How the imidazole ring modulates amyloid formation of islet amyloid polypeptide: A chemical modification study

Background The misfolding of human islet amyloid polypeptide (hIAPP) is an important pathological factor on the onset of type 2 diabetes. A number of studies have been focused on His¹⁸, the only histidine of hIAPP, whose imidazole ring and the protonation state might impact hIAPP amyloid formation, but the exact mechanism remains unclear. Methods We used diethylpyrocarbonate (DEPC) to specifically modify His¹⁸ and obtained mono-ethyloxyformylated hIAPP (DMI). Thioflavin T based fluorescence, transmission electronic microscopy, circular dichroism spectroscopy, fluorescence dye leakage, Fourier transform infrared spectroscopy and replica-exchange molecular dynamics (REMD) simulation were applied to study the impact of DEPC-modification on hIAPP amyloid formation. Results After an ethyl-acetate group was introduced to the His¹⁸ of hIAPP by diethylpyrocarbonate (DEPC) modification, the pH dependent hIAPP fibrillation went to the opposite order and the number of intra-molecular hydrogen bonds decreased, while the possibility of His¹⁸ participating in the formation of α-helical structures increased. Furthermore, the membrane-peptide interaction and ion-peptide interaction were both impaired. Conclusions The intramolecular hydrogen bond formation by His¹⁸ and the possibility of His¹⁸ participating in the formation of α-helical structures greatly modulated the manner of hIAPP amyloid formation. The imidazole ring directly participates in the hIAPP-membrane/ion interaction. General significance DEPC modification is an alternative approach to investigate the role of the imidazole ring during amyloid formation.