STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 173 



According to Bachhawat and Coon (1957) an AMP-CO2 compound reacts 

 with. ;8-liydroxyisovaleryl-Sco A to form^-hydroxy-^-methylglutaryl coenzyme 

 A, The adenylate is similarly generated by a COg-activating system which 

 catalyzes the exchange of PP and ATP. 



3. Carhoryl Activation of Amino Acids 



In similar reactions, amino acids catalyze the exchange of PP with ATP 

 in nonparticulate extracts of animal tissue (Hoagland, 1955; Cole et al., 1957), 

 bacteria (DeMoss and NoveUi, 1956), and yeast (Berg, 1956b). A similar 

 mechanism of amino acid activation appears to exist in plants (Webster, 

 1955, 1957a and b; Stephenson et al., 1956). 



In these reactions it has not been possible to demonstrate a net formation 

 of amino acid adenylate and it has been postulated that, as in pantoate 

 activation, the enzyme reacts with ATP: 



E + ATP ^ E . AI^IP . PP 



Pyrophosphate is displaced by the amino acid to give the bound amino acid 



E . AMP . PP + AA ^ E . AMP . AA + PP 



The amino acid adenylate does not dissociate from the enzyme to a signifi- 

 cant extent. However, hydroxylamine in some instances forms a dissociable 

 hydroxamate, dissociating this product plus AMP. 



E. AMP. AA + NH2OH > E. + AA-NH2OH + AMP. 



Regenerated enzyme is freed to recycle ATP and amino acid. Thus, in these 

 instances, in the presence of NHgOH, the net formation of activated amino 

 acids is indicated by the formation of amino acid hydroxamates and a stimu- 

 lated release of AMP and PP. The irreversibihty of hydroxamate production 

 accomits for the inhibition of PP exchange mider these conditions. The 

 vaUdity of this postulated mechanism in this series of reactions has recently 

 been confirmed in a study involvmg 0^^-labehng and transfer of this oxygen 

 from try|)tophan carboxyl to AMP (Hoagland et al., 1957b). 



Although it has not been possible to find a free mtermediate as a result of 

 amino acid carboxyl activation and it is considered that the appropriate 

 adenylate is strongly attached to the enzyme surface, exogenously supplied 

 synthetic amino acid-AMP derivatives, such as leucyl adenylate, are con- 

 verted to ATP m the presence of PP (DeMoss et al, 1956). 



The initial studies of Hoagland et al. (1956) indicated the existence of 

 separate enzymes for the activation of the amino acids. These workers 

 detected a specific system for methionine activation in rat liver, a result soon 

 obtained with yeast extracts (Berg, 1956b). This enzyme catalyzed the follow- 

 ing reaction: 



ATP + L-methionine ^ adenyl L-methionine + PP 



