Question 1

Which of the following statements is true of enzymes?
A)Required nonprotein cofactors generally alter the substrate specificity of enzymes.
B)Enzyme function is generally increased if the three-dimensional conformation of an enzyme is altered.
C)Enzyme function is generally independent of physical and chemical environmental factors such as pH and temperature.
D)Enzymes generally increase the rate of chemical reactions by lowering activation energy barriers.

Accepted Answer

Enzymes generally increase the rate of chemical reactions by lowering activation energy barriers. This is achieved through the enzyme's active site, which binds to the substrate and helps to facilitate the reaction. Enzyme activity can be affected by physical and chemical environmental factors, such as pH and temperature, but this does not necessarily mean that enzyme function is independent of these factors. Some enzymes also require nonprotein cofactors, which can either enhance or alter the enzyme's substrate specificity. However, this is not a universal requirement for all enzymes. Altering the three-dimensional conformation of an enzyme can also have varying effects on enzyme function, depending on the specific enzyme and the nature of the conformational change.

Question 2

The activity of many cellular enzymes is regulated by activators and inhibitors.Enzyme activity is also regulated in eukaryotic cells by which of the following mechanisms?
A)compartmentalization and restricting enzymes to specific organelles or membranes
B)limiting the availability of substrates
C)changing the primary structure of enzymes
D)secreting enzymes out of the cell

Accepted Answer

Compartmentalization and restricting enzymes to specific organelles or membranes is a key regulatory mechanism in eukaryotic cells, allowing for spatial control of enzyme activity and metabolic pathways.

Question 3

Figure 6.1 -Which of the following is the most correct interpretation of Figure 6.1? A)Inorganic phosphate is created from organic phosphate. B)Energy from catabolism can be used directly for performing cellular work. C)ADP + ℗_i are a set of molecules that store energy for catabolism. D)ATP is a molecule that acts as an intermediary to store energy for cellular work.

Accepted Answer

The figure shows the conversion of ADP to ATP and vice versa, indicating that ATP acts as an intermediary for storing energy that can be used for cellular work. Option A is incorrect as the figure does not depict the creation of inorganic phosphate from organic phosphate. Option B is incorrect as the figure only shows the conversion of ADP to ATP and does not directly demonstrate the use of energy for cellular work. Option C is incorrect as ADP and ℗_i are not shown to be a set of molecules that store energy for catabolism in the figure.

Question 4

In experimental tests of enzyme evolution,a gene encoding an enzyme was subjected to multiple cycles of random mutagenesis and selection for altered substrate specificity.The resulting enzyme with altered substrate specificity had multiple amino acid changes.Where in the enzyme might these amino acid changes be located?
A)only in or near the active site
B)only in the hydrophobic interior of the folded protein
C)only on surface sites distant from the active site
D)in or near the active site,in the hydrophobic interior,and/or at surface sites distant from the active site

Accepted Answer

The amino acid changes could be located in or near the active site, as this is where interactions with the substrate occur. They could also be in the hydrophobic interior, as this region helps to maintain protein folding and stability. Additionally, changes could occur at surface sites distant from the active site, as these can affect protein-protein interactions and overall stability. Therefore, the changes could be located in any of these regions, making D the best choice.

Question 5

The mechanism by which the end product of a metabolic pathway inhibits an enzyme that catalyzes an earlier step in the pathway is most precisely described as
A)competitive inhibition.
B)cooperativity inhibition.
C)irreversible inhibition.
D)feedback inhibition.

Accepted Answer

Feedback inhibition occurs when the end product of a metabolic pathway inhibits an enzyme that catalyzes an earlier step in the pathway, thereby regulating the overall pathway and preventing the accumulation of excessive levels of the end product. This is a reversible form of inhibition, as the inhibition is relieved when the concentration of the end product decreases. Competitive inhibition involves a molecule that competes with the substrate for binding to the active site of the enzyme, cooperativity inhibition involves other molecules binding to the enzyme and affecting its affinity for the substrate, and irreversible inhibition involves a molecule binding covalently to the enzyme and permanently deactivating it.

