Lactose takes years to break down on its own. But if exposed to the protein lactase, the reaction proceeds very quickly, while lactase itself remains unchanged. Lactase is an example of a(n) . High temperatures or changes in pH can an enzyme, causing it to lose its shape and biological activity. The specific location within an enzyme molecule where the substrate binds is called the . During an enzymatic reaction, a molecule of binds to the enzyme and is broken down into one or more molecules of , which are released. A(n) is a molecule that can bind to an enzyme and prevent the enzyme from working. An enzyme is considered a(n) because it speeds up a chemical reaction without being used up. Enzymes speed up chemical reactions by lowering the , which allows the reaction to proceed much more quickly. The between an active site and its substrate often strains bonds and helps the reaction proceed.