Question 1

Allosteric enzymes:

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:

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:

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:

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:

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?

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:

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:

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:

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:

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:

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):

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:

Accepted Answer

The answer of V_max for an enzyme-catalyzed reaction: ...

Question 14

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

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?

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 .

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:

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?

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:

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: