Cross-P sheets

Secondary Structure. The silkworm cocoon and spider dragline silks are characterized as an antiparaHel P-pleated sheet wherein the polymer chain axis is parallel to the fiber axis. Other silks are known to form a-hehcal (bees, wasps, ants) or cross- P-sheet (many insects) stmctures. The cross-P-sheets are characterized by a polymer chain axis perpendicular to the fiber axis and a higher serine content. Most silks assume a range of different secondary stmctures during processing from soluble protein in the glands to insoluble spun fibers. 

N 093 "Role of Interstrand Loops in the Formation of Intramolecular Cross-P-Sheets by Homopolyamino Acids"... 

Fig. 13 Cryo-TEM images of (a) 16, (b) I7 (inset magnified view of representative fibers), and (c) schematic representation of the proposed formation of fibers of Ig. The benzenedithiol core of building block 1 is shown in yellow and the peptide chain in blue. Stacks of hexamer are held together by the assembly of the peptide chains into elongated cross-P sheets. (Reproduced from [55])...

Although precursor proteins of amyloid are extremely diverse in sequence, the end-product fibrils in general share several structural features such as cross-P sheet assemblies, non-branched fibrils of similar length and structural organization. 

The aggregation of protein fibrils in organs is believed to be the cause of several degenerative disorders including Alzheimer s and Parkinson s disease. Amyloid fibrils are structures which, regardless of the identity of the protein, share a common cross-P-sheet core structure. Several Raman spectroscopic studies have focused on insulin amyloid fibrils (14-19). We have recently used DCDR to confirm the previously reported results and perform a more detailed difference spectroscopic analysis to quantify the principal Raman spectral features associated with insulin fibrillation (20). The following results demonstrate that very similar DCDR difference spectral features are observed upon fibrillation of lysozyme. 

Fig. 9. A de novo designed P sheet protein, betabellin, formed by the dimerization of two identical four-stranded -sheets and a disulfide linking the two sheets. This model is for betabeUins 9 and later progenies the earher betabeUins contained a two-armed cross-linker connecting the sheets (51).

The hairpin motif is a simple and frequently used way to connect two antiparallel p strands, since the connected ends of the p strands are close together at the same edge of the p sheet. How are parallel p strands connected If two adjacent strands are consecutive in the amino acid sequence, the two ends that must be joined are at opposite edges of the p sheet. The polypeptide chain must cross the p sheet from one edge to the other and connect the next p strand close to the point where the first p strand started. Such CTossover connections are frequently made by a helices. The polypeptide chain must turn twice using loop regions, and the motif that is formed is thus a p strand followed by a loop, an a helix, another loop, and, finally, the second p strand. 

We saw in Chapter 2 that the Greek key motif provides a simple way to connect antiparallel p strands that are on opposite sides of a barrel structure. We will now look at how this motif is incorporated into some of the simple antiparallel P-barrel structures and show that an antiparallel P sheet of eight strands can be built up only by hairpin and/or Greek key motifs, if the connections do not cross between the two ends of the p sheet. 

A more complex p helix is present in pectate lyase and the bacteriophage P22 tailspike protein. In these p helices each turn of the helix contains three short p strands, each with three to five residues, connected by loop regions. The p helix therefore comprises three parallel p sheets roughly arranged as the three sides of a prism. However, the cross-section of the p helix is not quite triangular because of the arrangement of the p sheets. Two of the sheets are... 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

Fig. 4.12(a). An outline structure of a protein (here the enzyme phospholipase A2), showing a-helical runs of amino acids as cylinders (A-E) and anti-parallel P-sheet runs as heavy black arrows. Disulfide cross-links are shown (the enzyme is extracellular), and runs of no a/p secondary structure appear as thin lines. The structure is relatively immobile, and binds calcium in a constrained loop. (Reproduced with permission from Professor J. Drenth.)... 

As anticipated from their sequence similarity, the (non-catalytic) a- and the (catalytic) P-type subunits have the same fold (Lowe et al. 1995 Groll et al. 1997) a four-layer a+p structure with two antiparallel five-stranded P sheets, flanked on one side by two, on the other side by three a helices. In the P-type subunits, the P-sheet sandwich is closed at one end by four hairpin loops and open at the opposite end to form the active-site cleft the cleft is oriented towards the inner surface of the central cavity. In the a-type subunits an additional helix formed by an N-terminal extension crosses the top of the P-sheet sandwich and fills this cleft. Initially, the proteasome fold was believed to be unique however it turned out to be prototypical of a new superfamily of proteins referred to as Ntn (N-terminal nucleophile) hydrolases (Brannigan et al. 1995). 

In order to clarify the relationship between cross peaks and carbon proton inter-atomic distances, molecular model for the anti-parallel p-sheet conformation of PG and PLV have been calculated using reference data (Wiithrich, et al.)117 by the X-PLOR 3.1 program, and we measured the carbon-proton distances from the modeled structure (Table 14). The distances between the carbons and their directly bonded protons are ca. 1.1 A and their signals can be observed in the FSLG C H HETCOR spectrum with contact time of 0.2 ms. Further, in the FSLG C H HETCOR spectrum with a contact time of 0.5 ms the signals corresponding... 

A spider s orb-web is formed by extrusion of a concentrated protein solution and stretching of the resulting fiber. The cross-strands, which are stronger than steel, resemble silkworm silk. The molecules contain microcrystalline p sheet domains that are rich in Gly-Ala repeats as well as polyalanine segments. The capture spiral is formed from much more elastic molecules that contain many -tum-forming sequences. These assume a springlike p spiral. See Box 2-B. 

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... 

Review Secs. 17.13 and 17.14. Secondary structures of importance are hydrogen bonds and the geometry of the peptide bond. The tertiary structures of fibrous sproteins are dominated by p-sheets (the presence of small amino acids with nonpolar side chains) and, in many cases, significant amounts of cysteine (the amino acid where R=CH2SH) cross-linked by disulfide bonds. 

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