Evidence for the lock-and-key model of enzyme action: Which observation most directly supports the classical lock-and-key hypothesis by demonstrating shape-specific recognition between enzyme and substrate?

Introduction / Context:
The lock-and-key model proposes that enzymes possess rigid, shape-complementary binding pockets that fit their substrates precisely. Although modern views incorporate induced fit, key experimental evidence still highlights shape-specific competition at the active site.

Given Data / Assumptions:

• The task is to identify the observation that most directly demonstrates shape complementarity.
• Competitive inhibitors mimic the substrate and compete for the active site.
• Enzymes accelerate reactions by lowering activation energy, not by changing equilibrium.

Concept / Approach:
If molecules that merely resemble the substrate can bind and block catalysis, it implies the active site recognizes specific structural features (shape and functional groups). This competitive inhibition is the clearest empirical demonstration of the lock-and-key concept. General statements about catalysis or rate increases do not directly evidence shape complementarity, and enzymes never change the reaction’s equilibrium position (direction).

Step-by-Step Solution:
List candidate observations and relate them to shape-specific binding. Identify that only competitive inhibition requires a substrate-like shape to bind the active site. Exclude generic catalytic effects and the incorrect notion that enzymes set reaction direction. Select competitive inhibition as best evidence.

Verification / Alternative check:
Kinetic analyses show that competitive inhibitors increase apparent Km without affecting Vmax, consistent with binding to the same active site as the substrate.

Why Other Options Are Wrong:
General catalysis (b, c) does not prove shape complementarity; enzymes do not change equilibrium (d); heat generation is not the mechanism of enzymatic rate enhancement (e).

Common Pitfalls:
Equating rate acceleration with proof of a specific binding geometry; only competition by structural analogs truly tests the model.

Final Answer:
Compounds structurally similar to the true substrate inhibit the enzyme's activity (competitive inhibition).