φ value analysis of heterogeneity in pathways of allosteric transitions: Evidence for parallel pathways of ATP-induced conformational changes in a GroEL ring

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Abstract
What are the mechanisms of ligand-induced allosteric transitions in proteins? A powerful method to characterize pathways and transition states of reactions is φ value analysis. A φ value is the ratio between the changes on a perturbation (e.g., mutation) in the activation and equilibrium free energies of a reaction. Here, φ value analysis is used to characterize the ATP-induced allosteric transitions of GroEL by using changes in ATP concentration as perturbations. GroEL consists of two stacked back-to-back heptameric rings that bind ATP with positive cooperativity within rings and negative cooperativity between rings. Evidence is presented for the existence of parallel pathways for the allosteric transition of each ring. In both allosteric transitions, there is an abrupt ATP-dependent switch from a pathway with ATP-binding sites in the transition state that are very similar to those of the initial T state (φ = 0) to a pathway with a φ value of ≈0.3. The φ value procedure outlined here should be useful in mapping the energy landscape of allosteric transitions of other proteins.