76 The Nature of Biological Diversity 



as other differences hetween the two enzymes, suggests that the 

 enzymes have some important differences in the mechanism of their 

 catalytic reactions. The situation is reminiscent of the familiar bio- 

 logical truism about diversity, namely, "there is more than one way 

 to skin a cat." 



A number of other yeast enzymes have not been found to have the 

 marked differences from their mammalian counterparts shown by the 

 aldolases. Thus both liver and yeast alcohol dehydrogenases contain 

 Zn as an essential component (14), and yeast and muscle glyceralde- 



Table 3. Some characteristics of yeast and muscle 

 aldolases 



sources: Refs. 11, 12, 13. 



hyde-3-phosphate dehydrogenases appear to have an essential — SH 

 group or groups at the catalytic site (15). A difference between yeast 

 and mammalian lactate dehydrogenases deserves mention however, if 

 only for its social implications. The yeast enzyme uses a flavin-contain- 

 ing cofactor, in contrast to the reaction of the mammalian enzyme, 

 which uses diphosphopyridine nucleotide. Were it not for this differ- 

 ence, lactic acid and not ethyl alcohol would be expected as a principal 

 metabolic product of yeast. 



Amino acid analog incorporation and activity 



An additional aspect of variation in composition as related to 

 enzyme function comes from studies on the specificity of protein 

 synthesis. Results and implications of recent researches are ably dis- 

 cussed in a review by Vaughan and Steinberg (16) ; studies of amino 

 acid analog incorporation appear to be of particular pertinence. 

 Various investigators working with bacterial systems have demon- 

 strated that amino acid analogs may be incorporated into tissue 

 proteins in place of naturally occurring amino acids. Recently in our 



