When we know the change in free energy (ΔG) of a reaction, we know where the equilibrium point of the reaction lies: the more negative the ΔG value is, the further the reaction proceeds to products and toward completion. However, ΔG tells us nothing about the rate of a reaction—how fast it moves toward equilibrium. When we measure reaction rates of biochemical reactions in the lab, it quickly becomes apparent that in terms of the rapid needs for the chemistry of life, reaction rates are very slow—in fact, so slow that the cells would not survive without a way to speed up the reactions. That is the role of catalysts: substances that speed up reactions without themselves being permanently altered. A catalyst does not cause a reaction to occur that would not proceed without it, but merely increases the rate of the reaction, allowing equilibrium to be approached more rapidly. This is an important point: No catalyst makes a reaction occur that cannot otherwise occur.

Most biological catalysts are proteins called enzymes. A biological catalyst is a molecular framework or scaffold within which chemical catalysis takes place. The enzyme binds the reactants and sometimes participates in the reaction itself; however, such participation does not permanently change the enzyme. That’s the hallmark of any catalyst: It ends up in exactly the same chemical condition after a reaction as before it.

In this section we will discuss the energy barrier that controls the rate of a chemical reaction. Then we will focus on the roles of enzymes: how they interact with specific reactants, how they lower the energy barrier, and how they permit reactions to proceed more quickly.