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.
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.