Enzymes

Enzymes are biological catalysts

An enzyme is a biological catalyst: a protein that speeds up a chemical reaction without being used up or permanently changed. Because it is not consumed, one enzyme molecule can be used again and again. Enzymes work by lowering the activation energy — the minimum energy needed to start a reaction — so the reaction can proceed quickly at the mild temperatures found in living cells.

How enzymes work: the active site

Each enzyme has a specially shaped region called the active site. The molecule it acts on is the substrate. The substrate fits into the active site like a key fits a lock — this is the "lock and key" idea. Because the shapes must match, each enzyme is specific: it usually catalyses only one type of reaction.

Factors that affect enzyme activity

• Temperature: activity rises as temperature increases up to an optimum (about 37 °C in humans). Beyond that the enzyme denatures — its shape is destroyed — and it stops working.
• pH: each enzyme has an optimum pH. Pepsin works best in the acidic stomach; salivary amylase prefers a neutral pH.
• Substrate concentration: more substrate speeds the reaction until all active sites are busy.

Intracellular and extracellular enzymes

Some enzymes work inside the cell that makes them; these are intracellular enzymes, such as the catalase and respiratory enzymes found in every cell. Others are made in a cell but released to work outside it; these are extracellular enzymes, such as the digestive enzymes secreted into the gut. In both cases the enzyme is not changed by the reaction, so a small amount can process a very large quantity of substrate over time. This reusability, together with their speed and specificity, is why living things depend on enzymes for almost every chemical change.