Question 6

Cooperativity is a form of allosteric activation in which
A)the product of a metabolic pathway serves as a competitive inhibitor of an early enzyme in the pathway.
B)all of the enzymes in a metabolic pathway are contained within a single multienzyme complex.
C)completion of one step in a metabolic pathway is required before a subsequent step can occur.
D)binding of a substrate molecule to one active site in a multisubunit enzyme stimulates the binding of substrate molecules to the active sites of other subunits.

Accepted Answer

Cooperativity occurs when binding of a substrate molecule to one active site in a multisubunit enzyme stimulates the binding of substrate molecules to the active sites of other subunits. This results in a positive feedback loop where the binding of one substrate molecule increases the likelihood of other substrate molecules binding to the enzyme, leading to a rapid increase in enzyme activity.

Question 7

An aminoacyl-tRNA synthetase is the enzyme that catalyzes the attachment of a particular amino acid to its corresponding tRNA.This reaction requires energy from ATP.The enzyme initially binds the amino acid and ATP,but it is unable to bind the tRNA at this point.Which of the following would be a likely next step in the reaction by which the enzyme ultimately binds the tRNA and attaches the amino acid?
A)Transfer of the ATP to the tRNA opens the active site,which allows the tRNA to bind.
B)Hydrolysis of ATP activates the amino acid,which is released,opening up the active site to allow binding of the tRNA,followed by binding of a new amino acid.
C)Hydrolysis of ATP phosphorylates the amino acid,which results in a conformational change in the active site that allows the tRNA to bind.
D)Alteration in the conformation of the tRNA allows it to bind to the active site along with the amino acid and ATP.

Accepted Answer

Hydrolysis of ATP phosphorylates the amino acid, resulting in a conformational change in the active site of the aminoacyl-tRNA synthetase. This new conformation of the active site allows the tRNA to bind and be attached to the amino acid.

Question 8

A mutation results in an amino acid substitution at a site distant from the active site of an enzyme.How might this amino acid change alter the active site and substrate specificity of the enzyme?
A)by changing the optimum temperature for the enzyme
B)by changing the intracellular location of the enzyme
C)by changing the binding site for an allosteric regulator
D)by changing the conformation of the enzyme

Accepted Answer

Amino acid substitutions can lead to changes in the enzyme's conformation, which can ultimately impact the active site and the enzyme's ability to interact with substrate(s). It is possible that this change could alter the substrate specificity, making the enzyme less or more effective at catalyzing the reaction of interest. Options A, B, and C are less likely to have a direct impact on the active site and substrate specificity of the enzyme.

Question 9

-In Figure 6.2,why does the reaction rate plateau at higher reactant concentrations?
A)Feedback inhibition by the product occurs at high reactant concentrations.
B)Most enzyme molecules are occupied by substrate at high reactant concentrations.
C)The reaction nears equilibrium at high reactant concentrations.
D)The activation energy for the reaction increases with reactant concentration.

Accepted Answer

At high reactant concentrations, most enzyme molecules are occupied by substrate, leading to a situation where adding more substrate does not increase the reaction rate because all the active sites of the enzymes are already full. This is known as enzyme saturation.

Question 10

Under a particular set of conditions in the lab,the enzyme in a chemical reaction is saturated.Which of the following alterations to the reaction will increase the rate at which substrate is converted to product?
A)increasing the concentration of substrate in the reaction
B)increasing the amount of enzyme in the reaction
C)increasing the volume of the reaction without increasing the amount of substrate or enzyme
D)decreasing the temperature of the reaction

Accepted Answer

When an enzyme is saturated, it means that all of its active sites are occupied by substrate molecules. Increasing the concentration of substrate will not increase the reaction rate because there are no free active sites on the enzyme. Increasing the amount of enzyme will provide more active sites for the substrate to bind to, thus increasing the rate of reaction. Increasing the volume of the reaction without increasing the amount of substrate or enzyme simply dilutes the reaction mixture, which does not affect the rate of reaction. Decreasing the temperature generally slows down reactions by decreasing the kinetic energy of the molecules involved.

