Molecular Mechanism Of Protein-solvent Interface Cation Regulation Of Melittin Folding And Assembly Structure

Folding and assembly are essential for the physiological activity of a protein.Ions play a crucial role in modulating the folding and assembled structure of a protein.For the free ions in bulk solution,ions are ranked as kosmotropic(structure-forming)and chaotropic(structure-breaking)to protein structure within the Hofmeister series.Currently,the impact of free ions in bulk solution on tuning the folding and assembly of protein has been intensively investigated.However,the effect of immobilized ions on a protein surface,although they are equivalently important,is less explored.Relative to the free ions,the immobilized ions at the solid/liquid interface are less hydrated and thus display unique physiochemical properties.A profound understanding of the impact of immobilized ions is beneficial for revealing the contribution of charged residues in protein primary structure to protein folding and assembly.In this study,we explored the influence of two immobilized cations(ammonium in the side chain of lysine and guanidinium in the side chain of arginine)on the folding and assembly of melittin.Melittin adopts an amphiphilic ?-helix structure and is driven by hydrophobic residues packing to associate into a helical bundle.To test the influence of immobilized cations on peptide/protein structure,we designed the homozygous isomers exclusively containing ammonium(melittin-K)or guanidinium(melittin-R)and compared the differences of melittin-K versus melittin-R in their folding,assembly,and molecular functions.The side chains of lysine and arginine differ in their influences on the folding and assembly of melittin.Specifically,the side chain of R increases the ?-helical propensity of melittin relative to that of K,following an inverse Hofmeister series.In contrast,the side chain of K favors the assembly of melittin relative to the side chain of R,which is in line with a direct Hofmeister series.The opposite regulatory effects of immobilized cations on the folding and assembly of melittin highlight the complexity of the noncovalent interactions that govern protein intermolecular architecture.