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MCAT Question A Day - 4/10/14 - Answer!

The conversion of threonine to isoleucine is a five-step enzymatic pathway. The end product, isoleucine, fits into the allosteric site of the enzyme at step 1, preventing its normal function. This is an example of

A.  enzyme specificity.
B.  competitive inhibition.
C.  enzyme enhancement.
D.  feedback inhibition.

The correct answer is (D). A basic understanding of enzyme function is needed. When the product of an enzymatic pathway inhibits any of the previous steps in the pathway, this is referred to as feedback inhibition. Enzyme specificity refers to the conditions appropriate for an enzyme to work (substrate, pH, temperature, etc.). Competitive inhibition occurs when a substance occupies the active site that normally binds to the substrate involved in the reaction. The question states that isoleucine fits into the allosteric site.

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