FrvDRor^vsis of pf.ptidf: bonds 



phase rule crilcria lor a pure protein (15), it wonUI .ippcar lli.K each of 

 these two proteins exhibits more than one distinet enzymic speeifiriiy. 

 The twofold specificity of the chymotrypsins may he ex|)lainecl on the 

 basis of the hypothesis presented above by assuming that each of the 

 protein molecules contains two distinct and different essential centers. 



The hypothesis of the predominant role of the essential center 

 as against that of the remainder of the enzyme molecule cannot be 

 considered the same as the well-known theory of Willstalter (34), who 

 assumed that an enzyme molecule consists of a colloidal carrier and a 

 prosthetic group, the latter being responsible for the enzymic activity 

 and specificity. On the basis of this theory, Kraut distinguishes be- 

 tween the pheron and the agon as the two components of enzymes 

 (20). Numerous workers refer to the prosthetic group as the co- 

 enzyme and the carrier as the apoenzyme (1), th6 dissociable flavo- 

 proteins frequently being regarded as the prototype of this postulated 

 dual structure. It should be recalled, however, that the flavin part 

 of the flavoprotcins usually represents one of the partners in the chemi- 

 cal reaction and that the essential catalytic activity resides in the 

 protein moiety; cf. also Parnas (27). In the case of proteolytic 

 enzymes that do not require activation by sulfhydryl compounds 

 (pepsin, trypsin, etc.), no evidence of a dual structure is available. 

 For the activatable proteolytic enzymes, our present knowledge indi- 

 cates that the active enzyme represents a dissociable combination of a 

 protein with one of several activators (19). However, in the case of 

 these latter enzymes, it cannot be claimed that the protein part of the 

 enzyme acts merely as a colloidal carrier for another active part of the 

 enzyme. On the contrary, it is the protein part of the activated 

 enzyme which contains the essential center and which determines the 

 specificity. No proteolytic enzyme is known in which the nature of 

 the activator determines the specificity type of the enzyme. 



Antipodal Specificily of Proteolylic Enzymes 



The majority of the known proteolytic enzymes of higher plants 

 and animals has been found to be adapted to the hydrolysis of sub- 

 strates in which the essential side chain belongs to an /-amino acid. 

 For example, chymotrypsin endopeptidase rapidly hydrolyzcs the 

 substrate benzoyl-/-tyrosylglycinamide, but does not hydrolyze bcnzoyl- 



129 



