Enzymes are chemical substances produced in the living organism. They are marvellous organic catalysts which are essential to life as they control all the chemical reactions that take place in a living system. Enzymes are part of all living cells, including those of plants and animals.

The term enzyme, which literally means in yeast', was coined following the demonstration of catalytic properties of yeast and yeast juices. Although enzymes are produced in the living cell, they are not dependent upon the vital processes of the cell and work outside the cell. Certain enzymes of yeast, for instance, when expressed from the yeast cells are capable of exerting their usual effect, that is, the conversion of sugar to alcohol.

A striking feature of enzymes is that while they enter into chemical reaction, they remain intact in the process. They however, act with maximum efficiency at a certain temperature. Lowering the temperature below or raising it above this level slows the reaction. A high degree of heat, that is above 60 o C, permanently destroys their action.

It has been estimated that there are over 20,000 enzymes in the human body. This estimate is based on the number of bodily processes that seem to require action. However, so far only about 1,000 enzymes have been identified. But their great role in nutrition and other living processes has been firmly established. They are protein molecules made up of chains of amino acids. They play a vital role and work more efficiently than any reagent concocted by chemists. Thus for instance, a chemist can separate proteins into their component amino acids by boiling them at 166 o C for over 18 hours in a strong solution of hydrochloric acid, but the enzymes of the small intestines can do so in less than three hours at body temperature in a neutral medium.

A feature which distinguishes enzymes from inorganic catalysts is that they are absolutely specific in their actions. This means that a particular enzyme can cause reactions involving only a particular type of substance or a group of closely related substances. The substance on which the enzyme acts is known as "substrate". The specificity of an enzyme is, however, related to the formation of the enzyme-substrate complex which requires that the appropriate groupings of both substrate and enzyme should be in correct relative position. The substrate must fit the enzyme like a key fits its lock.

Enzymes which are used in the cells which make them are called intracellular enzymes. Enzymes which are produced in cells which secrete them to other parts of the body are known as extracellular enzymes. Digestive juices are an example of the latter type.