Figure 2.
Landscape consequences of shifting PWT spectrin SH3 HX. (A) The conformational parameters of a two-state protein can be expressed in terms of the free energy landscape for unfolding from the native state (N) to the unfolded state (U) via a transition state ensemble (TS). The stability (ΔGU) and activation free energy barriers to unfolding (ΔG‡ U) and folding (ΔG‡ F) are calculated from ku, kf, and ku/kf using Equations 2, 3, and 4, respectively. Free energies are relative to U, arbitrarily set to 0. (B–D) Free energy landscape for PWT spectrin SH3 global unfolding calculated for ρ at the EX2 cutoff (ρ = −1.3), physiological (ρ = −0.5), EX1 cutoff (ρ = 1.3), and strongly EX1 (ρ = 3.3) conditions achieved using pH (B), denaturant (C), or temperature (D). Horizontal line across each landscape indicates apparent free energy barrier to intrinsic chemical exchange under matched conditions calculated from Equation 3 with kint in place of ku.