Enzymes are proteins consisting of long chains of amino acids held together by peptide bonds.
As catalytic agents, they help speed up chemical processes just by their presence in a reaction. They cannot function indefinitely, have limited stability, and are therefore used only once in commercial applications. Their catalytic activity was exploited long before they were recognized as enzymes. The making of beer, wine, cheese, and bread is thousands of years old, and each of these fermentation processes harnessed the activities of enzymatic organisms. Because a particular enzyme operates on only one substrate, the catalyzed reaction will generate only one product and no other.