Kinetic equivalence of the heat and cold structural transitions of λ_6-85

Heat- and cold-denatured proteins are considered separate thermodynamic states because temperature tuning requires the protein to pass through two 'soft' first-order phase transitions. When both pressure and temperature changes are allowed, the heat- and cold-denatured states of proteins can be interconverted without going through the native state. This raises the question of whether these states are distinguished from one another by their folding kinetics. For the Tyr22-Trp/Ala37Gly/Ala49Gly mutant of the 80 residue five-helix bundle protein λ_6-85, we show that viscosity-corrected folding rates do not distinguish the cold- and heat-denatured states. We attribute this to a folding mechanism dominated by hydrophobic collapse. Our 'temperature-symmetric' approach offers an alternative to viscosity tuning with solvent additives in such cases.