How Temperature and pH Affect Enzymes

Enzymes are fussy. The same enzyme that works beautifully at 37 °C in your body stops working in boiling water, and the digestive enzyme that thrives in stomach acid gives up in the small intestine. Four main factors control how fast an enzyme works: temperature, pH, substrate concentration and enzyme concentration. Each produces a characteristic graph, and each can be explained with two ideas you already know: collisions between molecules and the shape of the active site.

If you need a refresher on how enzymes work, start with enzymes: how they work.

Temperature

What happens

If you measure the rate of an enzyme reaction at different temperatures, you get a curve that:

1. rises gradually as temperature increases from cold,
2. reaches a peak at the optimum temperature, and
3. then falls steeply above the optimum, often to zero within a few degrees.

For most human enzymes, the optimum is around 37 to 40 °C.

Why it rises

At higher temperatures, enzyme and substrate molecules have more kinetic energy. They move faster and collide more often, and more collisions have enough energy to react. This is the same collision theory that applies to any chemical reaction (see reaction rates and catalysts). As a rough rule, many reactions, including enzyme reactions below their optimum, go about twice as fast for every 10 °C rise.

Why it crashes

Enzymes are proteins held in shape by weak interactions: hydrogen bonds, ionic bonds and hydrophobic interactions. As temperature rises, the atoms vibrate more strongly. Above the optimum, these vibrations start to break the weak interactions. The protein unfolds and the active site changes shape, so the substrate no longer fits. The enzyme is denatured (see protein denaturation).

Denaturation is usually irreversible: cooling the enzyme again doesn’t bring the activity back. That’s why the fall is so steep and why the two sides of the curve aren’t symmetrical.

Low temperatures are different

At low temperatures, enzymes are not denatured, just slow. Molecules move slowly and collide less often. Warm them up and they work again. That’s why food keeps longer in a fridge (enzymes in the food and in microbes slow down) and even longer in a freezer.

Enzymes from extreme environments

Optimum temperature depends on where an organism lives:

• Thermophiles in hot springs have enzymes with optima of 70 to 100 °C. The DNA polymerase from Thermus aquaticus (Taq polymerase) survives repeated heating to about 95 °C, which is why it’s used in PCR.
• Psychrophiles in polar seas have enzymes that work near 0 °C. Some are used in cold-wash laundry detergents.

pH

What happens

Plot enzyme activity against pH and you usually get a bell-shaped curve with a peak at the optimum pH. Activity falls on either side.

Enzyme	Location	Optimum pH (approx.)
Pepsin	Stomach	1.5–2
Salivary amylase	Mouth	6.7–7
Catalase	Most cells	7
Trypsin	Small intestine	7.5–8.5
Arginase	Liver	~9.5–10

Each enzyme is suited to its environment: pepsin works in stomach acid (see stomach acid chemistry), while trypsin works in the alkaline conditions created by pancreatic juice.

Why pH matters

pH is a measure of hydrogen ion concentration (see the pH scale explained). Changing it changes which groups on the enzyme carry a charge:

• In acidic conditions, basic side chains such as lysine (–NH₂ → –NH₃⁺) and acidic side chains such as aspartate (–COO⁻ → –COOH) pick up protons.
• In alkaline conditions, they lose them.

These charge changes:

• break ionic bonds that hold the enzyme’s shape, changing the active site;
• change the charges inside the active site, so the substrate may no longer bind or the catalytic groups may not work.

Small shifts away from the optimum usually cause reversible changes, and activity returns if the pH is restored. Large shifts can denature the enzyme permanently.

This is also why the body keeps blood pH so tightly controlled, between 7.35 and 7.45 (see the blood buffer system).

Substrate concentration

What happens

With a fixed amount of enzyme, the rate rises as substrate concentration increases, then levels off.

Why

At low substrate concentrations, many active sites are empty, so adding substrate means more enzyme–substrate collisions and a faster rate. At high concentrations, all the active sites are occupied almost all the time. The enzyme is working as fast as it can, and adding more substrate can’t help. The enzyme is saturated, and the rate has reached its maximum, V_max. The mathematical description is covered in enzyme kinetics.

Enzyme concentration

What happens

With plenty of substrate, the rate is directly proportional to enzyme concentration: double the enzyme, double the rate.

Why

More enzyme means more active sites available. As long as there’s excess substrate, every extra enzyme molecule adds to the rate. If substrate runs short, though, adding more enzyme stops helping, because there isn’t enough substrate to occupy the extra active sites.

Inhibitors

Molecules called inhibitors also reduce enzyme activity. Some compete with the substrate for the active site; others bind elsewhere and change the enzyme’s shape; some bind permanently. See competitive vs non-competitive inhibition.

Reading the graphs in exam questions

When describing or explaining an enzyme graph:

• Describe the trend with numbers: “The rate increases from 10 to 40 °C, reaches a maximum at about 40 °C, then decreases rapidly to zero by 60 °C.”
• Explain the rise with collision theory: more kinetic energy, more frequent successful collisions, more enzyme–substrate complexes formed.
• Explain the fall with denaturation: bonds holding the tertiary structure break, the active site changes shape, the substrate no longer fits, fewer enzyme–substrate complexes form.
• Use the right words: say “denatured”, not “killed”; say “active site changes shape”, not “enzyme dies”.
• Explain a plateau with saturation: all active sites occupied, or another factor (such as substrate or enzyme amount) now limiting.

Real-world applications

• Fever: a moderate fever may help fight infection, but very high temperatures (above about 41 °C) risk denaturing the body’s own enzymes.
• Cooking and blanching: vegetables are briefly boiled before freezing to denature enzymes that would otherwise spoil their colour and flavour.
• Biological detergents: designed to work at lower temperatures, saving energy, and not recommended for very hot washes where their enzymes denature.
• Food preservation: pickling in vinegar (low pH) and refrigeration (low temperature) both slow the enzymes of spoilage microbes.

Key takeaways

• Temperature: rate rises with temperature (more collisions) up to an optimum, then falls sharply as the enzyme denatures. Cold slows enzymes but doesn’t denature them.
• pH: each enzyme has an optimum pH; changing pH alters charges, breaks ionic bonds and changes the active site.
• Substrate concentration: rate rises then levels off when all active sites are occupied (saturation).
• Enzyme concentration: rate is proportional to enzyme concentration when substrate is in excess.
• Try these effects for yourself in investigating enzyme activity.