Otons involved in intermolecular b-sheet hydrogen bonds are labeled alternatively in the blue and gray monomers. Note that the b-sheet hydrogen bonding is continuous along the length of the fibril, so that the amide proton of T36 in the blue monomer is a hydrogen bond donor for the carbonyl of S35 in the next monomer below (not shown). (B) In the ssNMR model of amylin fibrils two columns of amylin b-hairpins stack against each other with C2 symmetry to form a purchase Pentagastrin protofilament [10]. The Cterminal strands (red and orange) constitute the packing interface between the two layers of b-sheets, whereas the N-terminal strands (green) are on the surface. Residues I26-L27 which were not assigned to strand b2 in the ssNMR model but which nevertheless show strong qHX protection are colored in light blue. The drawings were rendered in PyMOL [39]. doi:10.1371/MC-LR custom synthesis journal.pone.0056467.gshown to be in 23727046 good agreement with experimental B-factors determined by X-ray crystallography and to correlate with HX protection factors [34,42?4]. The theoretical B-factors calculated for the amylin fibril model are shown by the black symbols in Fig. 5a. The GNM calculations predict small B-factors indicative of reduced mobility for strands b1 and b2, as well as larger Bfactors for the N-terminal strand b1 compared to the C-terminal strand b2. Although the GNM calculations capture the features of the HX sequence profile (gray symbols in Fig. 5A) the quantitative correlation to the observed HX rates is poor (R-value = 0.17, r = 0.3 for n = 33).A better agreement (Fig. 5B) is seen when the HX rates are compared to theoretically predicted inhomogeneous frequency contributions to the 2DIR diagonal linewidths of amylin fibrils, Ci [45], calculated from an all-atom MD simulation [12] of the solvated ssNMR amylin fibril model. The Ci values were obtained by taking into account the fluctuating electric fields at a given site caused by the movement of all nearby atoms in the MD simulation. The Ci and log(kHX) data in Fig. 5B are pair-wise correlated with an R-value of 0.56 (r,0.001 for n = 33). The Ci values show a gradient of decreasing flexibility from the unstructured segment ending at C7 to about residue N14 in strandHydrogen Exchange in Amylin Fibrilsthe HX data suggests that strand b1 extends by one residue to H18 and strand b2 starts two residues earlier at L26. Differences in protection are observed within each b-strand, much like in native proteins. In the case of amylin fibrils these differences correlate with the packing of b-sheets into the higher-order protofilament structure. The N-terminal strand b1 on the surface 15900046 of the protofilament, shows weak protection until the last five residues. By contrast, amide protons are protected throughout the Cterminal strand b2, which is buried in the protofilament structure. The HX studies described herein set the foundation for investigations to determine if protection in fibrils accrues through intermediates or arises in an all-or-none fashion, to look at how fibril structure changes with solution variables such as pH or when complexed with accessory molecules (e.g. metals or glycosaminoglycans) and to determine binding sites for ligands and drugs that target fibril growth.Supporting InformationFigure S1 NMR experiments demonstrate that amylin is an unfolded monomer in DMSO. (A) 1D-1H NMR spectrum of 220 mM human amylin (with an amidated Cterminus) in 95 d6-DMSO/5 d2-DCA, pH* 3.5, 25uC. The large resonances at 2.5 and 6.7 ppm are.Otons involved in intermolecular b-sheet hydrogen bonds are labeled alternatively in the blue and gray monomers. Note that the b-sheet hydrogen bonding is continuous along the length of the fibril, so that the amide proton of T36 in the blue monomer is a hydrogen bond donor for the carbonyl of S35 in the next monomer below (not shown). (B) In the ssNMR model of amylin fibrils two columns of amylin b-hairpins stack against each other with C2 symmetry to form a protofilament [10]. The Cterminal strands (red and orange) constitute the packing interface between the two layers of b-sheets, whereas the N-terminal strands (green) are on the surface. Residues I26-L27 which were not assigned to strand b2 in the ssNMR model but which nevertheless show strong qHX protection are colored in light blue. The drawings were rendered in PyMOL [39]. doi:10.1371/journal.pone.0056467.gshown to be in 23727046 good agreement with experimental B-factors determined by X-ray crystallography and to correlate with HX protection factors [34,42?4]. The theoretical B-factors calculated for the amylin fibril model are shown by the black symbols in Fig. 5a. The GNM calculations predict small B-factors indicative of reduced mobility for strands b1 and b2, as well as larger Bfactors for the N-terminal strand b1 compared to the C-terminal strand b2. Although the GNM calculations capture the features of the HX sequence profile (gray symbols in Fig. 5A) the quantitative correlation to the observed HX rates is poor (R-value = 0.17, r = 0.3 for n = 33).A better agreement (Fig. 5B) is seen when the HX rates are compared to theoretically predicted inhomogeneous frequency contributions to the 2DIR diagonal linewidths of amylin fibrils, Ci [45], calculated from an all-atom MD simulation [12] of the solvated ssNMR amylin fibril model. The Ci values were obtained by taking into account the fluctuating electric fields at a given site caused by the movement of all nearby atoms in the MD simulation. The Ci and log(kHX) data in Fig. 5B are pair-wise correlated with an R-value of 0.56 (r,0.001 for n = 33). The Ci values show a gradient of decreasing flexibility from the unstructured segment ending at C7 to about residue N14 in strandHydrogen Exchange in Amylin Fibrilsthe HX data suggests that strand b1 extends by one residue to H18 and strand b2 starts two residues earlier at L26. Differences in protection are observed within each b-strand, much like in native proteins. In the case of amylin fibrils these differences correlate with the packing of b-sheets into the higher-order protofilament structure. The N-terminal strand b1 on the surface 15900046 of the protofilament, shows weak protection until the last five residues. By contrast, amide protons are protected throughout the Cterminal strand b2, which is buried in the protofilament structure. The HX studies described herein set the foundation for investigations to determine if protection in fibrils accrues through intermediates or arises in an all-or-none fashion, to look at how fibril structure changes with solution variables such as pH or when complexed with accessory molecules (e.g. metals or glycosaminoglycans) and to determine binding sites for ligands and drugs that target fibril growth.Supporting InformationFigure S1 NMR experiments demonstrate that amylin is an unfolded monomer in DMSO. (A) 1D-1H NMR spectrum of 220 mM human amylin (with an amidated Cterminus) in 95 d6-DMSO/5 d2-DCA, pH* 3.5, 25uC. The large resonances at 2.5 and 6.7 ppm are.
