Enzymes are biological catalysts, meaning that they are molecules capable of speeding up chemical reactions. They are large, protein-based compounds that bind to the reactants in a chemical reaction and, through one or more mechanisms, assist in the reaction without being changed at all themselves. Because chemical reactions are so diverse, the mechanisms by which enzymes speed up chemical reactions are also varied.
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Stabilization of the Transition State
In each chemical reaction, the reactants must pass through a temporary and highly unstable conformation called the “transition state.” While the transition state is technically the point in the reaction during which bonds are being broken while new bonds are being formed, it’s easier to picture this in terms of a physical analogy. If a person standing on one bank of a river wishes to jump to the other bank, the transition state would be the moment in time when the person is in the air, over the river. As with this analogy, chemical transition states exist for tiny fractions of time and are highly unstable. Enzymes can help stabilize a reaction’s transition state, making it easier for the chemicals to achieve that state—and from there, to continue on to form products. This speeds the rate of the reaction. In terms of the river-jumping analogy, an enzyme acts like a springboard, making it easier for the person jumping the river to get into the air, and from there to the other side.
Some molecular components of enzymes act as acids and bases. This is quite important, because the reactants in a chemical reaction must physically encounter each other in space in order to react. A good way to picture this is to imagine blue and red balls bouncing randomly around a room, where the blue balls represent one reactant and the red balls represent a second. If a blue ball hits a red ball, they will react. Since the bouncing is occurring randomly, the odds are that eventually, the right balls will hit each other. If the reaction requires acid or base as well, however, the situation becomes more complex. In such a case, a third color of ball—green, for instance, representing acid or base—must hit the red and blue ball at exactly the same moment at which they hit each other in order for the reaction to occur. The odds of this are very low. Enzymes have the ability to gather the right chemicals—the red and blue balls from the example, for instance—and also contain the acid or base necessary to complete the reaction, eliminating the need for the green balls and speeding the reaction.
Proximity and Orientation
While it’s convenient to imagine a chemical reaction as the collision of two bouncing balls, in reality it’s more complex. Not only must the reactants run into each other, they must do so with the right orientation. Rather than picturing balls colliding, it’s a bit more accurate to imagine keys and locks bouncing around a room, where the keys are one reactant and the locks are a second. For a reaction to occur, not only must a key and lock bump into each other, they must do so perfectly oriented in space, such that the key sticks into the lock. The odds of this are quite low, and this can make chemical reactions very slow. Enzymes help position reactants, adjusting the orientation of the key and the lock—the two reactants—so that when they run into each other, they do so with the appropriate orientation and are able to react.