The Nature and Diversity of Catalytic Protein- 83 



change of inorganic phosphate with ATP would he made to he ahout 

 equal, greater than, or less than the rate of interchange of glutamate 

 with glutamine (30). This led to some theoretical considerations of 

 the factors governing reaction rates at equilibrium. Some clear and 

 useful concepts have emerged (31). With regard to the glutamine 

 synthetase reaction, as depicted in Fig. 5, an explanation for the 

 inequalities of exchange rates is forthcoming if the slow steps in the 

 overall catalysis are not those in which covalent bonds are formed 

 or broken, but those of release of substrates from the enzyme. In- 

 equalities of exchange rates near equilibrium have been demonstrated 

 for the aldolase reaction as well (32), and studies with this and with 

 dehydrogenase reactions are in progress at Minnesota. The experi- 



Enzyme 



NH 3 -*- 



► NH- 



RC00~-<- 



ATP -*- 



RCONH 



*~RC00 



*-ATP 



RCONH, 



7P 



ADP 



FIG. 5. A scheme for the glutamate synthetase reaction showing substrate binding 

 and release steps. 



mental results indicate that the tacit assumption frequently made, 

 that the slow steps in enzymic catalyses are those in which the 

 covalent bonds are broken and formed, is open to serious question. 

 Recognition that there may be a considerable energy barrier be- 

 tween the free and bound forms of substrates, taken together with 

 the remarkable specificity of enzymic catalyses, suggests that the 

 important structural and electronic changes necessary for catal\-i- 

 accompany the binding, and that reaction of the bound substrates 

 occurs readily with little further change in their position and 

 properties. Such considerations lend support to the concept of the 

 synthetase and other enzymic reactions as concerted reactions i I. 33), 

 and actually give a simpler picture of the catalysis. This may be illus- 

 trated with a typical synthetase reaction, the acetate thiokinase re- 

 action, which also shows inequalities of substrate exchanges al 

 equilibrium (30 1. In line with the above discussion, the acetate 



