Glutamine synthetase mutants

X-ray crystal structures of glutamine synthetase from both Salmonella typhimuriuni and Mycobacterium tuberculosis are very similar. Structures of wild type enzymes and of active site mutants have been determined. All structures have been solved with Mn in the active site. There are twelve identical subunits arranged in two face-to-face symmetrical hexamers. The active sites are in funnel-shaped open-ended cavities located between adjacent subunits of the hexamer. These cavities are 45 A long, 30 A wide at the outer end, and 10 A wide at the inner end and the active site with the two Mn " ions is approximately halfway down the cavity. The metal-metal distance is 5.8 A. The more tightly bound Mn is coordinated to the side chains of Glu-131, Glu-212, Glu-220, and two water molecules, one of which is shared by both metal ions. Glu-129, Glu-357, His-269, and two additional water molecules are bound to the Mn + at the lower affinity site. A schematic view of the active site metal coordination is shown in Figure 36. 

Cock, J.M., Mould, R.M., Bennett, M.J. Cullimore, J.V. (1990). Expression of glutamine synthetase genes in roots and nodules of Phaseolus vulgaris following changes in the ammonium supply and infection with various Rhizobium mutants. Plant Molecular Biology 14, 549-60. 

These mutants demonstrate that glutamine synthetase is a potential target for herbicidal compounds and they indicate that photosynthetic tissue is most vulnerable for herbicidal damage caused by GS inhibitors. 

There is thus good evidence from the inhibitor studies that glutamine synthetase plays a key role in the process of photorespiration. Possible explanations for the rapid fall in photosynthetic rates will be discussed in the following section, which describes the properties of mutants lacking enzymes of the ammonia assimilation pathway. 

---