INTRODUCTION TO BIOCHEMICAL SYSTEMATICS 53 



Although the exact wording of the quoted paragraph empha- 

 sizes variation within individual organisms, variations among individ- 

 uals and among species occur. In the beginning, it is to be expected 

 that enzyme differences within individuals will comphcate taxonomic 

 appraisals of interspecific differences, but as the bases for such differ- 

 ences are better understood, the problem should be simpHfied. An 

 excellent illustration of the extent of variations in similar enzymes is 

 to be found in Fig. 4-3. 



Directly related to the remarks made above are implications 

 from studies of the effect of partial degradation of enzymes upon 

 their activity. Selected examples from those summarized by Anfinsen 

 are the following: 



(1) ACTH consists of 39 amino acid residues. 



(a) With carboxy peptidase, three C- terminal residues may 

 be removed without loss of activity. 



(b) With hmited pepsin digestion, eleven C-terminal resi- 

 dues may be removed without loss of activity. 



(c) With mild acid hydrolysis fifteen C-terminal residues 

 may be removed without loss of activity. 



(d) Loss of even one or two A'^-terminal residues results in 

 loss of activity. 



(2) Papain consists of 180 amino acids. 



(a) About eighty residues from the A/^-terminal end may 

 be removed without loss of activity. 



(3) Ribonuclease consists of 124 residues. (Fig. 4-2.) 



Figure 4-2 may be consulted to show the extensive modifica- 

 tions of ribonuclease which may be tolerated without loss of activity 

 of the enzyme. Note that it is at the extremes of the protein chain 

 wherein modification is permitted without loss of activity. 



The discovery that enzymes may have a rather large number 

 of nonessential amino acids (that is, nonessential with respect to the 

 overt action of the enzyme) associated with them and also substitu- 

 tions within the essential parts at some points without loss of activity 

 supports the earlier implications of serological findings: that the same 

 enzyme from two species may differ. This fact, again, provides for a 



^ Although enzyme heterogeneity is becoming recognized as commonplace, there 

 are also examples of enzymes from different sources which appear to be identical, at least 

 by serological criteria. For example, Fredrick (1961) has reported that a purified phos- 

 phorylase preparation from the bluegreen alga, Oscillatoria princeps, was serologically 

 active against other blue green algae. Yet, as noted, serological activity may not indicate 

 total similarity. 



