Cross-p-structures

If peptide chains can be oriented in a regular fashion, it may be useful to measure infrared linear dichroism.24 25 Absorption spectra are recorded by passing plane polarized light through the protein in two mutually perpendicular directions, with the electric vector either parallel to the peptide chains or perpendicular to the chains. Such a pair of spectra is shown in Fig. 23-3A for oriented fibrils of insulin. In this instance, the insulin molecules are thought to assume a P conformation and to be stacked in such a way that they extend transverse to the fibril axis (a cross-P structure). When the electric vector is parallel to the fibril axis, it is perpendicular to the peptide chains. Since the amide I band is dominated by a carbonyl stretching motion that is perpendicular to the... 

What is the nature of the insoluble forms of the prion protein They are hard to study because of the extreme insolubility, but the conversion of a helix to (3 sheet seems to be fundamental to the process and has been confirmed for the yeast prion by X-ray diffraction.11 It has been known since the 1950s that many soluble a-helix-rich proteins can be transformed easily into a fibrillar form in which the polypeptide chains are thought to form a P sheet. The chains are probably folded into hairpin loops that form an antiparallel P sheet (see Fig. 2-ll).ii-11 For example, by heating at pH 2 insulin can be converted to fibrils, whose polarized infrared spectrum (Fig. 23-3A) indicates a cross-P structure with strands lying perpendicular to the fibril axis >mm Many other proteins are also able to undergo similar transformation. Most biophysical evidence is consistent with the cross-P structure for the fibrils, which typically have diameters of 7-12 rnn."-11 These may be formed by association of thinner 2 to 5 nm fibrils.00 However, P-helical structures have been proposed for some amyloid fibrils 3 and polyproline II helices for others. 1 11... 

Insects and arachnoids produce well-known amyloids. Silk and spider webs, like P-keratin, also differ from amyloids in being fibrous P-sheet proteins composed of peptide strands that are parallel, rather than perpendicular, to the direction of the fibril axis. For the process of silk formation by spiders, it has been proposed that fibrils in the silk gland have an initial cross-P structure (Kenney et al. 2002 Table 3) that, when stretched, assume parallel P-structures. However, X-ray diffraction for a peptide derived from the central domain of the A class of chorion proteins, derived frovaAntheraea polyphemus eggshells, displayed P-sheets perpendicular to the fibril axis, the same cross-P structure that occurs in amyloid proteins (Iconomidou et al. 2000 Table 3). The stability and strength of the amyloid fibres provides mechanical and biological protection for the oocyte and developing embryo from a variety of environmental and predatory hazards. 

Using solid-state NMR correlation spectroscopy evidence for an extended cross-p structure was found in functional amyloid. " ... 

CD spectroscopy also proved to be useful for systematically studying the influence of the concentration on the conformation of oligopeptides Conformational transitions of the type P-structure - random coil can be compelled by diluting solutions of peptides with a potential to adopt a pleated sheet. This procedure permits the distinction between inter- and intramolecular p-structures ( cross-P-structures )- Cross-P-structures are characterized by a backfolding of the peptide chain the intramolecular aggregation is insensitive to disruption by dilution. 

---