Abstract
Glycosaminoglycans (GAGs) bind all known amyloid plaques and help store protein hormones in (acidic) granular vesicles, but the molecular mechanisms underlying these important effects are unclear. Here we investigate GAG interactions with the peptide hormone salmon calcitonin (sCT). GAGs induce fast sCT fibrillation at acidic pH and only bind monomeric sCT at acidic pH, inducing sCT helicity. Increasing GAG sulfation expands the pH range for binding. Heparin, the most highly sulfated GAG, binds sCT in the pH interval 3-7. Small angle x-ray scattering indicates that sCT monomers densely decorate and pack single heparin chains, possibly via hydrophobic patches on helical sCT. sCT fibrillates without GAGs, but heparin binding accelerates the process by decreasing the otherwise long fibrillation lag times at low pH and accelerates fibril growth rates at neutral pH. sCT·heparin complexes form β-sheet-rich heparin-covered fibrils. Solid-state NMR reveals that heparin does not alter the sCT fibrillary core around Lys(11) but makes changes to Val(8) on the exterior side of the β-strand, possibly through contacts to Lys(18) Thus GAGs significantly modulate sCT fibrillation in a pH-dependent manner by interacting with both monomeric and aggregated sCT.
Highlights
Since their first description in 1855 [1], amyloid deposits have been extensively studied both in vivo and in vitro
To address the role of heparin and other GAGs in the molecular steps leading to protein fibrillation, here we present a study of how GAGs affect fibrillation of the 32-residue peptide hormone salmon calcitonin used to treat osteoporosis in postmenopausal women [25]. sCT is more potent in this regard than human calcitonin [26], partly attributed to its reduced fibrillation propensity [27]
Our data clearly show that sCT fibrillation is affected by GAGs in a complex manner with pH having a large impact on sCT-GAG interactions
Summary
At low pH, hCT fibrillation is slowed by Asp protonation [32]. The decreased fibrillation propensity of sCT is attributed to the increased number of polar amino acids compared with hCT [33]. Address this is a study by Middleton and co-workers [23, 24] on heparin-amyloid  interactions where heparin binds on the outer surface of amyloid- fibrils, interacting closely with the loop region and the flexible N terminus. To address the role of heparin and other GAGs in the molecular steps leading to protein fibrillation, here we present a study of how GAGs affect fibrillation of the 32-residue peptide hormone salmon calcitonin (sCT) used to treat osteoporosis in postmenopausal women [25]. SCT is more potent in this regard than human calcitonin (hCT) [26], partly attributed to its reduced fibrillation propensity [27]. At neutral pH, hCT fibrillation is in part driven by hydrophobic interactions among
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