CHEMICAL PATHWAYS 



99 



change of ATP concentration is observed (De Moss et al., 1956), and no 

 free aminoacyl adenylate could ever be detected (Hoagland et al., 1956). 

 Using large amounts of pure tryptophan activation enzyme, Kingdon et al. 

 (1958) and Karasek et al. (1958) were able to liberate tryptophanyl adenylate 

 by denaturing the enzyme. The amount of anhydride obtained is com- 

 patible with the existence of one molecule of tryptophanyl adenylate per 

 molecule of enzyme. The aminoacyl adenylate might therefore be described 

 as a prosthetic group of the enzyme, rather than as a substrate or reaction 

 product. Actually, free aminoacyl adenylates would rapidly disappear in the 

 cell and be wasted in all kinds of reactions; it is quite certain that their 

 being bound to the activation enzyme protects them from reacting at 

 random (Askonas et ah, 1957; Moldave et al., 1959). They are anhydrides 

 of a carboxylic acid with adenylic acid, and belong therefore to the class of 

 mixed anhydrides of a carboxylic acid with a phosphoric group which 

 carries a substituent. Substances of this type have been shown to be rather 



OH OH 

 Fig. 27. Amino acyl adenylate. 



Stable in water, and much more so than the corresponding compound with 

 unsubstituted phosphoric group like acetylphosphate (Chantrenne, 1948). 

 On the other hand, they react very rapidly with amino acids to form a pep- 

 tide bond, even in 10"^ M solution at pH 74 and this is a purely non-enzymic 

 reaction (Chantrenne, 1947, 1948, 1949, 1950; Katchalski and Paecht, 

 1954; Avison, 1955; Moldave et al, 1959). Free aminoacyl adenylates in 

 neutral or slightly alkaline medium would rapidly form polypeptides at 

 random or produce a large variety of peptides by reacting with any amino 

 acid present. The formation of well-defined proteins from such substances 

 is possible only if they are in some way prevented from reacting at random 

 (Chantrenne, 1950). That their binding to the enzyme protects the amino- 

 acyl adenylates from reacting with amines is clearly shown by their behav- 

 iour in the presence of hydroxylamine. Whereas in the free state they would 

 react extremely readily with neutral 10~^ m hydroxylamine, it is necessary 

 to use at least 0-3 m, sometimes 3 m hydroxylamine to trap these 'activated' 

 carboxyl groups (Hoagland, 1955 ; De Moss and Novelli, 1955 ; Davie et al.. 



