The word enzyme comes from the Greek ensymo'. Here en' means at' or in' and simo' means leaven' or yeast'. The enzymes are biological catalysts. These are by and large proteins. There are also some RNAs that have catalytic activity. These are referred to as RNA enzymes or ribozymes. The enzymes bind temporarily to one or more of the reactants in a reaction and lower the amount of activation energy needed. In this way they speed up the reaction. The enzymes may speed up reactions by a factor of many millions.

If it were not for the enzymes then most of the chemical reactions in biological cells would occur too slowly. The products obtained from such reaction would also have been different. In this respect the enzymes are essential for sustenance of life. Again, a malfunctioning of a single enzyme can lead to a severe disease. Such malfunctioning may stem from mutation, overproduction, underproduction or deletion of the enzyme.

Changes in pH and temperature affect the activity of enzymes to a large extent. Every enzyme works at its optimum level at a certain temperature and pH. At temperature and pH levels above and below that point the activity of the enzyme decreases.

Other molecules can affect the activity of enzymes. There are certain naturally occurring or synthetically prepared molecules that decrease or even completely abolishing enzyme activity. These are called inhibitors. There are also activators, which increase activities of enzymes.

Apart from the use of enzymes for their biological role they are also commercially used for other purposes. There are many cleaners used in the household, which use enzymes to speed up chemical reactions. Breakings down of proteins or removing starch stains in clothes are facilitated by the use of enzymes. There are more than 5,000 known enzymes. The names of the enzymes characteristically include the suffix ase' added to the name of the substrate.

The enzymes are found in all tissues and fluids of the body. The intracellular enzymes catalyze the reactions of metabolic pathways, the plasma membrane enzymes regulates catalysis within cells in response to extra cellular signals while the enzymes of the circulatory system regulate the clotting of blood.