We recently concluded a study to ascertain whether glutamine synthetase was a viable target in antitubercular chemotherapy. The bacterial enzyme (in contrast to the mammalian enzyme) appears to have two distinct forms differing by the level of adenylation. If adenylation (via an adenyl transferase) is inhibited, bacterial survival is poor.
The discussion herein will illustrate key differences in the phosphoryl transfer mechanisms for each enzymatic subtype and the implications of these differences on the selective inhibition of the sub-types.
A key component of this study was the identification of the role of the C8-H of the purine residue of ATP on the rate of phosphoryl transfer as observed by a significant kinetic isotope effect at this position. The classical phosphoryl transfer model as elucidated primarily by Traxler suggests that the triphosphate should extend from the purine residue to the site to be phosphorylated. We will illustrate the potential for a mechanistically discrete process for phosphoryl transfer from ATP that implicates both the C8-H and carbon dioxide levels.
The proposed tertiary structure of ATP has be used as a basis for inhibitor design. Simple ATP site inhibitors effecting selective inhibition of one enzymatic sub-type have been prepared.