10. The ATP Molecule' 



Looking at the conventional structure formula of ATP (Fig. 

 12), one's first impression is that of great complexity. Nature does 

 not indulge in luxuries, so one may wonder why the cell uses such 

 a complex molecule if a P — O- — P link is all that is needed. A 

 much simpler inorganic polyphosphate should do just as well. 



The molecule has two ends: a phosphate-end and a purine-end. 

 The phosphate-end represents the energy store, (£); one may ask 

 whether the purine-end may not represent £*, thus providing the 

 molecule with the essential parts needed for the (£) -^ E* trans- 

 formation. The purine contains an extensive system of conjugated 

 double bonds with its nonlocalized tt electrons and five N-atoms, 

 each with its lone pair of electrons. As will be discussed later, 

 under conditions, this end of the molecule may also become 

 strongly fluorescent and thus conform to our demands of an E* 

 transmitter. The purine may thus be instrumental in transforming 

 the (E) of ^P into £*, when this (E) has to go into biological 

 action and drive the living machine. The whole ATP molecule 

 could thus be not only a storage battery but also a transformer. 



What is difficult to see in Fig. 12 is how the energy accepted 

 by the purine-end could be transmitted to the phosphate-end, and 

 v/ce versa, since the two are separated by the pentose, which has 

 no conjugated double bonds and no tt electrons. 



But does the ATP molecule really have this structure.^ Are we 

 not misled by the visual impression made by a structural formula, 



^ The contents of this chapter were presented at the International Enzyme 

 Symposium, Ford Foundation, Detroit, November 1-3, 1955, and pubHshed 

 in "Enzymes: Units of Biological Structure and Function," Oliver H. Gaebler, 

 Ed., Academic Press, New York, N.Y. 



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