82 The Nature of Biological Diversity 



cleavage, and that of the four possible modes of ATP cleavage leading 

 to release of one or both of the terminal phosphoryl groups of ATP, 

 only the two indicated in Fig. 4 occur (4, 25) . Such results give further 

 evidence of the mechanistic similarity of various enzymes catalyzing 

 similar reactions. 



One other aspect of the mechanism of enzymes catalyzing similar 

 reactions deserving brief mention pertains to enzyme specificity. An 

 unsettled question is whether enzymes catalyzing a similar reaction, 

 but with marked differences in the degree of specificity, do so by a 

 similar mechanism. Phosphatases are known, for example, with a high 

 degree of specificity, such as a phosphatase which will remove the 

 phosphoryl residue only from the 5' position of nucleotides. Other 

 phosphatases are known with considerably wider specificity, ranging 

 to the extracellular phosphatase from Escherichia coli, which Heppel 

 (29) has found to have a remarkable ability to hydrolyze a wide 

 variety of phosphate esters. Aside from their interest as examples of 

 the variability of enzyme specificity, such findings need study and 

 explanation on a mechanistic level. 



Possible generalities for enzyme tnechanistns 



Thus far students of enzyme mechanism have not uncovered any 

 properties of the protein molecule which serve as a general basis 

 for enzyme catalysis, and beyond the implications inherent in the 

 participation of a beautifully specific combination of enzyme and 

 substrate, one rapidly enters an area of speculation. With regard 

 to thermodynamic considerations, perhaps the most important finding 

 is that all enzymes appear to lower the energy of activation for the 

 reaction. Some kinetic findings of possible importance to enzyme 

 mechanisms in general have arisen from our studies of syntheses 

 coupled to ATP cleavage, and I would like to present some aspects 

 of these researches. 



In probing at the mechanism of glutamine synthetase, catalyzing 



ATP + glutamate + NH 3 ^± ADP + glutamine + Pi 



the above reaction, studies of the dynamic reaction rate at equilibrium 

 were undertaken, that is, the rate of interconversion of reactants to 

 products when no net reaction is occurring. Such rates are measurable 

 by adding traces of isotopically labeled substrates to the reaction 

 mixture at dynamic equilibrium. Somewhat to our surprise, we found 

 marked inequalities in the isotopic exchange rates at equilibrium. 

 Thus, with variations in equilibrium conditions, the rate of inter- 