Question 11

Cells use the ATP cycle shown in Figure 6.1 to perform which of the following processes?
A)recycle ADP and phosphate
B)recycle energy released by ATP hydrolysis
C)recycle the energy used for cellular work
D)move energy from ATP to ADP

Accepted Answer

Cells use the ATP cycle to recycle ADP and phosphate, which are the products of ATP hydrolysis, allowing them to be reassembled into ATP for further energy use.

Question 12

A noncompetitive inhibitor decreases the rate of an enzymatic reaction by
A)binding to the active site of the enzyme.
B)changing the ∆G for the reaction.
C)changing the shape of the enzyme active site.
D)decreasing the activation energy required for the reaction.

Accepted Answer

A noncompetitive inhibitor binds to a site on the enzyme that is not the active site, causing a conformational change in the enzyme that alters the shape of the active site. This prevents the substrate from binding to the enzyme and decreases the rate of the reaction. Choices A, B, and D all refer to mechanisms of competitive inhibition.

Question 13

An enzyme-catalyzed reaction is conducted in a test tube with a fixed amount of enzyme.Increasing the enzyme concentration in the test tube may overcome the effect of which of the following conditions?
A)denaturation of the enzyme
B)presence of a fixed amount of an allosteric inhibitor
C)presence of a fixed amount of a competitive inhibitor
D)a saturated enzyme population

Accepted Answer

A saturated enzyme population means that all enzyme active sites are already occupied by substrate molecules, so increasing the enzyme concentration would increase the number of active sites available for reaction and therefore overcome the saturation. Options A, B, and C are not affected by increasing the enzyme concentration and require other solutions to overcome their effects.

Question 14

Zinc,an essential trace element for most organisms,is required in the active site of the enzyme carboxypeptidase in order for the enzyme to function properly.The zinc most likely functions as a(n)
A)competitive inhibitor of the enzyme.
B)noncompetitive inhibitor of the enzyme.
C)allosteric activator of the enzyme.
D)cofactor necessary for enzyme activity.
E)coenzyme derived from a vitamin.

Accepted Answer

Zinc is serving here as a cofactor, which is a non-protein chemical compound required for enzyme activity. Without zinc, the enzyme carboxypeptidase won't function properly. Therefore, zinc is necessary for enzyme activity.

Question 15

-For the enzyme-catalyzed reaction shown in Figure 6.2,which of these treatments will cause the greatest increase in the rate of the reaction if the initial reactant concentration is 1.0 micromolar?
A)doubling the activation energy needed
B)cooling the reaction by 10°C
C)doubling the concentration of the reactants to 2.0 micromolar,while reducing the concentration of enzyme by half
D)doubling the enzyme concentration

Accepted Answer

Doubling the enzyme concentration increases the number of active sites available for the reaction, thus increasing the reaction rate. Changes in temperature, activation energy, or reducing enzyme concentration while increasing reactant concentration generally do not increase the reaction rate as effectively as increasing enzyme concentration.

Question 16

Why might a severe fever result in death if it is not brought under control?
A)It may destroy the primary structure of cellular enzymes.
B)It may alter the tertiary and quaternary structures of cellular enzymes.
C)It may increase the rate of cellular chemical reactions.
D)It may result in consumption of excess ATP.

Accepted Answer

A severe fever can cause changes in the tertiary and quaternary structures of cellular enzymes, which alters their function and can lead to cell dysfunction or death. This can ultimately result in organ failure and death if the fever is not brought under control.

Question 17

Which of the following is an example of cooperativity?
A)binding of an ATP molecule along with another substrate in an active site
B)binding of a molecule to one subunit of an enzyme,which promotes faster binding to each of the other subunits of the enzyme
C)the product of one enzyme in a metabolic pathway serving as the substrate for the next enzyme in the pathway
D)binding of the end product of a metabolic pathway to the enzyme that catalyzes the first step in the pathway

Accepted Answer

Cooperativity refers to the phenomenon where the binding or activity of one subunit of a protein influences the binding or activity of another subunit in the protein. In option B, binding of a molecule to one subunit of an enzyme promotes faster binding to each of the other subunits, which is an example of cooperativity. Option A describes substrate-level phosphorylation and option C describes a metabolic pathway, but neither involves cooperativity. Option D describes feedback inhibition, which is not an example of cooperativity.

