FRITZ LIPMANN 



consideration lies in the fact that here we have a case where chain 

 growth is terminal and is maintained in such a manner that each 

 addition prepares for the following by permitting reaction only 

 between two activated carboxyl derivatives, the energy of only 

 one of which is used in the process. The terminal activated 

 carboxyl remains intact for the next step of chain elongation. 

 Furthermore, although fatty acid synthesis impresses as a homo- 

 geneous process where long, straight, uniform chains are 

 synthesized from indentical two-carbon fragments, obviously 

 during the chain elongation the growing chain is changing con- 

 stantly. At least three different enzymes participate with 

 different specificity for activation of short, intermediate, and long 

 fatty acids (19). The process, therefore, is not a homogeneous 

 process, but a handling of a changing molecule which changes 

 through the process of elongation itself. Although a long- 

 chain fatty acid is homogeneous as compared to a polypeptide 

 chain, the two processes of chain elongation may have common 

 features. It is possible, though no indication has appeared so 

 far, that chain elongation of the polypeptide chain proceeds 

 similarly through linking of an activated carboxyl waiting at the 

 polypeptide terminal for a newly arriving, likewise activated 

 amino acid. 



However that may be, such considerations do not help much 

 in solving the cardinal question of how activated amino acids 

 are lined up in the specific sequence particular for the special 

 protein or enzyme. In all present considerations the mech- 

 anism of the linking process still presses into the foreground be- 

 cause there is some information present. The mechanism of 

 directing into proper sequence must be determined through 

 some kind of coding mechanism which seems to involve a trans- 

 lation, in Gamow's terminology (9), of specific nucleotide se- 

 quential into amino acid sequences. 



Conclusion 



The understanding of the basic mechanisms of joining mole- 

 cules together has opened the possibility of approaching the 



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