Evidence for cooperative Mn-ATP binding with Bacillus sp. glutamine synthetase

The binding of Mn-ATP with $\text{B. subtilis}$ glutamine synthetase, observed kinetically at 37°, pH 7.0, is cooperative (Hill $\text{n}\text{= 2.3, S}{}_{\mathrm{0·5}}\text{= 0.36mM)}$, a phenomenon overlooked in earlier studies. The Arrhenius plot is biphasic with a break at 26°C. Similar behavior is observed with the thermophilic $\text{B. stearothermophilus}$ enzyme, but is absent with the enzymes from $\text{E. coli}$, plant, and mammaliam sources under optimal assay conditions. The temperature dependence of the intrinsic fluorescence of the protein is also non-linear, and the intersection point of 18° shifts to 30° upon binding of substrates. These results are interpreted as indicating that $\text{Bacillus sp}$. enzymes can assume multiple, functionally important conformational states related to Mn-ATP binding at 37°. They also emphasize further that critical differences in mechanism exist among glutamine synthetases from different sources.