Enzymes

The essentials

Enzymes are biological catalysts that speed up reactions in living cells. Almost all enzymes are globular proteins. They are very specific: a given enzyme usually catalyses only one reaction or acts on one particular substrate. For example, maltase converts maltose into glucose, and invertase converts sucrose into glucose and fructose. Enzymes are usually named after the substrate or reaction, with the ending "-ase"; an enzyme that catalyses oxidation of one substrate with reduction of another is an oxidoreductase.

Key points

• Enzymes work under mild conditions of temperature and pH, and are needed only in small amounts.
• They lower the activation energy of a reaction, so the reaction becomes much faster at body temperature. For example, the activation energy for acid hydrolysis of sucrose is far higher than for its hydrolysis by the enzyme sucrase.
• Mechanism: the substrate binds to the active site of the enzyme, forming an enzyme–substrate complex. The reaction takes place on the enzyme, the products are released, and the enzyme is free to act again.
• The fit between enzyme and substrate is often compared to a lock and key: only a substrate of the right shape fits the active site, which explains specificity.
• Because enzymes are proteins, high temperature or extreme pH denatures them and stops their activity.
• Some enzymes need non-protein helpers (cofactors) to work.
• Some drugs act by blocking the active site of a particular enzyme (enzyme inhibitors).

Exam tips

• Draw the energy profile: the catalysed path has a lower peak; ΔH of the reaction is unchanged.

Common mistakes

• Saying enzymes shift equilibrium or are used up; they only make equilibrium reach faster and are regenerated.