Enzymes, produced by living things, are compounds that catalyze chemical reactions. Reactions involving enzymes may be said to proceed in two steps. In step 1, E + S ↔ ES (where E = enzyme, S = substrate, and ES = unstable intermediate complex that temporarily involves the enzyme). In step 2, ES + R ↔ P + E (where R = a substance in the substrate that reacts with the complex, P = the final product of the reaction, and E = enzyme liberated from the complex). Enzymes are critical to life because they have the ability to catalyze the chemical reactions that are important to life. Chemical reactions take place when the necessary reactants are present, but usually an energy input (activation energy) is required to start a particular reaction. The analogy usually given to illustrate this concept is that of a boulder located at the top of a hill. The boulder has the potential energy for rolling down the hill, but must first be pushed over the edge (see Fig. 8.1). The potential energy of the boulder could be great, depending on its mass and altitude. It could start a rock slide or landslide involving a great amount of energy. Although the energy required to push the boulder over the edge is insignificant compared to the total energy involved in the rolling of the boulder down the hill, that initial energy (called the activation energy) is nevertheless important, for without it there would be no landslide.
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