Physical Chemistry Third Edition

576 13 Chemical Reaction Mechanisms II: Catalysis and Miscellaneous Topics

Reactant

Enzyme

Lock-and-key

model

Enzyme-reactant complex

Figure 13.6 The Active Site of a Hypothetical Enzyme.

a ribozyme apparently has been found that cleaves an RNA molecule without such

ions.^10

Enzymes generally exhibitspecificity, which means that they catalyze only specific

reactions. Three kinds of specificity are recognized. The first isabsolute specificity,

which means that the enzyme catalyzes the reaction of only one substance. Urease

exhibits this kind of specificity, since it will not catalyze the reaction of anything other

than urea. The second kind of specificity isgroup specificity, which means that the

enzyme catalyzes any of a group of reactions. Protease catalyzes the hydrolysis of

various kinds of proteins, but will not catalyze the hydrolysis of fats or carbohydrates.

The third kind of specificity is calledstereochemical specificity, which means that an

enzyme will catalyze the reaction of one optical isomer but not its enantiomorph.

Protease catalyzes the hydrolysis of polypeptides made ofl-amino acids, but not

polypeptides made ofd-amino acids.

A typical enzyme molecule has anactive siteat which a reactant molecule can attach

itself. The active site is often like a socket into which the reactant molecule fits, like a

key in a lock, as shown schematically in Figure 13.6. Once in the active site, the reactant

molecule is rendered more reactive through conformational changes or polarizations

produced by interaction with the enzyme. The reactant is often put into a conformation

similar to a transition state.

(^10) S. R. Das and J. A. Piccirilli,Nat. Chem. Biol., 1 , 45 (2005).