19. Enzymes

Which of the following inhibitors could be a reversible competitive inhibitor for acetylcholinesterase if its substrate is acetylcholine?

How can you distinguish between a competitive inhibitor and an uncompetitive inhibitor experimentally?

Why are irreversible enzyme inhibitors referred to as poisons?

The text discusses three forms of enzyme inhibition: uncompetitive inhibition, competitive inhibition, and irreversible inhibition.

b. What kinds of bonds are formed between an enzyme and each of these three kinds of inhibitors?

What kind of inhibition (uncompetitive, competitive, or irreversible) is present in each of the following:

a. Penicillin is used to treat certain bacterial infections. Penicillin is effective because it binds to the enzyme glycopeptide transpeptidase and does not dissociate.

What kind of inhibition (uncompetitive, competitive, or irreversible) is present in each of the following:

c. The antibiotic deoxycycline inhibits the bacterial enzyme collagenase, slowing bacterial growth. Deoxycycline does not fit into the active site of collagenase and binds elsewhere on the enzyme.

One mechanism by which lead exerts its poisonous effect on enzymes can be stopped by chelation therapy with EDTA. Describe this type of lead poisoning and explain why it is reversible.

What kind of reaction product might be a competitive inhibitor for the enzyme that catalyzes its formation?

Explain how the following changes affect the rate of an enzyme-catalyzed reaction in the presence of an uncompetitive inhibitor:

(a) increasing the substrate concentration at a constant inhibitor concentration

Explain how the following changes affect the rate of an enzyme-catalyzed reaction in the presence of an uncompetitive inhibitor:

(b) decreasing the inhibitor concentration at a constant substrate concentration.

Indicate whether each of the following statements describes primary, secondary, tertiary, or quaternary protein structure: 

b. Protein chains of collagen form a triple helix.

Indicate whether each of the following describes a competitive or a noncompetitive enzyme inhibitor:

d. The structure of the inhibitor is not similar to the substrate.

Methanol and ethanol are oxidized by alcohol dehydrogenase. In methanol poisoning, ethanol is given intravenously to prevent the formation of formaldehyde that has toxic effects.

b. Would ethanol compete for the active site or bind to a different site?

In humans, the antibiotic amoxicillin (a type of penicillin) is used to treat certain bacterial infections.

a. Does the antibiotic inhibit enzymes in humans?

Oxaloacetate is an inhibitor of succinate dehydrogenase. 

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a. Would you expect oxaloacetate to be a competitive or a noncompetitive inhibitor? Why?

In humans, the antibiotic amoxicillin (a type of penicillin) is used to treat certain bacterial infections.

c. Is amoxicillin a reversible or irreversible inhibitor?

Ethylene glycol (HO—CH₂—CH₂—OH) is a major component of antifreeze. If ingested, it is first converted to HOOC—CHO (oxoethanoic acid) and then to HOOC—COOH (oxalic acid), which is toxic. (16.4, 16.5).

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b. The treatment for the ingestion of ethylene glycol is an intravenous solution of ethanol. How might this help prevent toxic levels of oxalic acid in the body?

How does an irreversible inhibitor function differently than a reversible inhibitor?

Indicate whether each of the following describes a competitive or a noncompetitive inhibitor.

e. Adding more substrate to the reaction restores the enzyme activity.

Indicate whether each of the following describes a competitive or a noncompetitive inhibitor.

a. The structure of the inhibitor is similar to that of the substrate.

Indicate whether each of the following describes a competitive or a noncompetitive inhibitor.

c. The inhibitor competes with the substrate for the active site.