Chapter 8 How Enzymes Work Answers

In general, enzymes are what kinds of molecules?

Answer: proteins

Enzymes work by _____.

 Answer: reducing EA

An enzyme _____.

 Answer:  is an organic catalyst

What name is given to the reactants in an enzymatically catalyzed reaction?

 Answer: substrate

As a result of its involvement in a reaction, an enzyme _____.

 Answer:  is unchanged

Under what conditions could some exothermic (ΔH<0) reactions be nonspontaneous (ΔG>0)?

A) The overall reaction reduces entropy (products have less entropy than reactants).

B) The overall reaction results in an increase in entropy (products have greater entropy than reactants).

C) The products have greater potential energy than the reactants.

D) The products and reactants have equal entropy.

 Answer:  A) The overall reaction reduces entropy (products have less entropy than reactants).

When is the overall free energy change ΔG in a reaction most likely to be negative (meaning that the reaction is exergonic)?

A) When products have higher potential energy and higher entropy than reactants

B) When products have lower potential energy and lower entropy than reactants

C) When products have higher potential energy and lower entropy than reactants

D) When products have lower potential energy and higher entropy than reactants

 Answer:  D) When products have lower potential energy and higher entropy than reactants

Living organisms increase in complexity as they grow, resulting in a decrease in the entropy of an organism. How does this relate to the second law of thermodynamics?

A) Living organisms do not follow the laws of thermodynamics.

B) Living organisms do not obey the second law of thermodynamics, which states that entropy must increase with time.

C) Life obeys the second law of thermodynamics because the decrease in entropy as the organism grows is exactly balanced by an increase in the entropy of the universe.

D) As a consequence of growing, organisms cause a greater increase in entropy in their environment than the decrease in entropy associated with their growth.

E) Living organisms are able to transform energy into entropy.

 Answer: D) As a consequence of growing, organisms cause a greater increase in entropy in their environment than the decrease in entropy associated with their growth.

What type of reaction breaks the bonds that join the phosphate groups in an ATP molecule?

A) Dehydration synthesis

B) Dehydration decomposition

C) Entropic

D) Hydrolysis

E) Anabolism

 Answer: D) Hydrolysis

Hydrolysis involves breaking bonds with the addition of water.

In general, enzymes are what kinds of molecules?

A) Minerals

B) Lipids

C) Carbohydrates

D) Proteins

E) Nucleic acids

 Answer: D) Proteins

Enzymes are proteins.

Enzymes work by _____.

A) Reducing EA

B) Increasing the potential energy difference between reactant and product

C) Adding a phosphate group to a reactant

D) Decreasing the potential energy difference between reactant and product

E) Adding energy to a reaction

 Answer: A) Reducing EA

Enzymes work by reducing the energy of activation

An enzyme _____.

A) Is a inorganic catalyst

B) Is a source of energy for endergonic reactions

C) Is an organic catalyst

D) Can bind to nearly any molecule

E) Increases the EA of a reaction

 Answer:  C) Is an organic catalyst

Enzymes are proteins that behave as catalysts.

What name is given to the reactants in an enzymatically catalyzed reaction?

A) Products

B) Active sites

C) Substrate

D) EA

E) Reactors

 Answer:  C) Substrate

This is the name given to the reactants in an enzymatically catalyzed reaction.

As a result of its involvement in a reaction, an enzyme _____.

A) Is unchanged

B) Permanently alters its shape.

C) Loses a phosphate group

D) Is used up

E) Loses energy

 Answer:  A) Is unchanged

Enzymes are not changed as a result of their participation in a reaction.

What is the correct label for “A”?

A) ATP

B) Energy of activation

C) Substrate energy

D) Uphill

E) Enzyme energy

 Answer:  B) Energy of activation

The energy of activation must be overcome in order for a reaction to proceed.

Can an enzyme make a nonspontaneous reaction occur spontaneously? Why or why not?

A) No, because enzymes do not lower the activation energy of the reaction.

B) Yes, because enzymes lower the activation energy.

C) No, because enzymes do not affect the overall ΔG of a reaction.

D) Yes, because enzymes lower the overall ΔG of a reaction.

 Answer:  C) No, because enzymes do not affect the overall ΔG of a reaction.

You have discovered an enzyme that can catalyze two different chemical reactions. Which of the following is most likely to be correct?

A) Either the enzyme has two distinct active sites or the reactants involved in the two reactions are very similar in size and shape.

B) The enzyme is subject to competitive inhibition and allosteric regulation.

C) The enzyme contains α-helices and β-pleated sheets.

D) Two types of allosteric regulation occur: The binding of one molecule activates the enzyme, while the binding of a different molecule inhibits it.

 Answer: A) Either the enzyme has two distinct active sites or the reactants involved in the two reactions are very similar in size and shape.

The lock-and-key analogy for enzymes applies to the specificity of enzymes _____.

A) As they form their tertiary structure

B) Binding to their substrate

C) Interacting with water

D) As they form their tertiary and quaternary structure

E) Interacting with ions

 Answer: B) Binding to their substrate

Complete this vocabulary exercise relating to the three types of enzyme inhibitors.

