Elementary Kinetics

When Enzymes Get Involved

Why should a budding biochemist be concerned with rates of chemical reactions? Perhaps most importantly, few things are at equilibrium in the body. You might have heard the expression "equilibrium equals death." That is certainly the case for chemical reactions in the body. If they were all at equilibrium, there would be no active metabolism. Thus, in examining metabolic processes, biochemists are examining reactions that are progressing at different rates.

These metabolic chemical reactions are catalyzed by enzymes. These special proteins really only do one thing, and they do it very well. Enzymes speed up how fast those chemical reactions happen. They do not add anything to the reaction. In fact, an important part of the definition of catalysts is that they themselves are not at all changed by the reactions they catalyze. Enzymes also cannot make reactions happen that are not thermodynamically possible, that is, that wouldn’t happen anyway even if the enzymes weren’t there (although those reactions would happen much more slowly without the enzymes). Furthermore, enzymes also cannot change the K_eq (equilibrium constant) of a chemical reaction. Remember that the equilibrium constant of a reaction is a number that tells us how much of the reactants, and how much of the products, are present when the chemical reaction reaches equilibrium.

Let’s review an example that illustrates these points. Say we lived in a world where apples sometimes magically "popped" into oranges (and back again).

The scientists in this "fruit world" have measured the K_eq of this reaction (K_eq = 3). The reaction would be written:

This means that at equilibrium, there are 3 times as many oranges as apples. So, if you started out with a dozen apples in your fruit bowl, at equilibrium there would be 3 apples and 9 oranges. At that point, the reaction doesn’t just stop, but the rates of apples popping into oranges and oranges popping into apples balance out so that the overall numbers of apples and oranges don’t change. Note how the reaction does occur spontaneously, even without the enzyme. But you really like oranges, and hope that by adding an enzyme to make the reaction go faster, you can have more oranges (say 11 oranges at equilibrium, with 1 apple left over for your baby brother). You get the enzyme that catalyzes this reaction and add it to your fruit bowl. Now, instead of taking a week to reach equilibrium, oranges and apples "pop" so fast that in less than a second, equilibrium has been established. The enzyme has greatly increased the rate of the reaction. However, much to your chagrin, you still have 9 oranges and 3 apples. The enzyme doesn’t change the K_eq of the reaction.