Enzyme

An enzyme is a biological catalyst, usually a protein, that speeds up a specific reaction in living things without being used up itself. Malay: enzim · Chinese: 酶.

Like any catalyst, an enzyme increases the rate of a reaction by providing an alternative path with a lower activation energy, and it is not consumed, so a small amount can process a large amount of reactant. What makes enzymes special is that they are made by living cells and are highly specific: each enzyme usually catalyses just one reaction or one type of reaction. Enzymes also work best within a narrow range of temperature and pH; outside that range the protein loses its shape and stops working.

Example. The enzyme catalase, found in liver and many other tissues, catalyses the breakdown of hydrogen peroxide into water and oxygen, the same reaction that manganese(IV) oxide speeds up in the laboratory. In everyday life, enzymes in yeast turn sugar into ethanol and carbon dioxide during fermentation, and enzymes in biological washing powders break down stains.

Confusion to avoid. Do not think enzymes are unlimited or indestructible. They are catalysts, so they are not used up in the reaction, but they can be destroyed by high temperature or the wrong pH, after which they no longer catalyse anything. Also, an enzyme is a biological catalyst; an inorganic catalyst such as manganese(IV) oxide is not an enzyme.

The enzyme shows that the idea of a catalyst reaches beyond the laboratory into biology, and understanding it as a catalyst that lowers activation energy links this term to the rest of the chapter in SPM Chemistry. A neat way to remember it is that every enzyme is a catalyst, but not every catalyst is an enzyme.