2.18: Enzymes are proteins that speed up chemical reactions.

Protein shape is particularly critical in enzymes, molecules that help initiate and accelerate the chemical reactions in our bodies. Enzymes emerge unchanged—in their original form—when the reaction is complete and thus can be used again and again. Here’s how they work.

Think of an enzyme as a big piece of popcorn. Its tertiary or quaternary structure gives it a complex shape with lots of nooks and crannies. Within one of those nooks is a small area called the active site (FIGURE 2-42). Based on the chemical properties of the atoms lining this pocket, the active site provides a place for the participants in a chemical reaction, the reactants or substrate molecules, to nestle briefly.

Enzymes are very choosy: they bind only with their appropriate substrate molecules, much like a lock that can be opened with only one key (see Figure 2-42). Only the substrate molecules are of the correct size and shape to fit into the active-site groove. Moreover, the exposed atoms in the active site are positioned to form weak interactions with specific atoms in the substrates. Once a substrate molecule is bound to the active site, a reaction can take place—and usually does so very quickly.

The chemical reactions that occur in organisms can either release energy or consume energy. But in either case, a certain minimum energy—a little “push”—is needed to initiate the reaction, called activation energy. And although enzymes don’t alter the amount of energy released by a reaction, they act as catalysts by lowering the activation energy, which causes the reaction to occur more quickly. For example, enzymes may lower the activation energy by holding substrate molecules in an orientation that stresses bonds that need to break or brings together atoms that need to bond.

By virtue of their catalytic capacities, enzymes are at the heart of the chemistry of living organisms. Taken together, all of the chemical reactions in a living organism are its metabolism.

Increasingly complex molecules are synthesized or degraded in a series of sequential reactions called a “metabolic pathway.” Each step of these metabolic pathways is catalyzed by an enzyme produced in the body. Proteins are the building blocks with which living organisms are built, but since nearly all enzymes are proteins, proteins can also be thought of as the builders of bodies, too.