CARBOXYPEPTIDASE, AMINOPEPTIDASES, DIPEPTIDASES 365 



the substrate site entirely, or possibly to two substrate sites, or even to 

 anionic groups outside the substrate site. The enzyme may have binding 

 sites for the imidazole ring of histamine since imidazole inhibits 11%, 

 imidazolelactate 20%, and histidine 4% at 6.7 mM when cadaverine is 

 3.3 mM (Zeller, 1941). Urate also competitively inhibits the oxidation of 

 histamine, but rather weakly. An excellent review of the structure-action 

 relationships among the amidine derivatives is by Fastier (1962). 



CARBOXYPEPTIDASE, AMINOPEPTIDASES, 

 AND DIPEPTIDASES 



Certain aspects of the inhibition of carboxypeptidase by substrate 

 analogs were discussed in Volume I (page 292) to illustrate how certain 

 interaction contributions could be estimated. We shall now attempt to 

 visualize more clearly the orientation of these analogs on the enzyme sur- 

 face. The data indicate that a three-point attachment of the substrate 

 is necessary for catalysis. The enzyme sites may be indicated as follows 

 (see Fig. 2-9 for hypothetical orientation of substrate): (A) the peptidatic 

 site contains the mechanism of the electron displacement necessary for hy- 

 drolysis and is probably positively charged, (B) the cationic site is a positively 

 charged group that interacts electrostatically with the C00~ group, and 

 (C) the electrokinetic site is perhaps a lipophilic region capable of reacting 

 with alkyl or phenyl groups by dispersion forces. It is easy to see why 

 D-substrates are not reacted since the peptide bonds would not be able to 

 approach the peptidatic site. There is also an enzyme region near the projec- 

 tion direction of the fourth asymmetric carbon bond that sterically prevents 

 attachment of groups larger than an amino group, and thus the D-isomers 

 usually do not bind and are not inhibitors. Only two-point attachment is 

 necessary for inhibitors, and most that have been studied bind at the cationic 

 and electrokinetic sites. 



The relative binding energies for inhibitors in Table 1-6-26 were calculated 

 from the data of Smith etal. (1951). Earlier studies by Elkins-Kaufman and 

 Neurath (1949) provide additional information on the competitive inhibitors 

 in the accompanying tabulation. It is interesting that D-phenylalanine is 



