THE SYNTHESIS OF PROTEINS II 



though such an effect would not be expected since only the respiratory 

 enzymes are known to be inactivated by HCN. The result might be due 

 to the fact that hydrolysis and synthesis are both caused by the same 

 enzyme. 



Schoenheimer's beautiful experiments (1942) have shown that pro- 

 teins are continuously being synthesized and hydrolyzed so that it is 

 quite possible that part of the energy for synthesis of proteins is ob- 

 tained from the hydrolysis of other proteins or peptides. 



The enzymes which hydrolyze proteins (pepsin, trypsin, papain, 

 cathepsin, etc.) do not carry digestion very far. The hydrolysis is com- 

 pleted by the various peptidases. Part of the energy liberated by the 

 hydrolysis of the peptides could, therefore, be returned to the protein 

 ^peptide system and so result in the synthesis of proteins. Such a 

 system is highly organized and hence would be difficult to isolate in good 

 working order. It could not function indefinitely without an external 

 source of energy. 



An interesting reaction which may be an example of this mechanism 

 has been studied by Behrens and Bergmann (1939). They found that 

 neither acetyl-J/-phenylalanyl glycine nor glycyl-/-leucine alone are 

 hydrolyzed by papain, but that glycine and leucine are formed when 

 both peptides are present. Similar results have been reported by Abder- 

 halden and Ehrenwall (1933). Behrens and Bergmann were also able 

 to show that an intermediate compound, acetyl phenyl-alanyl glycyl 

 glycyl leucine, is formed and then hydrolyzed. They consider that the 

 tripeptide acts as "co-substrate" for the hydrolysis of glycyl leucine 

 and that the energy required for the formation of the intermediate com- 

 pound is obtained from the hydrolysis of glycyl leucine. The tripeptide 

 appears to be a "co-enzyme," however, since its concentration is un- 

 changed at the end of the reaction, rather than a "co-substrate," which 

 is a substance changed by the reaction. 



The reaction is an excellent example of a catalytic reaction in which 

 the acceleration is due to the formation of an intermediate compound. 

 The overall change is glycyl leucine ^ leucine and glycine and hence 

 the formation of the pentapeptide does not enter into the energy relation 

 at all, since thermodynamic equilibria are entirely independent of the 

 path by which they are reached. As Petrie (1943) has pointed out, the 

 reaction is actually an hydrolysis and does not by itself serve as a source 

 of the synthetic substance. Nevertheless, the reaction is of interest since 

 it shows that the enzyme does catalyze the synthetic reaction. If a mech- 



