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

Replacing a positively charged lysine in an enzyme's active site with the nonpolar amino acid leucine leaves the overall fold intact but nearly abolishes substrate binding. Which explanation best accounts for the loss?

A.Leucine blocks the exit channel, so products accumulate and jam the site.
B.Lysine supplied mechanical rigidity that kept the whole protein from collapsing.
C.Leucine raised the activation energy by donating electrons to the substrate.
D.The substrate lost the electrostatic attraction that a positively charged side chain provided at a key docking position.

Explanation

Core Concept

Active sites recognize substrates through many simultaneous weak contacts whose patterns match the substrate's surface. A positively charged lysine can attract a negatively charged group on the substrate through ionic interaction, pinning the molecule in a productive orientation. Swapping in nonpolar leucine removes both the charge and the hydrogen-bonding ability, so that anchor vanishes while the rest of the pocket barely moves. Binding affinity collapses because recognition depends on the sum of these precise contacts, not merely on having an open cavity.

Correct Answer

DThe substrate lost the electrostatic attraction that a positively charged side chain provided at a key docking position.

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