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Human islet amyloid polypeptide (hIAPP) forms amyloid fibrils in pancreatic islets of patients with type 2 diabetes mellitus. It has been suggested that the N-terminal part, which contains a conserved intramolecular disulfide bond between residues 2 and 7, interacts with membranes, ultimately leading to membrane damage and β-cell death. Here, we used variants of the hIAPP1–19 fragment and model membranes of phosphatidylcholine and phosphatidylserine (7:3, molar ratio) to examine the role of this disulfide in membrane interactions. We found that the disulfide bond has a minor effect on membrane insertion properties and peptide conformational behavior, as studied by monolayer techniques, 2H NMR, ThT-fluorescence, membrane leakage, and CD spectroscopy. The results suggest that the disulfide bond does not play a significant role in hIAPP–membrane interactions. Hence, the fact that this bond is conserved is most likely related exclusively to the biological activity of IAPP as a hormone. [ABSTRACT FROM AUTHOR]

Although adsorption-induced conformational changes of proteins play an essential role during protein adsorption on interfaces, detailed information about these changes is lacking. To further the current understanding of protein adsorption, in this study, the orientation, conformation, and local stability of bovine α-lactalbumin (B LA) adsorbed on polystyrene nanospheres is characterized at the residue level by hydrogen/deuterium exchange and 2D NMR spectroscopy. Most of the adsorbed BLA molecules have conformational properties similar to BLA molecules in the acid-induced molten globule state (A state). A folding intermediate of BLA is thus induced and trapped by adsorption of the protein on the hydrophobic interface. Several residues, clustered on one side of the adsorbed folding intermediate of BLA, have altered amide proton exchange protection factors compared to those of the A state of BLA. This side preferentially interacts with the interface and includes residues in helix C, the calcium binding site, and part of the β-domain. Local unfolding of this interacting part of the adsorbed protein seems to initiate the adsorption-induced unfolding of BLA. Adsorption-induced protein unfolding apparently resembles more the mechanical unfolding of a protein than the global unfolding of a protein as induced by denaturant, pH, or pressure. 2D macromolecular crowding prevented the minority of adsorbed BLA molecules, which arrived late at the interface, to unfold to the A state. Protein adsorption is a novel and challenging approach to probe features of the free energy landscapes accessible to unfolding proteins. [ABSTRACT FROM AUTHOR]