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Free MCAT Practice Question, Round 3 – Biology

MCAT Sample Question

The MCAT loves to ask about enzymes and cells – the building blocks of life.  When it comes to enzymes, a popular topic is how enzymes function. Try your hand at this sample question:


Item 7

Carbonic anhydrase is the enzyme in the human circulatory system that catalyzes the following reaction:

CO_{2}+H_{2}O rightleftharpoons H_{2}CO_{3}

This enzyme functions best at an approximately neutral pH. When placed in an acidic environment, the catalytic activity of carbonic anhydrase decreases. The most likely reason for this is:

A) the instability of carbonic acid at low pH.

B) reduced sythesis of carbonic anhydrase by the ribosomes of the rough endoplasmic reticulum.

C) the structure of the enzyme is changed.

D) the increased protonation of water into hydronium ion.



Every enzyme has a particular pH and temperature range that are optimal. If you deviate from those ranges, the enzyme starts to denature – its quaternary and tertiary structures begin to break down. Needless to say, that denaturation causes enzymatic activity to drop off rapidly.

In this case, if you place an enzyme that normally prefers a slightly basic pH (as is found in the blood) and put it in an acidic environment, it’s going to lose its overall structure, making (C) the right answer.

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