7. Energy and Metabolism

Enzyme Inhibition

Enzymes, Feedback Inhibition, and Allosteric Regulation

 No. In feedback inhibition, accumulating product inhibits one of the early enzymes in the metabolic pathway. 

 No. In this form of competition, a molecule similar to the substrate blocks the active site. 

 No. Binding of a noncompetitive inhibitor would stabilize the enzyme in a form that is less active, not more active. 

 No. In cooperativity, binding occurs at the active site. The binding of the substrate to one active site increases the affinity of other active sites on the same enzyme for the substrate. 

Think about the different mechanisms by which enzyme activity can be regulated.

The process of stabilizing the structure of an enzyme in its active form by the binding of a molecule outside the active site is an example of __________.

A Level Biology Revision "Competitive Inhibitors"

 They are sensitive to environmental conditions. 

 No. Although this is a true statement, is it the only correct statement? 

 They exist in active and inactive conformations. 

 None of the first three listed responses is correct. 

 No. All of the first three statements are true of allosteric proteins. 

 All of the first three listed responses are correct. 

Recall the characteristics of allosteric regulation of proteins.

Which of the following statements about allosteric proteins is/are true?

 Succinylcholine must be an allosteric regulator for this enzyme. 

 No. Although this is remotely possible, the observations are much more consistent with a different role for succinylcholine. 

 The active site must have the wrong configuration to permit succinylcholine binding. 

 No. The observations are consistent with acetylcholine and succinylcholine competing for the active site of the enzyme. 

 Succinylcholine must be a noncompetitive inhibitor. 

 No. Noncompetitive inhibition occurs when a molecule dissimilar to the substrate interacts with the enzyme at a site that is separate from the active site. If succinylcholine were a noncompetitive inhibitor, then the increased rate of acetylcholine seen upon subsequent addition of acetylcholine would not have been observed. 

 The presence of succinylcholine changes the conditions in the solution, resulting in a denaturation of the enzyme. 

 No. If this explanation were correct, then adding more acetylcholine would not restore the original rate. 

 Succinylcholine must be a competitive inhibitor with acetylcholine. 

The nearly identical structures of succinylcholine and acetylcholine are key to answering this question.

Succinylcholine is structurally almost identical to acetylcholine. If succinylcholine is added to a mixture that contains acetylcholine and the enzyme that hydrolyzes acetylcholine (but not succinylcholine), the rate of acetylcholine hydrolysis is decreased. Subsequent addition of more acetylcholine restores the original rate of acetylcholine hydrolysis. Which of the following correctly explains this observation?

 The allosteric inhibitor binds to the active site, preventing the substrate from binding. 

 No. Allosteric regulators bind at sites that are different from the active site. This statement describes how a competitive inhibitor behaves. 

 The allosteric inhibitor causes free energy change of the reaction to increase. 

 No. The allosteric inhibitor will cause a change in the shape of the active site of the enzyme. 

 The allosteric inhibitor lowers the temperature of the active site. 

 No. The temperature of the active site is the same as the environment that surrounds it. 

 The allosteric inhibitor binds to the substrate and prevents it from binding at the active site. 

 No. Although anything that binds to the substrate would alter its ability to bind at the active site, this is not the way that allosteric regulators work. 

 The allosteric inhibitor causes a structural change in the enzyme that prevents the substrate from binding at the active site. 

Think about how an allosteric regulator works to change enzyme activity.

The binding of an allosteric inhibitor to an enzyme causes the rate of product formation by the enzyme to decrease. Which of the following best explains why this decrease occurs?

BioInteractive: Gleevec

By binding to the active site of the enzyme, thus preventing binding of the normal substrate.

By binding to an allosteric site, thus changing the shape of the active site of the enzyme.

By decreasing the free-energy change of the reaction catalyzed by the enzyme.

By binding to the substrate, thus changing its shape so that it no longer binds to the active site of the enzyme.

How does a noncompetitive inhibitor decrease the rate of an enzyme-catalyzed reaction?a) By binding to the active site of the enzyme, thus preventing binding of the normal substrate.b) By binding to an allosteric site, thus changing the shape of the active site of the enzyme.c) By decreasing the free-energy change of the reaction catalyzed by the enzyme.d) By binding to the substrate, thus changing its shape so that it no longer binds to the active site of the enzyme.

Animation: Enzymes: Noncompetitive Inhibition

 A competitive inhibitor binds to the enzyme at a place that is separate from the active site. 

 No. A competitive inhibitor competes with the substrate for binding at the active site. 

 A noncompetitive inhibitor does not change the shape of the active site. 

 No. A noncompetitive inhibitor changes the shape of the enzyme and its active site by binding to a site different from the active site. 

 When the product of an enzyme or an enzyme sequence acts as its inhibitor, this is known as positive feedback. 

 No. This is an example of negative feedback. 

 Inhibition of enzyme function by compounds that are not substrates is something that only occurs under controlled conditions in the laboratory. 

 No. Almost all biological reactions or pathways are regulated by noncompetitive or allosteric interactions. 

 The action of competitive inhibitors may be reversible or irreversible. 

Think about the different ways that enzyme inhibitors work.

Which of these statements about enzyme inhibitors is true?

A Level Biology "Non-Competitive Inhibitors"

A competitive inhibitor binds to the substrate and inhibits it from binding to the active site of the enzyme.

A competitive inhibitor binds to a site other than the active site and inhibits the substrate from binding.

A competitive inhibitor binds to the active site and degrades the enzyme.

A competitive inhibitor binds to the active site of an enzyme and inhibits the substrate to bind.

Which of the following statements correctly describes competitive inhibition?a) A competitive inhibitor binds to the substrate and inhibits it from binding to the active site of the enzyme.b) A competitive inhibitor binds to a site other than the active site and inhibits the substrate from binding.c) A competitive inhibitor binds to the active site and degrades the enzyme.d) A competitive inhibitor binds to the active site of an enzyme and inhibits the substrate to bind.

Enzyme Examples, Cofactors/Coenzymes, Inhibitors, and Feedback Inhibition

Enzyme Function and Inhibition

Enzyme Inhibition IB Biology

Animation: Enzymes: Competitive Inhibition

Which of the following types of enzyme inhibition is overcome by increasing the substrate concentration?a) The need for a coenzyme.b) Noncompetitive inhibition.c) Competitive inhibition.d) None of the above.

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