What changes the rate of a reaction

Enzymes work in a narrow range

Enzymes are proteins, so they are sensitive to their surroundings. Four things change the rate of an enzyme-catalysed reaction: the temperature, the pH, the concentration of the substrate, and the presence of an inhibitor. We look at each in turn.

Temperature

Each enzyme shows its highest activity at one temperature, its optimum temperature. Below the optimum, the rate rises as the temperature rises. Above the optimum, the rate falls steeply and soon drops to nothing.

• Low temperature does not destroy the enzyme. It only makes it inactive. When the temperature rises again, the enzyme works again.
• High temperature destroys activity. The protein is denatured: its fold is lost, so the active site no longer fits the substrate.

Enzymes of animals work best near body temperature. Enzymes of organisms that live in hot springs and deep-sea hot vents work best at very high temperatures.

pH

Each enzyme also has an optimum pH. A change in pH changes the charges on the side groups of the amino acids, and this changes the shape of the chain. Away from the optimum, the active site no longer fits well.

Pepsin works in the stomach, in strong acid, with an optimum near pH $2$. The amylase of saliva works best at a nearly neutral pH. Trypsin works in the mildly alkaline small intestine, with an optimum near pH $8$. So a pepsin taken to the small intestine would hardly work.

Concentration of the substrate

Keep the amount of enzyme fixed and add more and more substrate. At first the rate rises quickly. Then it rises more slowly, and finally it levels off. The enzyme molecules are all busy, with every active site holding a substrate, so more substrate cannot make the reaction go faster.

The top of the curve is the maximum rate, called Vmax. The substrate concentration at which the rate is half of Vmax is called the Km, or the Michaelis constant. An enzyme with a small Km reaches half its top speed with very little substrate.

The program in the simulation uses a formula that fits this curve. You do not need to learn the formula. The shape of the curve, the maximum rate and the Km are what matter.

Inhibitors

An inhibitor is a chemical that shuts off the activity of an enzyme. A competitive inhibitor looks very like the substrate. It fits the active site, so it competes with the substrate for the site. While the inhibitor sits there, the substrate cannot bind, and the reaction slows down.

A classic case is succinate dehydrogenase, which turns succinic acid into fumaric acid. Malonate looks very like succinic acid. It binds to the active site and blocks the enzyme, so succinic acid is not oxidised.

Competitive inhibition is reversible. If you add much more substrate, the substrate wins the site more often, and the effect of the inhibitor fades. That is what the simulation shows: the top speed is the same, but more substrate is needed to reach it. Many medicines work as competitive inhibitors. For example, statins slow an enzyme that makes cholesterol.

Try it in Python

The program takes a set of measurements and finds the optimum temperature. It also draws a bar for each rate.

Practise with new numbers

Common mistakes

• "A cold enzyme is destroyed." It is only inactive, and it works again when warmed. Heat is what denatures it.
• "Rate keeps rising with substrate for ever." It levels off at Vmax, when every active site is busy.
• "Km is the maximum rate." Km is a substrate concentration: the one that gives half of Vmax.
• "A competitive inhibitor binds far from the active site." It binds in the active site, in place of the substrate.
• "More substrate cannot help against an inhibitor." For a competitive inhibitor, more substrate does help.

Remember these

• The optimum temperature and the optimum pH give the highest rate.
• Cold makes an enzyme inactive. Heat denatures it.
• The rate rises with substrate and then levels off at Vmax.
• Km is the substrate concentration at half of Vmax.
• A competitive inhibitor looks like the substrate and blocks the active site. More substrate reduces its effect.
• Malonate competes with succinic acid for succinate dehydrogenase.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A cold enzyme works again when warmed, but a boiled one does not. Why?

2. As the substrate concentration rises from zero to a very high value, how does the rate change? Why does it level off?

3. What is the Km of an enzyme?

4. How does a competitive inhibitor work, and how can its effect be reduced?

5. Pepsin has an optimum near pH $2$. Would it work well in the small intestine, which is mildly alkaline?