Enzymes are proteins that catalyse biochemical reactions in the cells, though some nucleic acids called ribozymes also exhibit catalytic power. Like other proteins, enzymes possess primary, secondary, and tertiary structures. The tertiary structure creates a crevice or pocket known as the active site, into which the substrate fits. This specific fitting allows enzymes to exhibit substrate specificity.

Enzymes function by lowering the activation energy of reactions, which greatly enhances the rate of chemical conversions. They differ from inorganic catalysts as they do not work efficiently at high temperatures and pressures; instead, enzymes get damaged or denatured at high temperatures (above 40°C).

The activity of an enzyme is sensitive to temperature. Each enzyme shows its highest activity at a specific optimum temperature. Low temperature preserves the enzyme in a temporarily inactive state, whereas high temperature destroys enzymatic activity. However, enzymes isolated from thermophilic organisms are stable and retain catalytic power even at high temperatures (80°-90°C).

Enzymes also function within a narrow range of pH. They exhibit maximum activity at a particular optimum pH, and activity declines both below and above this optimum value.