Effects of temperature and pH

Biology · Enzymes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An enzyme works fastest at its optimum temperature and optimum pH. You can follow the reaction by measuring how fast the substrate disappears or how fast a product appears.

As the temperature rises towards the optimum, the molecules gain kinetic energy and move faster. Enzyme and substrate collide more often, so more effective collisions happen each second and the rate goes up.

Above the optimum the enzyme starts to lose its shape. The active site no longer fits the substrate. The enzyme is denatured and this cannot be reversed. A pH far from the optimum changes the shape of the active site in the same way. Cold only slows an enzyme down; it does not denature it.

Rules to remember

Below the optimum: higher temperature → more kinetic energy → more frequent effective collisions → faster reaction.
Above the optimum: the active site changes shape, the substrate no longer fits, the enzyme is denatured and the rate falls quickly.
Denaturation is permanent. Low temperature makes an enzyme inactive, but it works again when warmed.
Each enzyme has its own optimum pH: about pH 2 for pepsin in the stomach, about pH 7 for salivary amylase.
A shorter time to finish the reaction means a faster rate.
Fair test: change one factor only; keep enzyme concentration, substrate concentration and volumes the same.

Common mistake

Saying that enzymes are killed by heat, or that cold denatures them. Enzymes are not alive, so write denatured, and remember that only high temperature or an unsuitable pH denatures them.

Worked example

A protease is tested at different pH values. The times taken to clear a cloudy protein suspension are: pH 5, 10 min; pH 6, 5 min; pH 7, 2 min; pH 8, 4 min; pH 9, 9 min. What is the optimum pH, and why is the reaction slow at pH 5?

The fastest reaction is the one with the shortest time.
The shortest time is 2 min, at pH 7, so the optimum is about pH 7.
At pH 5 the shape of the active site is changed, so the protein fits less well.
Fewer enzyme-substrate complexes form each second, so clearing takes longer.

Answer: Optimum about pH 7; at pH 5 the active site has changed shape, so the rate is lower.

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