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MCAT Question A Day - 8/11/13 - Answer!

Substance A is produced during a reaction catalyzed by Enzyme B. As the concentration of Substance A increases, it binds to Enzyme B, causing it to change its shape and become inactive. This is an example of

I.  feedback inhibition only.
II.  allosteric inhibition only.
III.  competitive inhibition only.

A.  I and II
B.  I and III
C.  I, II, and III

The correct answer is (A). When the presence of a substance prevents an enzyme from further catalyzing the reaction that produced that substance, this is called feedback inhibition. In addition, when a substance binds to an enzyme in a place other than its active site, in a way that causes the inactivation of the enzyme, this is called allosteric inhibition. Competitive inhibition occurs when a substance binds to an enzyme’s active site so that its normal substrate cannot bind there.