An enzyme, a molecule that serves as a catalyst in living things, regulates the speed at which chemical reactions take place without changing the nature of the reaction itself.
An enzyme is defined as an organic catalyst for biochemical processes.
A catalyst is a substance that speeds up a chemical reaction but is not consumed or altered in the process.
In the absence of enzymatic catalysis, most biochemical reactions are so slow that they would not occur under the mild conditions of temperature and pressure that are compatible with life.