The Nature and Diversity of Catalytic Proteins 71 



other means of handling cellular energy and reproductive require- 

 ments are not possible? The enzymological unity of the forms of life 

 we recognize might find its explanation in one of the following: 



1. A common origin of all living cells. This is implicit in theories 

 of the origin of life based on the occurrence of at least one highly 

 improbable event. 



2. Given the changing chemical environment to which earth has 

 been exposed, the development of life is a logical consequence. This 

 implies that the basic chemistry common to various cells is uniquely 

 required for life under our present conditions. 



3. Living forms with different chemical bases have existed on earth, 

 but that form with a chemistry as we know it today has, through 

 adaptability, been the only form to survive. 



Means are not at hand to distinguish among these or other pos- 

 sibilities, and it is not my purpose to dwell further upon such matters 

 but to turn attention to certain questions for which pertinent informa- 

 tion is at hand. Some questions about the diversity of enzymes to 

 which the bulk of this lecture will be addressed are: 



Do enzymes catalyzing the saiiie or similar reactions in various cells 

 have structural features in common? Features about which 

 meager information is becoming available pertain to size, amino 

 acid composition, amino acid sequences in catalytically important 

 areas, and the relation of degradation and of modification of 

 composition to catalytic activity. In this lecture, some results on 

 amino acids present at catalytic sites of hydrolytic enzymes and 

 on degradation of enzymes in relation to activity will be pre- 

 sented. 



Do enzymes from different sources catalyzing the same reaction do 

 so by the same or similar mechanism? At this stage the answer 

 appears to be yes for a number and perhaps for most enzymes, 

 but with some enzymes there is strong suggestion that different 

 mechanisms hold. Attention will be directed toward some results 

 with aldolases, and with several types of enzymes involved in 

 phosphorylation reactions. 



Are there additional common features of enzyme mechanisms which 

 are as yet unrecognized? Study of mechanism has many levels, 

 and as finer details of mechanism are revealed, examination for 

 possible similarities in the chemistry of various catalyses is in 

 order. Some of our studies on syntheses coupled to ATI* cleavage 

 have given indications of previously unrecognized features of 

 enzymic catalyses, and these implications, together with an anal- 



