Vmax, Km and inhibitors
Vmax and Km
- When every active site is full, the reaction hits a top speed — the maximum rate, $V_{max}$.
- The curve of rate against substrate concentration climbs and then flattens at $V_{max}$.
What does V_max represent?
V_max is the top rate; the curve flattens there because every active site is busy.
What Km tells you
- The Michaelis–Menten constant, $K_m$, is the substrate concentration that gives half of $V_{max}$.
- It measures the enzyme's affinity (pulling power) for its substrate:
- low $K_m$ → reaches half-speed at low substrate → high affinity.
- high $K_m$ → needs lots of substrate → low affinity.
- So $K_m$ lets you compare how strongly different enzymes hold their substrates.
An enzyme with a LOW Km has:
Low Km means half of V_max is reached at a low substrate concentration — a high affinity for the substrate.
Reversible inhibitors
- An inhibitor slows an enzyme; a reversible one can leave again. Two types:
| Type | Binds | Add more substrate? | $V_{max}$ | $K_m$ |
|---|---|---|---|---|
| competitive | the active site (similar shape to substrate) | out-competes it — effect drops | unchanged | rises |
| non-competitive | a different site (changes the active site) | does not help | falls | unchanged |
A competitive inhibitor:
It competes for the active site; more substrate out-competes it, so V_max is still reached (higher Km).
For a non-competitive inhibitor, adding more substrate does not help because it:
It binds away from the active site and alters the enzyme's shape; extra substrate cannot reverse it, so V_max falls.
Immobilised enzymes
- An immobilised enzyme is fixed in place — e.g. trapped in alginate beads — while substrate flows past.
- A free enzyme works a little faster, but immobilising it brings big practical wins:
- it is not washed away, so it can be reused.
- the product is pure (not mixed with enzyme).
- it is more stable to changes in temperature and pH.
- the process can run continuously.
A key advantage of immobilising an enzyme (e.g. in alginate beads) is that:
Immobilised enzymes are not washed away (reusable), keep the product enzyme-free, are more stable, and allow continuous processing.
You've got it
- $V_{max}$ = top rate (all active sites full); $K_m$ = substrate concentration giving half $V_{max}$
- low $K_m$ = high affinity; high $K_m$ = low affinity
- competitive inhibitor: active site, raises $K_m$, $V_{max}$ unchanged (more substrate overcomes it)
- non-competitive inhibitor: other site, lowers $V_{max}$, $K_m$ unchanged
- immobilised enzymes: reusable, pure product, more stable, continuous