Enzymes are proteins that catalyse biochemical reactions in the cells. Almost all enzymes are proteins, though some nucleic acids called ribozymes also behave like enzymes. Like any protein, enzymes have primary, secondary, and tertiary structures. The tertiary structure creates crevices or pockets, one of which is the active site.

The active site is a crevice or pocket into which the substrate fits. Enzymes exhibit substrate specificity, meaning the substrate specifically binds to the active site. Through this active site, enzymes catalyse reactions at a high rate, converting the substrate (S) into product (P).

Enzymes lower the activation energy of reactions and greatly enhance the rate of reactions. This allows biochemical conversions to occur efficiently within cells.

Enzymes require optimum temperature and optimum pH for maximal activity. They generally function in a narrow range of temperature and pH. Low temperature preserves enzymes in a temporarily inactive state, while high temperature destroys enzymatic activity as proteins get denatured by heat (above 40°C).

Enzyme catalysts differ from inorganic catalysts. Inorganic catalysts work efficiently at high temperatures and high pressures, while enzymes get damaged at high temperatures. However, enzymes isolated from thermophilic organisms (those living in hot vents and sulphur springs) are stable and retain catalytic power even at high temperatures (80°-90°C).