An enzyme is a type of protein that speeds up a certain chemical reaction. Also, while it is acting like a catalyst, it tries to produce a biochemical reaction. For example, in an E. coli bacterium, a enzyme is responsible for breaking down maltose molecules into simpler glucose molecules. That enzyme can only break down maltose and does nothing else. In the picture above, one enzyme that milk carries lactose, which is broken down by lactase. Lactase is a glycoside hydrolase enzyme that cuts lactose into it’s constituent sugars, galactose and glucose. Without sufficient production of lactase enzyme in the small intestine, some people become lactose intolerant.