Bond Dissociation Energy glycine

For the first two peptides, CysS radicals abstract hydrogen atoms from the or-carbon of glycine with 7 = (1.0 to 1.1) x 10 s , while the reverse reaction proceeds with = (8.0 to 8.9) x 10 s . For the latter peptide, CysS radicals abstract hydrogen atoms from the ce-carbon of alanine with = (0.9 to 1.0) x 10 s while the reverse reaction proceeds with k -j = 1.0 x 10 s" The order of reactivity, Gly > Ala, is in accordance with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that < k y suggests that some secondary structure prevents the adoption of extended conformations for which calculations of homolytic bond dissociation energies would have predicted k j > k y. 

In summarizing, it is of interest to compare GSH synthesis to other systems which utilize ATP for the formation of —CONH— bonds. In contrast to the acetylation reactions, the condensation of glutamylcysteine and glycine is accomplished by a single enzyme and in the absence of a dissociable cofactor. While both the acetylation reaction (13) and GSH synthesis are initiated by a reaction of ATP and enzyme, the products of the interaction are different in the two cases. Thus there must be at least two ways in which the energy of ATP is utilized in carboxyl activation, (a) the splitting of ATP to AMP and pyrophosphate, and (6), the splitting to ADP and Pi. 

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