Enzymes for the IMAT: activation energy vs reaction energy
Enzymes increase reaction rates by lowering the activation barrier; they do not change the reaction's overall free-energy difference or equilibrium constant. To interpret an enzyme question, separate the energy barrier from the energy change, then identify what limits the rate. Substrate concentration, temperature and inhibitors can affect activity through different mechanisms, so the same slower rate need not have the same cause.
An enzyme question can describe a faster reaction without describing a more favourable reaction. Keeping those two ideas separate is the starting point for interpreting energy diagrams and rate experiments.
Lower the barrier, not the endpoint
Activation energy concerns the barrier on the route from reactants to products. The overall free-energy change, ΔG, compares the initial and final states. An enzyme changes the route, not those states.
Consider an original, simplified energy-diagram example: reactants are at 20 kJ/mol, products at 5 kJ/mol, and the uncatalysed transition state at 70 kJ/mol. The forward barrier is 50 kJ/mol; the overall change is −15 kJ/mol.
If a catalysed route has its transition state at 40 kJ/mol, the forward barrier becomes 20 kJ/mol. The overall change is still −15 kJ/mol. The enzyme has not made the products lower in energy.
This distinction also explains why a catalyst helps a system approach equilibrium without changing where that equilibrium lies. See OpenStax’s treatment of activation and free energy.
Why more substrate eventually stops helping
An enzyme binds substrate at its active site, and binding can involve changes in shape: the induced-fit model is more flexible than a rigid lock and key.
In a simple saturation experiment, keep enzyme amount and other conditions fixed while increasing substrate concentration. Initially the rate rises. At sufficiently high substrate concentration, the enzyme is working close to its maximum rate; adding more substrate then has little effect.
Do not confuse this rate-versus-substrate plateau with a product-versus-time plateau. The latter could reflect substrate depletion or approach to equilibrium. Read the axes before explaining the shape. OpenStax describes enzyme binding and regulation.
Competitive versus pure noncompetitive inhibition
For a simple Michaelis–Menten enzyme, these idealised reversible models predict different changes:
| Feature | Competitive | Pure noncompetitive |
|---|---|---|
| Binding relationship | Substrate and inhibitor binding are mutually exclusive | Inhibitor binds free enzyme and enzyme–substrate complex equally well |
| Apparent maximum rate, Vmax | Unchanged | Lower |
| Apparent Km | Higher | Unchanged |
| Can abundant substrate restore the uninhibited maximum rate? | Yes, in this model | No |
Km is the substrate concentration at half the maximum rate in this model. It should not always be treated as a direct binding-affinity measurement.
The word pure matters. Binding away from the active site does not automatically establish pure noncompetitive behaviour: mixed inhibition can change Km as well as Vmax. The Assay Guidance Manual distinguishes these models.
Temperature is a different variable
Warming can increase activity over a suitable range, but sufficiently high temperatures can disrupt a protein enzyme’s structure. Cooling commonly slows activity without causing the same structural damage. Likewise, pH can change the charge states needed for binding and catalysis. Neither variable has one optimum shared by every enzyme.
Check the distinction
A treatment lowers the observed maximum rate even at very high substrate concentration. Does that prove pure noncompetitive inhibition?
No. It is compatible with that model, but fewer active enzyme molecules or other mechanisms could also lower the maximum. You need additional information, including the behaviour of Km, to distinguish explanations.
The recurring traps are changing ΔG when only the barrier changes, assuming more substrate always helps, and identifying an inhibitor solely from the location where it binds. For each, state what the evidence shows before naming a mechanism.
Sources
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