Metabolic Pathways 107 



of a fixed order; and (c) competition between alternative 

 pathways. 



One more factor should be added. Granting that there 

 are reaction sequences of such a nature that the first step 

 of what happens to a substance determines, for a long path 

 thereafter, what must subsequently happen to it; granting 

 that competition between alternative pathways exists, it 

 follows that the competitive sites are not critical at each 

 reaction step but only at points where fixed path reaction 

 sequences cross. That is, one has lines of metabolism, but 

 at points where these lines cross — at the nexus or node, as 

 it were — there the competitive forces exert their far-reach- 

 ing effect. 



One further question may now be asked of Nature: Are 

 the lines or paths of metabolism many or few? Of the 

 thousands of substances acted upon by the living cell, are 

 there thousands of pathways? The experimental answer is: 

 there are few. Not just one, but a few. There are not a 

 thousand paths from glucose to pyruvate, but there is more 

 than one — three, perhaps five, but no more. The main 

 lines of metabolism of carbohydrate, for example, are no 

 more than perhaps ten at the most. The degradation of an 

 array of substances, at first sight utterly dissimilar, eventu- 

 ally proceeds through the same central intermediary metab- 

 olites. The shoes, the ships, the sealing wax, degrade, in 

 principle, through one or another of the major pathways, 

 and their breakdown is not different from that of sugar 

 and spice; only the early enzymatic maneuvers have to be 

 slightly different. 



With synthesis, it may be otherwise, but we suspect not. 

 In fact, one may propose that synthesis begins at such 

 crossroads of metabolic paths. In short, there is a certain 

 unity of process in all living forms; not identity, but not 

 wholesale diversity. In a sense, life has been fashioned, not 

 from an identical mold but from a mold designed by the 

 same craftsman using the same tools in the same way. It is 



