How Enzymes Work If exergonic reactions occur spontaneously, what keeps molecules from breaking apart and cell chemistry from racing out of control? For any reaction to occur, even a downhill reaction, some energy must be added to get the reaction going. This energy is needed to break bonds in the reactant molecules. The energy needed to start a chemical reaction is called the energy of activation (EA). This required energy input represents a barrier that prevents even energy-releasing exergonic reactions from occurring without some added energy. How does a living cell overcome the energy barrier so that its metabolic reactions can occur quickly and precisely? A special kind of protein called an enzyme is the answer. An enzyme serves as a biological catalyst, increasing the rate of a reaction without being changed into a different molecule. An enzyme does not add energy to a reaction; instead, it speeds up a reaction by lowering the energy barrier. An enzyme is very selective. Its three-dimensional shape allows it to act only on specific molecules, referred to as the enzyme's substrates. As the substrates bind to the enzyme's active site, they are held in a position that facilitates the reaction. This takes less activation energy than the unaided reaction. Products form and are released. The enzyme emerges unchanged from the reaction. Because of the specific fit between enzyme and substrate, each enzyme can catalyze only one kind of reaction involving specific substrates. Thousands of different enzymes may be required to carry out all of a cell's metabolic processes.