Enzymes are biological catalysts that increase the rate of chemical reaction in the body
They are all produced inside cells
Some are intracellular and they catalyse reactions within the cells
Others are extracellular and are secreted out of the cells where they work e.g. digestive enzymes

Properties of Enzymes

Enzymes are protein in nature
Enzymes are specific to the type of reaction they catalyse
This is referred to as substrate specificity
Enzymes work in very small amounts
They remain unchanged after the reaction
They catalyse reversible reactions
They work very fast (high turnover numbers) e.g. the enzyme catalase works on 600 thousand molecules of hydrogen peroxide in one second