According to the science of biology, living things are differentiated from non-living things by their capacity to perform chemical reactions that help them survive. This can be found in even one-celled organisms, which—despite their very simple structures—can actually have hundreds of these reactions going on in their miniscule cell walls.
These chemical reactions are made possible through biological, protein-based catalysts called enzymes. Enzymes help in either triggering or quickening a reaction. The human body has about 75,000 enzymes which perform processes at astounding speed. Without the enzymes it would take the body several hundred years just to convert a product (such as a slice of bread) into another (glucose, for the body to use).
Enzymes are known to work in 3 ways. In “substrate orientation” the enzymes help molecules to bond. In “physical stress” it breaks the molecule apart. In “substrate reactivity” it alters the molecules electron placement so that it bonds with other molecules. One can say therefore that enzymes act like the body’s little (but oh so powerful) engineers.
The body would not be able to function properly without enzymes, and in fact a lot of conditions are caused just because one enzyme of the 75,000 is not working as it should. For example in phenylketonuria (PKU) where one enzyme is missing can cause severe retardation and even death among babies. On the other hand some medicines are effective because they bond with enzymes of the disease causing organism and prevents them from spreading, which is the case with penicillin.