46 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



2. E^ JAMP — PP|_ + AA = iEi JAMP — AA|_ + PP 



activated amino pyrophosphate 

 acid 



The activated amino acid can then react with hydroxylamine accord- 

 ing to the reaction : 



3. EijAMP — AA|_ + NH2OH = ^1 + AA — NHgOH + AMP 



Under physiological conditions, hydroxylamine would be replaced 

 by an amino acid or a peptide bound to the microsomes. 



Later work by De Moss et al. (1956) has given some indication 

 about the biochemical mechanism of Hoagland's (1955) reaction. 

 In the case of the amino acid leucine, a leucyl-AMP compound acts 

 as the intermediate promoting the exchange reaction between ATP 

 and pyrophosphate, since synthetic leucyl-AMP is active in the 

 absence of the enzyme. Amino acid-AMP compounds thus repre- 

 sent the activated amino acids, which will ultimately become part 

 of the protein. 



Hoagland et al. (1956) also obtained definite evidence showing 

 that their soluble enzyme really activates the carboxyl group of the 

 amino acids. 



More recent work has shown that the amino acid becomes 

 attached to the ribose moiety of adenylic acid. It has also been 

 shown clearly that there are many distinct activating enzymes — 

 possibly one for each of the amino acids. One of them, the enzyme 

 which activates tryptophane, has even been crystallized. 



b. The role of RNA in protein synthesis 



In a very important paper, Hoagland etal.(\ 957) have shown that 

 the incorporation of amino acids into proteins occurs in three 

 successive steps. After the formation of the amino acyl-AMP com- 

 pound, the activated amino acid is transferred to the RNA present 

 in the soluble fraction. This second step is very sensitive to ribo- 

 nuclease. Guanosine triphosphate finally acts as an intermediate 

 in the transfer of this activated amino acid to a peptide linkage, via 

 the microsomes, by a mechanism which is as yet unknown. 



The existence of a transfer of the activated amino acid to soluble 



