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Chapter 9: Enzymes and Enzyme Kinetics

The interconversion of kJ/mol and kcal/mol in the following questions can be done with the formula: 1kcal/mol = 4.184kJ/mol.

 

You are studying the effects of a new drug on the activity of your favorite enzyme. The results of your assays indicate that the drug acts as a noncompetitive inhibitor. Which of the following changes to the Km and Vmax allowed that determination to be made regarding the drugs actions?

A. Km decreased, Vmax increased

B. Km decreased, Vmax unchanged

C. Km increased, Vmax decreased

D. Km unchanged, Vmax decreased

E. Km unchanged, Vmax unchanged

Answer D: Noncompetitive inhibitors bind to enzyme or ES complex other than at the catalytic site and thus have no effect on substrate binding. However, the resultant ESI complex cannot form products. This type of inhibition cannot be reversed by the addition of more substrate. Under these conditions Km appears unaltered, while Vmax is decreased proportionately to inhibitor concentration.

The drug acetazolamide is used in the treatment of glaucoma since it decreases the production of aqueous fluid, thereby reducing intraocular pressures. This drug is a noncompetitive inhibitor of carbonic anhydrase. Which of the following statements most accurately relates to the actions of this type of inhibitor?

A. decreases both Vmax and apparent Km

B. decreases apparent Km

C. decreases Vmax

D. increases apparent Km

E. increases Vmax

Answer C: Noncompetitive inhibitors bind to enzyme or ES complex other than at the catalytic site and thus have no effect on substrate binding. However, the resultant ESI complex cannot form products. This type of inhibition cannot be reversed by the addition of more substrate. Under these conditions Km appears unaltered while Vmax is decreased proportionately to inhibitor concentration.

Drugs that inhibit enzymes in a competitive manner will have which of the following effects on the Michaelis-Menten equation derived parameters: Vmax and/or Km?

A. decrease both Vmax and Km

B. decrease Km but no effect on Vmax

C. decrease Vmax but no change on Km

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