GENERAL CONSIDERATIONS OF MULTIENZYME SYSTEMS 377 



the tricarboxylic acid cycle, electron transport, and phosphate transfer) 

 the values generally fall between 2000 and 25,000 molecules of substrate 

 reacting per enzyme site per minute under optimal conditions. Since the 

 enzymes of a sequence are seldom saturated with substrate and inasmuch 

 as the over-all rate relates particularly to the slowest step, a turnover 

 number of around 5000 may be selected as representing an active multi- 

 enzyme system. At this rate, one molecule of substrate and each inter- 

 mediate is reacted per enzyme site each 12 msec. If molecules are to be 

 transferred from enzyme to enzyme as rapidly as reacted, so that no marked 

 accumulation occurs, the transit time must be less than this value. Table 

 7-1 shows that this will be the case, even with molecules of protein size, 

 when the distance between enzymes is less than 10,000 A or 1 //. Within 

 the cell the distances would certainly be less, so that diffusion would not be 

 limiting unless permeability barriers between the enzymes existed. In an 

 isolated homogeneous enzyme preparation, where each enzyme of the se- 

 quence is present at lO"'^ M, the average distance between consecutive 

 enzymes would be 0.25 //. Thus it is unlikely that diffusion will be limiting 

 even in these preparations unless the turnover number of an enzyme is 

 very high and the diffusing intermediate is large, or the enzyme concen- 

 tration is much less that 10"'^ M. 



Compartmentalization 



The effect of restriction of multienzyme sequences to limited regions 

 of the system on the maintenance of the steady state during inhibition has 

 been discussed. However, there are many possible types of compartmental- 

 ization in cells and it is probable that this is often an important factor in 

 the response to inhibition. Recent studies of cell structure indicate that 

 there may be more compartmentalization than previously believed. The 

 demonstration of a system of membranes within the cytoplasm (endo- 

 plasmic reticulum), the probability of two or more distinct regions in mito- 

 chondria, and the unsuspected double nature of most membranes all point 

 to the likelihood that the cell consists of a very complex structure of dif- 

 ferent compartments separated by barriers with different properties. Evi- 

 dence is accumulating that metabolic systems exist in these various re- 

 gions and can, within limits, operate independently. 



The subject of compartmentalization with all its ramifications is much 

 too extensive and complex to be taken up here in any detail, but it may be 

 useful to outline in a qualitative fashion the changes brought about by two 

 basic types of compartmentalization (Fig. 7-38). Each of the three systems 

 — the homogeneous (I), the single compartment (II), and the double com- 

 partment (III) — contains the same total amount of enzymes and substrate, 

 the only difference being the restriction of the enzymes to certain regions. 

 Two general situations may be distinguished: the compartment membranes 



