Enzymes are biological catalysts that constitute the largest and most highly specialized
class of protein molecules. Enzymes act as catalysts to increase the rates of chemical reactions,
but they do not cause a reaction to occur that would not proceed spontaneously without the
enzyme; that is, the reaction must have ΔG < 0. The reactions of metabolism would occur at
extremely slow rates at normal body temperature and pH in the absence of enzymes. An
appreciation of the catalytic efficiency of enzymes can be gained by realizing that under
optimal conditions, most enzymatic reactions proceed 108 to 1011 times more rapidly than the
corresponding reactions without enzymes. Without the enzymes in our digestive tract for
example, it would take us about 50 years to digest a single meal!
One very important property of enzymes is their specificity. Any one enzyme will only
catalyze a single class of chemical reactions. Some enzymes act on one substrate only; other
enzymes act on a family of related molecules. Enzymes participate in the reaction that they
catalyze, but they emerge unchanged at the end of the reaction, i.e., they are not used up.
Thus, a few enzyme molecules can go a long way. This reaction can be represented by the
following cartoon: