Length Dependence of the Coil reversible β-Sheet Transition in a Membrane Environment

摘要

The most abundant structural element in protein aggregates is the β-sheet. Designed peptides that fold into a β-sheet structure upon binding to lipid membranes are useful models to elucidate the thermodynamic characteristics of the random coil reversible β-structure transition. Here, we examine the effect of strand length on the random coil reversible β-sheet transition of the (KIGAKI)_n peptide with the total chain length varying between 7 and 30 amino acids. The β-sheet content of the peptides in the presence and absence of membranes was measured with circular dichroism spectroscopy. The peptides were titrated with small unilamellar lipid vesicles, and the thermodynamic binding parameters were determined with isothermal titration calorimetry (ITC). Membrane binding includes at least two processes, namely (ⅰ) the transfer of the peptide from the aqueous phase to the lipid surface and (ⅱ) the conformational change from a random coil conformation to a β-sheet structure. CD spectroscopy and ITC analysis demonstrate that β-sheet formation depends cooperatively on the peptide chain length with a distinct increase in β-structure for n > 10-12. Binding to the lipid membrane is an entropy-driven process as the binding enthalpy is always endothermic. The contribution of the β-sheet folding reaction to the overall process was determined with analogues of the KIGAKI repeat where two adjacent amino acids were replaced by their D-enantiomers. The folding reaction for peptides with n ≥ 12 is characterized by a negative free folding energy of △G_β~o ≈ -0.15 kcal/ mol per amino acid residue. The folding step proper is exothermic with △H_β~o ≈ -0.2 to -0.6 kcal/mol per residue and counteracted by a negative entropy term T△S_β~o = -0.1 to -0.5 kcal/mol per residue, depending on the chain length (18 ≤ n ≤ 30). For a short chain with n = 12, β-sheet formation is unfavorable with △G_β~o ～ +0.08 kcal/mol per residue. Small changes of environmental parameters like pH or temperature can thus be anticipated to have profound effects on aggregation reactions, leading to amyloid fibril formation.