who discovered it?
Anselme Payen
Enzyme
French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. 
what is an enzyme? Enzymes are Proteins that act as biological catalysts
wiki:1625291
Enzyme [SEP] Etymology and history
By the late 17th and early 18th centuries, the digestion of meat by stomach secretions and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified. French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. A few decades later, when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells." In 1877, German physiologist Wilhelm Kühne (1837–1900) first used the term "enzyme", which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process.