Researchers can learn about the active site of an enzyme (the place on the enzyme where the reaction is catalyzed) by treating the enzyme with a substance that fits into the active site and then covalently binds to it. Such a reaction almost always irreversibly inhibits the enzyme, but leaves a "tag" on the enzyme at the active site. Such "tagging" is called affinity labeling because the molecule must have an affinity for the active site in order to bind there. An affinity labeling compound that is inactive until the enzyme acts on it is called a suicide inhibitor. (see also Table 11.4, Figure 11.24.