no THE BIOSYNTHESIS OF PROTEINS 



different enzymes in the preparation, one for each type of nucleotide. It is 

 conceivable that these enzymes catalyse the activation of amino acids by 

 a process of the type illustrated in the synthesis of glutamine or of gluta- 

 thione. The enzymes involved in the formation of these peptides have been 

 partly purified (Elliot, 1951, 1953; Snoke and Bloch, 1952, 1955; Webster 

 and Varner, 1954). The carboxyl group of glutamic acid must be activated 

 in the process, for in the presence of ATP and hydroxylamine, glutamyl 

 hydroxamic acid is formed. No free glutamic acid derivative with a reactive 

 carboxyl could ever be isolated however, and it is assumed that the activated 

 compound is bound to the enzyme. A bound glutamyl-phosphate inter- 

 mediate is postulated (Webster and Varner, 1954; Varner and Webster, 

 1955 ; Kowalsky et ah, 1956), because heavy oxygen is transferred stoichio- 

 metrically to inorganic phosphate, and not at all to the ADP moiety of 

 ATP; besides, the exchange of inorganic phosphate with the terminal 

 phosphate of ATP requires the presence of both glutamic acid and the 

 acceptor (e.g. ammonia in the synthesis of glutamine). The following 

 scheme accounts for these experimental facts, although the detailed work- 

 ing of the enzyme cannot be considered as completely clarified. 



Enzyme + ATP + Glu ^====i ( y- Glu~P) Enzyme + ADP 



( Y- Glu~P) Enzyme +NH3 ^ = ===^ y- GIU-NH2 + Pi + Enzyme 



Fig. 29. 



A similar reaction scheme may be assumed for the 'incorporation 

 enzyme'. It would account for the exchange data and for the fact that 

 amino acids cause a net liberation of inorganic phosphate from the nucleo- 

 side triphosphates in the presence of the enzyme preparation. It would also 

 explain the transfer of heavy oxygen from the amino acid to inorganic 

 phosphate in bacterial preparations (Bernlohr and Webster, 1958). An 

 interesting observation is that the individual amino acids do not cause the 

 liberation of phosphate from all four nucleoside triphosphates equally well. 

 Thus glycine appears to react readily with ATP, less with GTP, leucine 

 reacts with UTP and CTP, phenylalanine with CTP only (Beljanski, 

 I960). 



It is indeed impossible at present to decide whether these reactions play 

 a part in protein synthesis or not. One favourable indication is that the 

 amino acid promoted liberation of inorganic phosphate is inhibited by 

 chloramphenicol, a typical inhibitor of protein synthesis in bacteria. The 

 possibility that the 'incorporation enzyme' might be involved together 

 with the activation enzymes must be kept in mind. 



(b) The S-protein. An additional factor in amino acid incorporation in 

 certain systems, is a protein material isolated by Sacks (1957) from the 

 soluble fraction of liver homogenate, but v/hich does not contain any 

 activation enzymes. This S-protein enhances the incorporation of amino 



