110 Essays in Biochemistry 



/^-alanine to form pantothenic acid. Lipmann 4 has suggested that a 

 mechanism of this type may be involved in the synthesis of the poly- 

 peptide chains of proteins, but no experimental evidence for or against 

 this possibility is available at present. 



To the two mechanisms of "amino acid activation" proposed on the 

 basis of studies on hippuric acid and pantothenic acid must be added 

 the conclusions of Bloch and his associates, from their important work 

 on the biosynthesis of glutathione. 5 The synthesis of the two CO-NH 

 bonds proceeds in separate steps, in each of which one equivalent of 

 ATP is required; all efforts to demonstrate a role for CoA appear to 

 have been unsuccessful. In contrast to the synthesis of pantothenic 

 acid, the formation of glutathione from the component amino acids is 

 accompanied by the liberation of one equivalent of phosphate, and 

 not pyrophosphate, per CO-NH bond formed, indicating that ATP 

 is cleaved at different pyrophosphate bonds in the two processes. 

 Furthermore, the studies of Speck and of Elliott have shown that, in 

 the biosynthesis of glutamine from glutamic acid and ammonia, which 

 also requires the participation of ATP, inorganic phosphate is formed, 

 as in the synthesis of y-glutamylcysteine from glutamic acid and 

 cysteine in glutathione formation. 



These studies on the biosynthesis of pantothenic acid, glutathione, 

 and glutamine have all led to the working hypothesis that the role 

 of ATP is to make possible the formation of a reactive form of an 

 amino acid, as for example in a "pantoyl enzyme" or a ' y-glutamyl 

 enzyme," where the acyl group is attached to the enzyme protein at 

 a suitable site (e.g., the sulfhydryl group of cysteine, the imidazolyl 

 ring of histidine, etc. ) . However, no general scheme for the role of 

 ATP in the formation of the acyl enzyme compounds can be offered 

 at present. Nevertheless, the hypothesis that such reactive acyl 

 enzyme compounds are intermediates in the biosynthesis of the peptide 

 chains of proteins is an extremely attractive one and finds support 

 in work to be discussed later in this essay. 



In the face of the fragmentary knowledge currently available, it 

 would seem desirable to extend the study of the synthesis of CO-NH 

 bonds in simple compounds to an examination of the metabolism of 

 a-amides of amino acids. Until recently, compounds of this type were 

 not considered to occur in nature, but the demonstration of a terminal 

 glycinamide in oxytocin and vasopressin 6 confers upon such a-amides 

 increased biological interest. Calorimetric studies have shown that the 

 A// of hydrolysis for the CO-NH2 bonds of acylamino acid amides 

 is of the same order of magnitude as that found for inorganic pyro- 



