pH & Enzyme Function

An enzyme is a biological catalyst — a protein that speeds up a reaction without being used up. Each enzyme has a 3-D shape with an active site that the substrate fits into, like a key into a lock.

The pH of the surroundings affects how well the active site works:

• Each enzyme has its own optimum pH — the pH at which it works fastest.
• At the optimum pH the active site is the perfect shape for the substrate, so the rate of reaction is highest.
• Move the pH above or below the optimum and the rate decreases on both sides. This gives the typical peak (bell-shaped) curve when you plot rate against pH.

Different enzymes have different optimum pH values because they work in different parts of the body:

• Pepsin — a protein-digesting enzyme in the stomach — has an optimum of about pH 2 (strongly acidic, matching stomach acid).
• Amylase — found in saliva and the small intestine — has an optimum of about pH 7 (neutral).

Exam tip. A common 1-mark gift is "state the optimum pH" — read it straight off the peak of the graph.

Plotting rate of reaction (y-axis) against pH (x-axis) gives a curve that rises to a peak at the optimum pH and then falls back down. Two enzymes that work in different places peak at different pH values.

How to read it:

• The pH at the top of the peak = the optimum pH.
• The height of the curve at any pH = the rate at that pH.
• As pH moves away from the optimum the curve drops, showing the rate slowing.

Exam tip. When asked to describe the graph, use the data: "the rate increases up to pH 7, is highest at pH 7, then decreases" — quote the values from the axes.

This is the part examiners want explained, not just stated. The reasoning chain is:

1. An enzyme's active site has a specific 3-D shape held together by forces within the protein molecule.
2. A pH far from the optimum (too acidic or too alkaline) disrupts these forces and changes the shape of the active site — the enzyme becomes denatured.
3. With a different-shaped active site, the substrate no longer fits — fewer (or no) enzyme–substrate complexes form.
4. So the rate of reaction falls, and if the active site is badly changed the enzyme stops working.

Key points for full marks:

• The substrate is no longer complementary to the active site — it does not fit.
• Denaturing by extreme pH is normally permanent (the active site does not return to its original shape when pH is corrected).
• This is the same kind of damage that very high temperature causes — both denature the enzyme by changing the active site shape.

Watch the wording. The enzyme is denatured, NOT "killed" — enzymes are not alive. Avoid "the enzyme dies".

The two named enzymes show why optimum pH matches the environment:

Enzyme	Where it works	Optimum pH	Why
Pepsin	Stomach	about pH 2	The stomach makes hydrochloric acid, so pepsin is adapted to work in strong acid.
Amylase	Saliva and small intestine	about pH 7	The mouth and small intestine are roughly neutral, so amylase peaks near pH 7.

What happens if you put an enzyme in the wrong place?

• Pepsin in a neutral environment (pH 7) works slowly — it is far from its optimum.
• Amylase in the acidic stomach (pH 2) is denatured — the strong acid changes its active site, so it stops digesting starch.

This is why amylase from the mouth stops working once swallowed into the acidic stomach, while pepsin is well suited to the acid there.

Exam tip. If asked why an enzyme's optimum is a particular value, link it to where it normally works (e.g. "pepsin's optimum is pH 2 because the stomach contains acid").