Temperature and guanidine induced unfolding of dodecameric glutamine synthetase from E. coli

Glutamine synthetase (GS) of 622000 M(r) from E. coli is composed of 12 identical subunits which are structurally arranged in two superimposed hexagonal rings with active sites at subunit interfaces. The enzyme undergoes reversible, thermally induced, partial unfolding without dissociation of subunits at pH 7 in the presence of 100 mM KCl and 1.0 mM MnCl2. Cooperative interactions link partial unfolding reactions of all subunits within the Mn.GS dodecamer (DELTAH(cal) = 750 kJ/mol) and only two, two-state transitions with similar T(m) values (324+/-2 K) are observed. Enthalpies at 310 K for subunit dissociation and subsequent unfolding were estimated to be approximately 61 and approximately 55 J/g, respectively, or approximately 100-fold the value of DELTAH for thermal unfolding.