As noted above, enzymes are orders of magnitude more effective (faster) than chemical catalysts. The secret of their success lies in a fundamental difference in their mechanisms of action. Every chemistry student has had hammered into their heads the fact that a catalyst speeds a reaction without being consumed by it. In other words, the catalyst ends up after a reaction just the way it started so it can catalyze other reactions, as well. Enzymes share this property, but in the middle, during the catalytic action, an enzyme is transiently changed. Such changes may be subtle electronic ones or more significant covalent modifications. It is also important to recognize that enzymes are not fixed, rigid structures, but rather are flexible. Flexibility allows movement and movement facilitates alteration of electronic environments necessary for catalysis. Enzymes are, thus, much more effcient than rigid chemical catalysts as a result of their abilities to facilitate the changes necessary to optimize the catalytic process.