Reversible pH-induced dissociation of glucose dehydrogenase from Bacillus megaterium. II. Kinetics and mechanism

Glucose dehydrogenase from Bacillus megaterium exists as a stable, active tetramer at pH 6.5. By shifting the pH to 9, the enzyme is, completely and reversibly, dissociated into four inactive protomers. Kinetics and mechanism of this pH-induced dissociation have been studied, at various enzyme concentrations, by ultraviolet absorption, circular dichroism, normal and stopped-flow fluorescence as well as by light scattering and activity measurements. Dissociation of the fully active tetramer proceeds via three distinct kinetic steps: (1) fast conformational rearrangement of the tetramer, without any loss of activity (t1/2 0.0075 sec); (2) slow isomerization to a tetramer with lower specific activity (t1/2 27 sec); (3) subsequent dissociation of this rearranged tetramer into inactive monomers (t1/2 114 sec) with still intact native secondary structure. All three processes follow first-order kinetics. Both rate and extent of the dissociation are reduced, with a concomitant shift to higher reaction orders, by increasing the NaCl concentration in the buffer. This suggests the establishment of a dissociation/association equilibrium, due to the concentration-dependent stabilization of the tetrameric enzyme state by NaCl.