According to the lock-and-key hypothesis, which pair must be intrinsically compatible to enable specific binding?

Introduction / Context:
The lock-and-key model is one of the earliest explanations of enzyme specificity. It emphasizes pre-existing complementarity between the active site (lock) and the substrate (key) before binding, in contrast to the induced-fit model that stresses conformational change upon binding.

Given Data / Assumptions:

• The active site possesses a shape and chemical environment complementary to the substrate.
• Specific interactions (hydrogen bonds, hydrophobic contacts, ionic pairs) drive binding.
• Product formation and release occur after binding and catalysis.

Concept / Approach:
Compatibility in lock-and-key refers specifically to enzyme–substrate pairing. While product may also fit transiently, the model’s defining point is that binding recognition is determined by the substrate’s fit to the unbound active site.

Step-by-Step Solution:

Verification / Alternative check:
Modern views add induced fit and conformational selection, but the core compatibility statement of lock-and-key remains between enzyme and substrate.

Why Other Options Are Wrong:

• Enzyme and product: product generally has reduced affinity to allow release.
• Enzyme and ES complex: tautological; complex forms after compatibility is established.
• ES and product: not the defining pair.
• Two enzymes: irrelevant to the model.

Common Pitfalls:
Assuming the model denies flexibility; it only asserts initial complementarity, not rigid immobility.

Final Answer:
Enzyme and substrate