TEMPERATURE AND GUANIDINE INDUCED UNFOLDING OF DODECAMERIC GLUTAMINE-SYNTHETASE FROM ESCHERICHIA-COLI

Glutamine synthetase (GS) of 622000 M(r) from E. coli is composed of 1 2 identical subunits which are structurally arranged in two superimpos ed hexagonal rings with active sites at subunit interfaces. The enzyme undergoes reversible, thermally induced, partial unfolding without di ssociation of subunits at pH 7 in the presence of 100 mM KCl and 1.0 m M 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 subsequ ent unfolding were estimated to be approximately 61 and approximately 55 J/g, respectively, or approximately 100-fold the value of DELTAH fo r thermal unfolding.