A catalyst is a chemical that increases the rate of a chemical reaction without itself being changed by the reaction. The fact that they aren't changed by participating in a reaction distinguishes catalysts from substrates, which are the reactants on which catalysts work. Enzymes catalyze biochemical reactions. They are similar to other chemical catalysts in many ways:
- Enzymes and chemical catalysts both affect the rate but not the equilibrium constant of a chemical reaction. Reactions proceed downhill energetically, in accord with the Second Law of Thermodynamics. Catalysts merely reduce the time that a thermodynamically favored reaction requires to reach equilibrium. Remember that the Second Law of Thermodynamics tells whether a reaction can occur but not how fast it occurs.
- Enzymes and chemical catalysts increase the rate of a chemical reaction in both directions, forward and reverse. This principle of catalysis follows from the fact that catalysts can't change the equilibrium of a reaction. Because a reaction at equilibrium occurs at the same rate both directions, a catalyst that speeds up the forward but not the reverse reaction necessarily alters the equilibrium of the reaction.
- Enzymes and chemical catalysts bind their substrates, not permanently, but transiently—for a brief time. There is no action at a distance involved. The portion of an enzyme that binds substrate and carries out the actual catalysis is termed the active site.
Enzymes differ from ordinary chemical catalysts in several important respects:
Enzymes are specific. Chemical catalysts can react with a variety of substrates. For example, hydroxide ions can catalyze the formation of double bonds and also the hydrolysis of esters. Usually enzymes catalyze only a single type of reaction, and often they work only on one or a few substrate compounds.
Enzymes work under mild conditions. Chemical catalysts often require high temperature and/or pressure to function. For example, nitrogen can be reduced to ammonia industrially by the Haber process, catalyzed by iron at 500° C. and at 300 atmospheres pressure of N 2. In contrast, the same reaction is carried out enzymatically at 25°C. and less than 1 atmosphere pressure of N 2. These gentle conditions of temperature, pressure, and pH characterize enzymatic catalysis, especially within cells.
Enzymes are stereospecific. Chemical catalysis of a reaction usually leads to a mixture of stereoisomers. For example, the addition of acid‐catalyzed water to a double bond leads to an equimolar (50:50) mixture of D and L isomers where the water is added. In contrast, catalysis of water addition by enzymes results in complete formation of either the D or L isomer, but not both.
Enzymes are macromolecules. The macromolecules are composed of protein, or in a few cases, RNA. Most chemical catalysts are either surfaces, for example, metals like platinum, or else small ions, such as hydroxide ions.
Enzymes are often regulated. The regulation occurs either by the concentration of substrates, by binding small molecules or other proteins, or by covalent modification of the enzymes' amino acid side chains. Thus, an enzyme's effectiveness can be altered without changing the concentration of the enzyme; on the other hand, the effectiveness of a chemical catalyst is generally determined by its overall concentration.