Thermodynamic characterisation of two transition states along parallel protein folding pathways

Recent work has shown that a β-sandwich domain from the human muscle protein titin (TI I27) unfolds via more than one pathway, providing experimental evidence for a long-standing theoretical prediction in protein folding. Here we present a thermodynamic analysis of two transition states along different folding pathways for this protein. The unusual upwards curvature previously observed in the denaturant-dependent unfolding kinetics is increased at both high and low temperatures, indicating that the high denaturant pathway is becoming more accessible. The transition states in each pathway are structurally distinct and have very different heat capacities. Interestingly the nucleation-condensation pathway is dominant at all physiologically relevant temperatures, supporting the suggestion that pathways with diffuse rather than localised transition states have been selected for by evolution to prevent misfolding.