70 The Nature of Biological Diversity 



some concepts or points, considerable use will be made of data from 

 our group at Minnesota because I am most familiar with them and 

 can speak most freely of them, not because these data are necessarily 

 the best to illustrate a particular point. 



From the viewpoint of an enzymologist, the most striking charac- 

 teristic of different living forms is not their diversity but their similar- 

 ity. Morphological and functional differences among organisms and 

 cells are large compared to chemical differences, and in particular, 

 compared to differences in cellular enzyme chemistry. Some generali- 

 zations about the nature of enzyme action, which are so well recog- 

 nized that their importance may be overlooked, are as follows: 



Catalytic proteins (enzymes) are found in all cells. This and the fol- 

 lowing generalization are absolute in that no known exceptions 

 have been found. Thus biologists would predict with a high degree 

 of confidence that any living form found on earth would have 

 enzymes in its cells. 



All cells have in common some basic enzymic reactions. All known 

 types of cells, including many diverse unicellular organisms, have 

 enzyme systems for the formation and use of certain key chemical 

 substances, such as amino acids, particular organic phosphates, 

 and ribonucleic acids. 



Various enzymic reactions have important common features of mech- 

 anism. The most prominent of these features is that enzymic 

 catalyses occur by specific combination of the substrate mole- 

 cules with particular regions of the enzyme molecule. Possibilities 

 of some other common features in finer details of mechanism 

 will be examined later. 



Enzyme cof actors have similar roles in different cells. This has been 

 an important generalization in the elucidation of metabolic func- 

 tion of vitamins. Biochemists have frequently taken advantage 

 of the knowledge that a cofactor role found with microorganisms 

 will apply to larger, more complex forms in an identical manner 

 or one which is closely related chemically. 



Among the implications of the first two generalizations are those 

 related to the origin of life itself, and a few comments of a phylo- 

 genetic nature may be in order, particularly in this day of interest in 

 planets other than earth. Existence of living material without the 

 presence of catalysts therein is difficult to visualize, but one may ask 

 if the enzyme protein is the only possible way for a variety of cat- 

 alysts to be produced from chemical elements. Are the properties 

 of phosphate and phosphate compounds, and of nucleic acids such that 



