Definition of Enzymes

The living machine is essentially a chemical engine, dependent for its growth, maintenance and energy on chemical reactions. These reactions are controlled by catalysts. One of the most striking achievements of modern biology and biological chemistry is the isolation of more and more of these catalysts so important for the vital process. Once isolated, it is possible to study their behavior in test tubes or other suitable containers. In other words, one can detach from the living material certain non-living substances capable of causing or promoting the complex chemical transformations which constitute a major part of the mystery of protoplasm.

Originally, enzymes were defined as "catalytic substances produced by living cells." With increase in knowledge and understanding, it has become necessary to restrict this definition. The original concept was that enzymes were not only produced by living things but were peculiar to them. There are some rather simple substances in protoplasm which can act catalytically; so for example, ascorbic acid, glutathione or as a matter of fact, even the hydrogen ion. The term enzyme was never meant to apply to these. Hence in modern usage an enzyme is defined as a "catalyst of biological origin, possessing a high molecular weight." In the light of what we now know, it is generally believed that all enzymes are proteins.

In as much as enzymes are catalysts, it is important to remember that they hasten rather than initiate reactions, and that when they act on reversible reactions they accelerate the progress of the reaction in either direction. Thus the same enzyme may synthesize as well as break down a complex chemical compound, and the very enzyme which digests proteins or fats or carbohydrates also serves, under the proper conditions, to build up these compounds from their breakdown products.