In simple terms, an enzyme functions by binding to one or more of the reactants in a reaction. The reactants that bind to the enzyme are known as the substrates of the enzyme. The exact location on the enzyme where substrate binding takes place is called the active site of the enzyme. The shape of the active site just fits the shape of the substrate, somewhat like a lock fits a key. In this way only the correct substrate binds to the enzyme.
Once the substrate or substrates are bound to the enzyme, the enzyme can promote the desired reaction in some particular way. What that way is depends on the nature of the reaction and the nature of the enzyme. An enzyme may hold two substrate molecules in precisely the orientation needed for the reaction to occur. Or binding to the enzyme may weaken a bond in a substrate molecule that must be broken in the course of the reaction, thus increasing the rate at which the reaction can occur.
An enzyme may also couple two different reactions. Coupling an exothermic reaction with an endothermic one allows the enzyme to use the energy released by the exothermic reaction to drive the endothermic reaction. In fact, a large variety of enzymes couple many different endothermic reactions to the exothermic reaction in which ATP is converted by hydrolysis to ADP. In this way, ATP serves as the molecular fuel that powers most of the energy-requiring processes of living things.
E-mail instructor: Sue Eggling
Clackamas Community College