Enzymes react with substrates to give products
A substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed.
Enzymes make possible all the biochemical reactions proceed at very high rates even under mild conditions of temperature and pH characteristic of living organism. To effect biochemical reactions, very powerful catalysts are needed. These essential biological catalysts are collectively known as enzymes. Without them, the living process would be too slow to sustain life. In fact, the human body would not exist without enzymes because the chemical reactions required to maintain the body simply would not occur fast enough.
For example, just think about the juice you drank few moments ago before reading the magazine. The sugar in the juice was converted to CO2, H2O, and chemical energy within seconds of being absorbed by our cells, and this chemical energy enabled us to see, think, and move. However, the 2.2-kilogram (5-pound) bag of sugar in our kitchen cabinet can sit for years and still not be converted to CO2 and H2O. Thus, the human body speeds the overall reaction through a series of enzyme-mediated steps. The key is in the catalytic power of enzymes to drive reactions on a time scale required to digest food, relay signals via the nervous system, and contract muscles. To date, more than 3000 enzymes have been identified and they constitute the largest and most highly specialized class of proteins.