Enzymes are biological catalysts produced by the living cells and they catalyze several reactions in the body. They are proteins in nature and specific in action. The enzymes mainly catalyze the metabolic pathways in the human body. The deficiency of the enzyme results in metabolic errors. Most of the enzymes are of intracellular enzymes excluding the extracellular enzymes. The enzyme binds with its specific substrate to form enzyme-substrate complex. The active site of an enzyme is the region where substrate binds. Enzymes require coenzymes, which are small organic molecules that transport chemical groups from one enzyme to another. The factors which affect enzyme activity are pH, temperature, concentration of substrate and concentration of enzyme. Enzymes are classified into six classes depending on their functions. The phenomenon of the decrease in the rate of enzymatic reaction brought about by the addition of a chemical substance is called enzyme inhibition. The substances, which inhibit the enzyme, are called as inhibitors. The inhibition is of competitive and noncompetitive. The competitive inhibition is utilized in the drug therapy. Most of the enzymes are regulated by allosteric regulation to control the metabolic reactions.