Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologymediummcq1 pt

An enzyme works best near pH 7. Moving it to pH 2 sharply reduces its activity, but activity fully recovers when the solution is returned to pH 7. Which explanation fits these observations?

A.Peptide bonds throughout the enzyme hydrolyzed rapidly and then reformed when conditions improved.
B.The substrate changed shape at low pH and never regained the ability to bind.
C.Cold temperatures slowed molecular motion until warming restored the original rate.
D.Extra protons temporarily altered the ionization of side chains that stabilize the active site.

Explanation

Core Concept

Side chains such as histidine, aspartate, and lysine gain or lose protons depending on pH. At very low pH, extra protonation disrupts the ionic attractions and hydrogen bonds holding the active site in its productive shape, so substrate binding and catalysis collapse even though covalent bonds remain intact. Restoring neutral pH removes the excess protons, the stabilizing interactions re-form, and the native conformation returns along with full activity. Reversible charge changes rather than permanent destruction explain the observed pattern.

Correct Answer

DExtra protons temporarily altered the ionization of side chains that stabilize the active site.

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