152 S. S. COHEN 



potential, or A F, when tte transfer of the phosphoryl group containing this 

 phosphorus is effected from ATP to water or some other acceptor, such as 

 glucose, a nucleotide, etc. In this case, the reaction proceeds spontaneously, 

 and under standard conditions the reaction results in a change of chemical 

 potential or of free energy (A F) of — 7 kilocalories per mole. 

 ATP*- + H^O^ ADP2- + HPO42- 



In such reactions, then, we are concerned primarily with group 

 transfers and indeed the energetics of polymer biosynthesis are pri- 

 marily concerned with group-transfer potentials. Although the reaction 

 presented above is essentially irreversible in practice, group-transfer reactions 

 are frequently readily reversible. The extent of such reversibihty is determin- 

 able from the relation of AF to the equihbrium constant (K) of any reaction, 

 i.e., — AF = RT InK. The energetics of group transfer has been clearly 

 discussed by Klotz (1957). 



Compounds embodying high group-transfer potentials serve to activate 

 amino acids, nucleotides, sugars, fatty acids, etc., and thereby permit their 

 utilization in biosynthesis. Compounds such as ATP arise in the breakdown 

 of foods; their formation constitutes mechanisms for the conservation of the 

 energy contained in the bonds of these nutrients. Thus, the conversion of 

 glucose to 2 moles of lactate via the Embden-Meyerhof pathway of anaerobic 

 glycolysis leads to the net production of 2 moles of ATP. The oxidation of 

 acetate to 2 moles each of CO 2 and HgO leads to the formation of 15 moles 

 of high-energy jjhosphate. General aspects of these routes of energy meta- 

 bohsm are discussed extensively in modern texts of biochemistry (cf. Fruton 

 and Simmonds, 1953; White et al., 1954); comparative aspects of these 

 reactions in animals and microorganisms have been summarized by Krebs 

 (1954). 



Assuming the ready availabihty of some high-energy intermediates, such 

 as ATP, we shaU outhne the known mechanisms of activation of inter- 

 mediates of polymer biosynthesis and proceed to a brief summary of the 

 apparent course of these syntheses. However, the activation of components 

 derived from nutrients does not always require the degradation of meta- 

 bohtes to the smallest possible components and the resjoithesis from scratch 

 of the essential activated molecules. Thus, many group-transfer reactions, 

 e.g., phosphorylytic, pyrophosphory lytic, and hydrolytic reactions, are useful 

 in the scavenging of transferable groups from foods in the fabrication of 

 essential metabohtes suitable for polymer biosynthesis. For example, an 

 ingested nucleic acid may be degraded to nucleotides, perhaps providing a 

 surfeit of uridyhc acid, when adenyhc acid is needed for biosyntheses. A. 

 pyrophosphorylytic cleavage of uridyhc acid permits the formation of a 

 pyrophosphoryl sugar phosj)hate which can react with adenine to form the 

 required nucleotide: 



