When enzymes catalyse a chemical reaction, enzyme and substrate molecules must bind to each other at the correct orientation to form an enzyme-substrate complex.
Within the enzyme-substrate complex, chemical reaction occurs and a product is formed. The product leaves the enzymes' active sites, making the active sites available for substrate molecules to bind to them.
As enzymes are not used up during chemical reactions and they remain chemically unchanged, small amounts of enzymes are required to catalyse chemical reactions.