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|Chapter Two: The Chemistry of Biology|
Enzymes are very special types of proteins for all living things; they are called organic catalysts. The first part makes sense, since enzymes are proteins, and proteins are organic, but let's make clear what it means to be a catalyst. A catalyst is any substance that speeds up the rate of a chemical reaction but is itself not affected once the reaction is completed. Enzymes are used in organisms to increase the rate of the chemical reactions that are necessary for life.
For an enzyme to catalyze a reaction, it must join with one or more of the molecules in the reaction; the molecules that an enzyme attaches to are called substrates, and when they join they form an enzyme-substrate complex. However, enzymes are able to attach only to certain substrates, a fact which is explained by the structure of an enzyme. Enzymes are proteins which have folded onto themselves several times to create a complex, three-dimensional structure. Each enzyme has an area called the active site where the substrate will join, but, like a lock and a key, only certain substrates will fit into the active site.
Unfortunately, it's not as simple as just a lock and a key. Sometimes, a substrate doesn't fit exactly into the active site of an enzyme, but the match is fairly close. In these cases, the enzyme is induced (persuaded) to change the shape of its active site slightly so that the substrate fits. This explanation of an enzyme's activity is called the "induced-fit hypothesis" and is generally accepted by most biologists.
Coenzymes are organic molecules which are not proteins like enzymes but still play a role in reactions catalyzed by enzymes. In cells, coenzymes frequently serve as electron acceptors; they bond with electrons released by chemical reactions in the cell. In Chapter Four, you will see how coenzymes are especially important for the process of cell respiration.