ENZYME COMPLEXES 



conceptual and experimental inadequacies. How these potenti- 

 alities are translated into actualities, however, can be determined 

 only in the presence of the other components of the system. It 

 is trite to state here and it has been argued more convincingly 

 by others that different observational principles are operative, 

 and different experimental approaches are fruitful, at different 

 levels of organization. I should like to go one step further. I 

 believe it to be inherently impossible (or at least to be accom- 

 panied by such formidable experimental and arithmetical 

 hurdles as to be essentially and pragmatically fruitless) to arrive 

 at a completely determined description of the interaction of 

 individual units at one observational or abstractional level using 

 only the observations and abstractions of that level. Thus I 

 doubt whether it is possible to arrive at a knowledge of molecular 

 structure and inter- and intramolecular interactions on the 

 basis of a knowledge, no matter how thorough, of the properties 

 and structures of the individual atoms making up the molecules, 

 in the complete absence of any observations on molecules. 

 Similarly an all-embracing study of individual enzymatic 

 reactions can tell us very little about their interactions and about 

 the more complex array of enzymes from which they must have 

 been derived. Investigations solely of this kind do not permit 

 us to form a clear picture of the catalytic efficiency of this array, 

 the nature of the forces holding together the array, and the 

 control mechanisms operative within the array. Yet it is pre- 

 cisely these concepts of efficiency and control that are probably 

 the most distinguishing features in any comparison of a group 

 of enzymes acting jointly within an array as compared to a 

 mixture of these same enzymes in solution. Efficiency means 

 increased turnover at a given substrate concentration: the 

 array permits the transfer of key coenzymes (such as CoA) and 

 of intermediates (the products of any preceding enzymic step 

 which become the substrates of the succeeding one, as in the 

 reaction catalyzed by pyruvic dehydrogenase, vide infra) in an 

 essentially intramolecular manner, from one site to the next. 

 Free coenzyme or intermediate in solution never comes into 



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