Drag the words on the left to the appropriate blanks in the sentences on the right. Each word is used only once.

 Answer: Competitive inhibitors compete physically and structurally with the substrate for an enzyme’s active site; they can be outcompeted by adding extra substrate. Noncompetitive inhibitors do not compete for the active site, but inhibit the enzyme by binding elsewhere and changing the enzyme’s shape. Irreversible inhibitors bind directly to the active site by covalent bonds, which change the structure of the enzyme and inactivate it permanently. Most medications are enzyme inhibitors of one kind or another.

A(n) ________ inhibitor has a structure that is so similar to the substrate that it can bond to the enzyme just like the substrate.

 Answer: competitive

A(n) ________ inhibitor binds to a site on the enzyme that is not the active site.

 Answer:  noncompetitive

Usually, a(n) _______ inhibitor forms a covalent bond with an amino acid side group within the active site, which prevents the substrate from entering the active site or prevents catalytic activity.

 Answer:  irreversible

The competitive inhibitor competes with the substrate for the _________ on the enzyme.

 Answer: active site

When the noncompetitive inhibitor is bonded to the _________, the shape of the enzyme is distorted.

enzyme

Enzyme inhibitors disrupt normal interactions between an enzyme and its _________.

 Answer:  substrate

You have added an irreversible inhibitor to a sample of enzyme and substrate. At this point, the reaction has stopped completely.

What can you do to regain the activity of the enzyme?A) Removing the irreversible inhibitor should get the reaction working again.

B) The enzyme is inactive at this point. New enzyme must be added to regain enzyme activity.

C) Adding more substrate will increase the rate of reaction.

D) Adding more inhibitor should get the reaction up to speed again.

 Answer:  B) The enzyme is inactive at this point. New enzyme must be added to regain enzyme activity.

Because they bind directly to the active site by covalent bonds, irreversible inhibitors permanently render an enzyme inactive. Some drugs are irreversible inhibitors, including the antibiotic penicillin (which inhibits an enzyme involved in bacterial cell-wall synthesis) and aspirin (which inhibits cyclooxygenase-2, the enzyme involved in the inflammatory reaction).

You have an enzymatic reaction proceeding at the optimum pH and optimum temperature. You add a competitive inhibitor to the reaction and notice that the reaction slows down.

What can you do to speed the reaction up again?A) Add more inhibitor to speed up the reaction.

B) Add more substrate; it will outcompete the inhibitor and increase the reaction rate.

C) Increase the temperature.

D) Increase the pH.

 Answer:  B) Add more substrate; it will outcompete the inhibitor and increase the reaction rate.

Competitive inhibition can be overcome by adding more substrate to outcompete the inhibitor. Many drugs used to treat different medical conditions, including hypertension, are competitive inhibitors. It is fairly easy to make a molecule that is similar in structure to a particular substrate because the known enzyme’s shape can be used as a model of what the molecule needs to look like. It is more difficult to make a noncompetitive inhibitor because it is less obvious what the noncompetitive inhibitor’s shape and structure should be.

An enzyme inhibitor that is roughly the same shape as the substrate and binds at the active site is termed a(n) _____ inhibitor.

A) Catalytic

B) Noncompetitive

C) Competitive

D) Allosteric

 Answer:  C) Competitive

Competitive inhibitors prevent substrate binding by mimicking the substrate and competing for the active site.

A noncompetitive inhibitor decreases the rate of an enzyme reaction by _____.

A) Acting as a coenzyme for the reaction

B) Binding at the active site of the enzyme

C) Changing the free energy change of the reaction

D) Decreasing the activation energy of the reaction

E) Changing the shape of the enzyme’s active site

 Answer:  E) Changing the shape of the enzyme’s active site

How might a change of one amino acid at a site, distant from the active site of an enzyme, alter an enzyme’s substrate specificity?

A) By changing the enzyme’s pH optimum

B) By changing the shape of an enzyme

C) By changing the enzyme’s stability

D) By changing the enzyme’s location in the cell

E) An amino acid change away from the active site cannot alter the enzyme’s substrate specificity

 Answer:  B) By changing the shape of an enzyme

What is a transition state?

A) The place where an allosteric regulatory molecule binds to an enzyme

B) The shape adopted by an enzyme that has an inhibitory molecule bound at its active site

C) An interaction between reactants with high kinetic energy, due to high temperature

D) The complex formed as covalent bonds are being broken and re-formed during a reaction

 Answer:  D) The complex formed as covalent bonds are being broken and re-formed during a reaction

If you were to expose glucose to oxygen on your lab bench, why would you not expect to see it burn as shown in the figure below?

A) Energy is required for the sugar and oxygen to reach their transition state.

B) The reaction is endergonic and requires an input of energy.

C) The sugar must first be phosphorylated to increase its potential energy.

D) The reaction is not spontaneous unless an enzyme is added to the substrates.

 Answer:  A) Energy is required for the sugar and oxygen to reach their transition state.