Characterization of the structure and dynamics of amyloidogenic variants of human lysozyme by NMR spectroscopy

The structures and dynamics of the native states of two mutational variants of human lysozyme, I56T and D67H, both associated with non-neuropathic systemic amyloidosis, have been investigated by NMR spectroscopy. The H-1 and N-15 main-chain amide chemical shifts of the I56T variant are very similar to those of the wild-type protein, but those of the D67H variant are greatly altered for 28 residues in the beta -domain. This finding is consistent with the X-ray crystallographic analysis, which shows that the structure of this variant is significantly altered from that of the wild-type protein in this region. The H-1-N-15 heteronuclear NOE values show that, with the exception of V121, every residue in the wild-type and I56T proteins is located in tightly packed structures characteristic of the native states of most proteins. In contrast, D67H has a region of substantially increased mobility as shown by a dramatic decrease in heteronuclear NOE values of residues near the site of mutation. Despite this unusual flexibility, the D67H variant has no greater propensity to form amyloid fibrils in vivo or in vitro than has I56T. This finding indicates that it is the increased ability of the variants to access partially folded conformations, rather than intrinsic changes in their native state properties,. that is the origin of their amyloidogenicity.