EFRAIM RACKER 



these specific interactions in intact cells (5). This investigation 

 represents a new and important approach to the steady-state 

 kinetics of intracellular metabolism. 



THE MICROCYCLES OF ENZYME-SUBSTRATE INTERACTIONS 



Catalysis may be looked upon, in many instances, as a cyclic 

 process in which the catalyst undergoes reversible changes. 

 Evidence for this thesis has been obtained in the case of the few 

 enzymes which have been closely examined from this point of 

 view. The enzymologist, hardened by his experiences with com- 

 plex metabolic cycles, now begins to turn to the exploration of 

 enzymes as "microcycles." 



6-l-P E-P E G-l,6-P 



G-6-R 



Figure 2. 



Phosphoglucomutase catalyzes the reversible transformation 

 of glucose- 1 -phosphate (G-l-p) to glucose-6-phosphate (G-6-p). 

 The reaction was first written as G-l-p<=^G-6-p. When the func- 

 tion of glucose-l,6-diphosphate (G-l,6-p) as a coenzyme was 

 discovered (28), the reaction was written as: G-l-p + G-l,6-p«=^ 

 G-1 ,6-p + G-6-p. With the elucidation of the role of the enzyme 

 protein as phosphate acceptor (32), the reaction sequence can be 

 represented by a cyclic process which includes all reactants, as 

 shown in Figure 2. The enzyme occurs in muscle extracts as 

 phosphoenzyme (E-p). In the presence of the substrate (either 

 G-l-p or G-6-p), the phosphate group is transferred and glu- 

 cose- 1,6-diphosphate is formed. The latter then returns the 

 phosphate to the enzyme and forms the product (G-6-p or 

 G-l-p). 



230 



