Because enzymes are proteins they exhibit all of the structural properties of proteins: primary structure with covalent bonding, secondary structure holding the molecule in coils (frequently an alpha helix) with hydrogen bonding, and tertiary structure which bonds the helical arrangements together. Secondary and tertiary structure is permanently destroyed by heating these molecules too much. When that happens, the enzyme is inactive and is said to be denatured. These denatured enzymes no longer act as catalysts because the structure has been rearranged. Cooking food denatures the enzymes in foods helping to preserve the foods. As temperature increases, the rate of the denaturing reaction increases which decreases the concentration of active enzyme.

When an enzyme catalyzes a reaction, the overall rate of that reaction decreases as the enzyme denatures because the concentration of the active catalyst is decreasing. Most enzymes have a temperature where the reaction rate is maximum before denaturing occurs. If the enzyme is from tissue in warm blooded animal with a temperature of about 36°C or 98°F, the maximum rate is usually close to that temperature.