DISTRIBUTIVE SYSTEMS 



347 



Dixon and Webb, 1958, p. 565). A generalized system may be represented 

 as: 



A -> i^r - X ^ D 



(7-33) 



where A, B, C, D, F, and G are substrates that can accept the group X 

 from the central molecule M-X, each substrate requiring a different trans- 

 ferring enzyme. Any step may be reversible or essentially irreversible. 

 In the latter case no satisfactory thermodynamic equilibrium of X between 

 the various substrates will be achieved but the system may be in a steady 

 state, M-X being formed by one of the reactions and utilized by the rest, 

 at least one of the products, such as C-X, reacting further outside the sys- 

 tem. It is unlikely that distribution of X will be commonly determined by 

 the equilibrium constants of the individual reactions, but rather by the 

 relative rates, since systems in the cell are usually not static. The central 

 compound and the enzymes associated with it in some important systems 



are given in the tabulation (following Dixon and Webb). With regard to 

 kinetics, these systems are really multibranched chains when M-X is 

 formed at a constant rate and the effects of inhibitors as discussed above 

 apply. 



Inhibition of a single enzyme in such a system would cause the distribu- 

 tion of X to become abnormal and thus may modify the pattern of meta- 

 bolic flow in the cell. The steady-state concentration of M-X will be more 

 or less stabilized, in the sense that alterations of enzyme activity will have 

 relatively little effect on it. For this reason it is often difficult to relate levels 

 of ATP in the cell to inhibitions of enzymes involved in its formation or 

 utilization, the concentration of ATP being rather unresponsive to such in- 

 hibition over an appreciable range. 



The inhibition of enzymes in distributive systems may have complex 

 effects on the over-all metabolism. When one of the acceptors can serve as 

 a reserve of X, which, upon cessation of the formation of M-X by the usual 



