A Levels Biology (9700)•9700/13/M/J/24

Explanation
Enzyme-Substrate Binding in a Reaction Mixture
Steps:
- Identify molecule P as the enzyme based on its role in facilitating reactions with other molecules.
- Observe that mixing leads to R and S attaching to P's active site, forming a complex.
- Rule out inhibition or product formation by checking for no blocking or breakdown evidence.
- Confirm C matches the diagram's depiction of direct active site binding.
Why C is correct:
- Enzymes bind substrates at the active site to form an enzyme-substrate complex, initiating catalysis as per the Michaelis-Menten model.
Why the others are wrong:
- A: Binding to the active site decreases, not increases, activation energy by stabilizing the transition state.
- B: Non-competitive inhibitors bind outside the active site, not forming a typical enzyme-substrate complex.
- D: Products result from reaction completion, not initial binding without evidence of P's breakdown.
Final answer: C
Topic: Mode of action of enzymes
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