Normally chemical reactions do not proceed spontaneously, but require the help of a catalyst.
    A catalyst accelerates a chemical reaction without itself being changed. For example, the reaction of hydrogen with oxygen to produce water requires the addition of the metal platinum. These days we encounter the concept of a catalyst most often in connection with technology for cleaning up the exhaust fumes from our automobiles, where platinum and rhodium catalyze the breakdown of polluting nitrogen oxides.
    Chemical reactions within living cells must also be catalyzed. Biological catalysts are called enzymes. There is, for instance, an enzyme in our saliva which converts starch to a simple sugar, which is used by the cell to produce energy, and another enzyme which degrades the excess lactic acid produced when we overexert ourselves. All green plants contain enzymes which convert carbon dioxide in the air to nutritious carbohydrates such as sugar and starch. Without enzymes life would not be possible!
    Enzymes are highly selective. Among the thousands of different compounds in a cell, an enzyme can recognize the right molecule (substrate) and transform it into a new product. This property arises from the special three-dimensional structure of each enzyme. One can compare an enzyme and its substrate with a lock and its key.
    Enzymes are very effective catalysts. A chemical reaction might require several months to reach completion without a catalyst, but only a few seconds with the help of an enzyme. Since the enzyme remains unchanged, one enzyme molecule can catalyze the transformation of millions of substrate molecules.
    Up until the beginning of the 1980’s, all enzymes were thought to be proteins. We now know that proteins do not have a monopoly on biocatalysis. RNA molecules can also function as enzymes.