In the lock and key model, the active site is described as a keyhole that requires what characteristics?

• A. Complementary size, shape, and charge
• B. The same shape as the receptor
• C. High lipid solubility only
• D. A flexible structure that adapts after binding

Answer: (A)

In the lock-and-key view, the active site acts like a precisely shaped keyhole that only accepts a substrate with the exact complementary features. The substrate must have the right size to fit into the pocket, the right shape to nestle into the active site, and the right charge distribution so that charged or polar groups can form favorable interactions with the residues in the site. These complementary properties—size, shape, and charge—together promote specific binding and proper alignment for catalysis.

That’s why this option is the best: it captures the idea of a rigid, specific fit between enzyme and substrate. Saying the active site has the same shape as the receptor isn’t correct because the binding involves the substrate fitting into the enzyme’s pocket, not matching the receptor’s shape. Requiring high lipid solubility alone misses the key point of specific molecular fit. Describing a flexible active site that adapts after binding corresponds to the induced-fit model, which is a different concept from the rigid lock-and-key view.