Vmax, Km and inhibitors · ⁨Vmax、Km 和抑制剂⁩

Vmax and Km

• When every active site is full, the reaction hits a top speed — the maximum rate 最大速率, $V_{max}$.
• The curve of rate against substrate concentration climbs and then flattens at $V_{max}$.

What Km tells you

• The Michaelis–Menten constant 米氏常数, $K_m$, is the substrate concentration that gives half of $V_{max}$.
• It measures the enzyme's affinity 亲和力 (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.

Both reach the same $V_{max}$, but the low-$K_m$ enzyme gets to half-speed on far less substrate — it grips its substrate more tightly.

Reversible inhibitors

• An inhibitor slows an enzyme; a reversible one can leave again. Two types:

Immobilised enzymes 固定化酶

• 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.
• Used industrially — e.g. lactase beads make lactose-free milk.

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