Reversible Unfolding of β-Sheets in Membranes: A Calorimetric Study

The hexapeptide acetyl-Trp-Leu5 (AcWL5) has the remarkable ability to assemble reversibly and spontaneously into β-sheets on lipid membranes as a result of monomer partitioning followed by cooperative assembly. This system provides a unique opportunity to study the thermodynamics of protein folding in membranes, which we have done using isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC). The results, which may represent the first example of reversible thermal unfolding of peptides in membranes, help to define the contribution of hydrogen bonding to the extreme thermal stability of membrane proteins. ITC revealed that the enthalpy change for partitioning of monomeric, unstructured AcWL5 from water into membranes was zero within experimental error over the temperature range of 5 °C to 75 °C. DSC showed that the β-sheet aggregates underwent a reversible, endothermic, and very asymmetric thermal transition with a concentration-dependent transition temperature (Tm) in the range of 60 °C to 80 °C. A numerical model of nucleation and growth-dependent assembly of oligomeric β-sheets, proposed earlier to describe β-sheet formation in membranes, recreated remarkably well the unusual shape and concentration-dependence of the transition peaks. The enthalpy for thermal unfolding of AcWL5 β-sheets in the membrane was found to be about 8(±1) kcal mol−1, or about 1.3(±0.2) kcal mol−1 per residue.