The folding unit of phosphofructokinase-2 as defined by the biophysical properties of a monomeric mutant

© 2015 Biophysical Society. Escherichia coli phosphofructokinase-2 (Pfk-2) is an obligate homodimer that follows a highly cooperative three-state folding mechanism N<inf>2</inf> → 2I → 2U. The strong coupling between dissociation and unfolding is a consequence of the structural features of its interface: a bimolecular domain formed by intertwining of the small domain of each subunit into a flattened β-barrel. Although isolated monomers of E. coli Pfk-2 have been observed by modification of the environment (changes in temperature, addition of chaotropic agents), no isolated subunits in native conditions have been obtained. Based on in silico estimations of the change in free energy and the local energetic frustration upon binding, we engineered a single-point mutant to destabilize the interface of Pfk-2. This mutant, L93A, is an inactive monomer at protein concentrations below 30 μM, as determined by analytical ultracentrifugation, dynamic light scattering, size exclusion chromatograph