A proenzyme (or zymogen) is an inactive enzyme precursor (thus the prefix, pre- meaning "before"). In order for a proenzyme to become active, it requires a biochemical change (change in the molecular shape or configuration). When a biochemical change occurs, the active part of the enzyme (called the active site), becomes available for reactions that are to take place. Enzymes, as you may know, speed up the rate of reactions, that would, otherwise, proceed at a very slow rate. For example, the enzymes that are produced in the pancreas help us digest the food we consume at a much faster rate than would proceed without their assistance.