Question 18

Figure 6.3 Activity of various enzymes (a)at various temperatures and (b)at various pH.
-Which curves on the graphs in Figure 6.3 may represent the temperature and pH profiles of an enzyme taken from a bacterium that lives in a mildly alkaline hot spring at temperatures of 70°C or higher?
A)curves 1 and 5
B)curves 2 and 4
C)curves 2 and 5
D)curves 3 and 4
E)curves 3 and 5

Accepted Answer

The answer of  
  Figure 6.3 Activity of...

Question 19

An enzyme-catalyzed reaction is conducted in a test tube with a fixed amount of enzyme.Increasing the substrate concentration in the test tube may overcome the effect of which of the following conditions?
A)denaturation of the enzyme
B)presence of a fixed amount of an allosteric inhibitor
C)presence of a fixed amount of a competitive inhibitor
D)a saturated enzyme population

Accepted Answer

Increasing the substrate concentration can overcome the effect of a competitive inhibitor because both the substrate and the inhibitor compete for the same active site on the enzyme. With more substrate present, the likelihood of substrate molecules binding to the enzyme instead of the inhibitor increases, thus mitigating the inhibitor's effect.

Question 20

Some of the drugs used to treat HIV patients are competitive inhibitors of the HIV reverse transcriptase enzyme.Unfortunately,the high mutation rate of HIV means that the virus rapidly acquires mutations with amino acid changes that make them resistant to these competitive inhibitors.Where in the reverse transcriptase enzyme would such amino acid changes most likely occur in drug-resistant viruses?
A)in or near the active site
B)at an allosteric site
C)at a cofactor binding site
D)in regions of the enzyme that determine packaging into the virus capsid

Accepted Answer

Amino acid changes that make viruses resistant to competitive inhibitors would most likely occur in or near the active site of the HIV reverse transcriptase enzyme, which is the site where the drugs bind and inhibit the enzyme. Mutations in this region would disrupt the binding of the drug and enable the virus to continue replicating.

Quiz 6: An Introduction to Metabolism

Which of the following statements is true of enzymes?

The activity of many cellular enzymes is regulated by activators and inhibitors.Enzyme activity is also regulated in eukaryotic cells by which of the following mechanisms?

Figure 6.1 -Which of the following is the most correct interpretation of Figure 6.1?

The mechanism by which the end product of a metabolic pathway inhibits an enzyme that catalyzes an earlier step in the pathway is most precisely described as

Cooperativity is a form of allosteric activation in which

A mutation results in an amino acid substitution at a site distant from the active site of an enzyme.How might this amino acid change alter the active site and substrate specificity of the enzyme?

-In Figure 6.2,why does the reaction rate plateau at higher reactant concentrations?

Under a particular set of conditions in the lab,the enzyme in a chemical reaction is saturated.Which of the following alterations to the reaction will increase the rate at which substrate is converted to product?

Cells use the ATP cycle shown in Figure 6.1 to perform which of the following processes?

A noncompetitive inhibitor decreases the rate of an enzymatic reaction by

An enzyme-catalyzed reaction is conducted in a test tube with a fixed amount of enzyme.Increasing the enzyme concentration in the test tube may overcome the effect of which of the following conditions?

Zinc,an essential trace element for most organisms,is required in the active site of the enzyme carboxypeptidase in order for the enzyme to function properly.The zinc most likely functions as a(n)

-For the enzyme-catalyzed reaction shown in Figure 6.2,which of these treatments will cause the greatest increase in the rate of the reaction if the initial reactant concentration is 1.0 micromolar?

Why might a severe fever result in death if it is not brought under control?

Which of the following is an example of cooperativity?

Figure 6.3 Activity of various enzymes (a) at various temperatures and (b) at various pH. -Which curves on the graphs in Figure 6.3 may represent the temperature and pH profiles of an enzyme taken from a bacterium that lives in a mildly alkaline hot spring at temperatures of 70°C or higher?

An enzyme-catalyzed reaction is conducted in a test tube with a fixed amount of enzyme.Increasing the substrate concentration in the test tube may overcome the effect of which of the following conditions?