;human cells
E)aldolase;bacteria

Accepted Answer

Penicillin and related drugs specifically target the transpeptidase enzyme found in bacterial cell walls, inhibiting the bacterial cell wall synthesis and leading to bacterial death. This enzyme is produced by bacteria, not human cells, making option C the correct choice.

Question 17

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.

Accepted Answer

Transition-state analogs are designed to resemble the transition-state structure of the normal enzyme-substrate complex, not the substrate itself, which allows them to bind more tightly to the enzyme than the substrate does.

Question 18

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 an allosteric modulator,cAMP,causing dissociation into active trypsin monomers. E)Two inactive trypsinogen dimers pair to form an active trypsin tetramer.

Accepted Answer

Proteolysis of trypsinogen, usually by an enzyme called enterokinase or enteropeptidase, cleaves off a section of the molecule to yield active trypsin. This process is important for regulating the amount of trypsin in the digestive system and preventing premature activation.

Question 19

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.

Accepted Answer

The larger size of glucose allows for better binding to the enzyme, inducing a conformational change that brings active-site amino acids into position for catalysis. This increased binding affinity results in glucose being a better substrate for the enzyme than water.

Question 20

Define the terms "cofactor" and "coenzyme."

Accepted Answer

The answer of Define the terms "cofactor" and "coenzyme."...

Quiz 6: Enzymes

Allosteric enzymes:

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

In a plot of l/V against 1/[S] for an enzyme-catalyzed reaction,the presence of a competitive inhibitor will alter the:

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 good transition-state analog:

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?

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:

The allosteric enzyme ATCase is regulated by CTP,which binds to the T-state of ATCase.CTP is a:

In competitive inhibition,an inhibitor:

To calculate the turnover number of an enzyme,you need to know:

Blood coagulation involves:

A small molecule that decreases the activity of an enzyme by binding to a site other than the catalytic site is termed a(n) :

V_max for an enzyme-catalyzed reaction:

Which of the following statements about allosteric control of enzymatic activity is false?

Which of the following has not been shown to play a role in determining the specificity of protein kinases?

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

A transition-state analog:

How is trypsinogen converted to trypsin?

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:

Define the terms "cofactor" and "coenzyme."

Quiz 6: Enzymes

Allosteric enzymes:

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

In a plot of l/V against 1/[S] for an enzyme-catalyzed reaction,the presence of a competitive inhibitor will alter the:

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 good transition-state analog:

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?

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:

The allosteric enzyme ATCase is regulated by CTP,which binds to the T-state of ATCase.CTP is a:

In competitive inhibition,an inhibitor:

To calculate the turnover number of an enzyme,you need to know:

Blood coagulation involves:

A small molecule that decreases the activity of an enzyme by binding to a site other than the catalytic site is termed a(n) :

Vmax for an enzyme-catalyzed reaction:

Which of the following statements about allosteric control of enzymatic activity is false?

Which of the following has not been shown to play a role in determining the specificity of protein kinases?

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

A transition-state analog:

How is trypsinogen converted to trypsin?

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:

Define the terms "cofactor" and "coenzyme."

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

V_max for an enzyme-catalyzed reaction: