Inhibition of amyloid fibrillation of hen egg-white lysozyme by the natural and synthetic curcuminoids

Amyloid fibrillogenesis of almost twenty different proteins followed by deposition of amyloid fibrils in organs results in several neurodegenerative disorders such as Alzheimer's, Parkinson's, Huntington's diseases and type II diabetes. Natural polyphenolic compounds are introduced as appropriate candidates for inhibition of amyloid formation. In the present study, the inhibitory activities of curcumin (CUR) as a natural polyphenolic compound and diacetylcurcumin (DAC) as a synthetic derivative of CUR on the amyloid fibrillation of hen egg white lysozyme (HEWL) as an appropriate model protein for in vitro fibrillation were investigated using thioflavin T (ThT) fluorescence, atomic force microscopy (AFM) and circular dichroism spectroscopy. Also, the binding interaction evaluations were carried out using fluorescence quenching, fluorescence resonance energy transfer (FRET), synchronous fluorescence, circular dichroism spectroscopy and molecular docking approaches. The binding interaction evaluations revealed that two hydrogen bonds between CUR and HEWL formed via one of the phenolic OH groups and the carbonyl of the beta-diketone moiety of CUR with Arg-112 and Leu-56 of HEWL, whereas, two hydrogen bonds formed via two acetyl groups of DAC with Asn-39 and Trp-63 of HEWL. Both amyloid fibrillation and binding results indicated that interaction of DAC with HEWL is stronger than that of CUR, and amyloid fibrillation of HEWL was inhibited more effectively by DAC than CUR. It can be inferred from these results that the acetyl groups of DAC can play a similar role to the hydroxyl groups of CUR in the interaction with amino acid residues of protein and interrupting amyloid structure. The present results demonstrate that as one of the strategies for synthesis of novel amyloid inhibitors, chemical modification of the natural curcuminoids framework can be introduced.