Lock and Key Model of Enzyme-Substrate Binding

Steps:

• Enzymes catalyze reactions by binding substrates at a specific region called the active site.
• In the lock and key model, the enzyme's active site has a shape complementary to the substrate.
• The enzyme acts as the lock, with the active site as its keyhole, and the substrate as the key that fits precisely.
• Choice A matches: active site on enzyme, lock (enzyme) on enzyme, key (substrate) on substrate.

Why A is correct:

• By definition in the lock and key hypothesis, the active site is on the enzyme (lock), the enzyme is the lock, and the substrate is the key.

Why the others are wrong:

• B: Places active site on substrate, reversing the model where enzyme binds substrate.
• C: Repeats substrate locations for active site and key, ignoring enzyme's role as lock.
• D: Puts active site and lock on substrate, contradicting enzyme-centric binding.

Final answer: A