488 ANIMAL BIOCHEMISTRY 



R K' R" R'" 



II 11 

 +Hn,NCHCONHC:HCOO- + +HnNCHCONHC:HCOO- 



jr 



R R" R'" R' 



III 1 

 +H3NCHCONHCHCONHCHCOO- + +H3NCHCOO- ^ 



R R" 



1 1 



+H3NGHCON HCHCOO- 



+ 

 R' R'" 



1 I 



+ H3Nc:hc:()nhc;ik;()0- 



liaiisicis ()c( Ills with the free cneigy of one peptide bond exchanged 

 for that of anolfier. By this means it is possible to conceive of a step- 

 wise increase of molecular size at the expense of the ATP needed to 

 form the original peptide bonds like those in glutathione. Again 

 there is no direct experimental indication that such a process partic- 

 ipates in protein synthesis. 



More recently a second type of anhydride has been proposed for the 

 activation of amino acids. ATP and other nucleotide phosphates can 

 function in this process, where AMP represents the acyl form of 



R R 00- 



l 1 11 1 



+H3NCHCOO- + ATP ±^ +H3NCHCO— AMP + HOPOPOH 



1 II 

 0-0 



pyrophosphate 



adenosine monophosphate (adenylic acid). These reactive anhydrides 

 have been synthesized and polymerize spontaneously to yield poly- 

 peptides. Since a spontaneous reaction would be random, some system 

 must be required to control specificity as discussed below. 



Before leaving the energetics of peptide bonds, however, it is neces- 

 sary to consider the stability of the completed bond. As pointed out 

 earlier, the peptide bond is rather unstable with respect to hydrolysis, 

 and at equilibrium there would be little peptide. To provide peptides 

 and proteins for the purposes of cells, three possibilities appear to 

 exist. First, hydrolysis is extensive but is compensated for by rapid 

 and continuous synthesis. Since this procedure would necessitate the 

 consumption of large quantities of ATP and other high-energy sources 

 without recovery of this energy on hydrolysis, the inefficiency of such 

 a system is unattractive. Furthermore, the energy demands of cells 



