ENZYMES 133 



for peptide bonds (as far as is known), but these peptide bonds must 

 involve certain definite amino acids. Nevertheless, a wide variety of 

 different substrate molecules is attacked. At the other extreme, urease 

 catalyzes only one known reaction, 



H2NCONH2 + H2O -^ 2NH3 + H2CO3 



Such absolute specificity as this is relatively rare. Usually the degree 

 is somewhere intermediate between these two extremes. It is common 

 for an enzyme to catalyze the reaction of one or at most a few sub- 

 strates in living systems. In the test tube, this enzyme can promote 

 reaction of synthetic substrates closely related to the naturally occur- 

 ring one. For example, catalase decomposes hydrogen peroxide and 

 presumably serves to prevent accumulation of toxic concentrations of 

 this product of the action of certain oxidases. Catalase accelerates the 

 breakdown of ethyl hydrogen peroxide, C2H5OOH, a compound un- 

 known and unlikely in biological sources. Structurally related com- 

 pounds may thus often be used as substrates for an enzyme. 



Stereochemical specificity is widely encountered. One optical isomer 

 serves as a substrate; the other is completely inert. A peptidase, car- 

 boxypeptidase, hydrolyzes the synthetic peptide derivative carbo- 

 benzoxyglycyl-L-phenylalanine but has no effect on the d isomer. Mal- 

 tase hydrolyzes maltose and several other a-glucosides but no ^-gluco- 

 sides. |S-Glucosidase splits the (3 but not the a isomers. D-Amino acid 

 oxidase is widely distributed in animal cells and is specific for the 

 D forms. The less common L-amino acid oxidases occur in quantity in 

 snake venoms and are equally specific. This high degree of optical 

 specificity is commonplace and is important, since cells seem to make 

 a preferred optical form whenever two or more forms of metabolic 

 compounds are possible. Indeed it is suspected that the optical specific- 

 ity of enzymes may be primarily responsible for the occurrence of 

 optical specificity among the other biological molecules. 



When related substrates are attacked, as by catalase in the example 

 given, the rates often differ. Sometimes the natural substrate reacts 

 more rapidly, sometimes less rapidly, than the synthetic substances. 

 The reasons for these differences are obscure but probably lie in the 

 chemical reactivities of the substrate molecules and in the matching 

 of the geometry and force fields of substrate and enzyme. It seems 

 likely that enzymatic catalysis always involves the formation of a com- 

 plex between enzyme and substrate. If the structures of the enzyme 

 and substrate do not properly correspond, they cannot fit together to 

 form a reactive complex. When the correspondence is not perfect, 

 catalysis may be possible but less efficient. Thus the presence of 



