Microbeam X-ray Diffraction Study on Insulin Spherulites

Insulin is a hormone with a molecular weight of 5.7 kDa, that is composed of two polypeptide chains. In the native state, its secondary structure is primarily αhelical. However, forms a spherical precipitate called spherulite (Fig. 1), which is made of β-amyloid fibrils (Fig. 2), at low pHs and high temperatures (pH 2.0 and 37 100 oC). No chemical modification is involved in the assembly of insulin to form spherulite, and insulin retains its three disulfide bridges (two between chains and one within one chain) in spherulite. Amyloids are self-aggregates of insoluble, fibrillar assemblies of protein molecules. Amyloids can be harmful to organisms, causing pathologies such as Alzheimer's disease, Parkinson's disease, Huntington's disease, type II diabetes, and prion diseases. Several nonpathological proteins such as insulin, lysozyme and short peptides self-assemble into amyloid-like fibrils, suggesting that amyloid formation is an inherent property of polypeptide chains. Denatured proteins in amyloid f ibers commonly take a cross-β structure even when their native form is α-helical [1].