Extended structures antiparallel-chain pleated sheet

The application of these structural principles and the use of accurate values for interatomic distances and bond angles permitted the exact description of several possible configuration of the polypeptide chain, the alpha helix and the two pleated sheets. In particular, it was found that acceptable sheet structures of polypeptide chains could not be formed by fully extended polypeptide chains instead, the chains need to be contracted somewhat, and stiffened in the direction perpendicular to the fiber axis and the material hydrogen bonds. The predicted length of the two-residue unit of a completely extended polypeptide chain is 7.23A, that for the antiparallel chain pleated sheet is 7.00A, and that for the parallel chain pleated sheet is 6.6A. 

FIGURE 4-7 The /8 conformation of polypeptide chains. These top and side views reveal the R groups extending out from the /3 sheet and emphasize the pleated shape described by the planes of the peptide bonds. (An alternative name for this structure is /3-pleated sheet.) Hydrogen-bond cross-links between adjacent chains are also shown, (a) Antiparallel /3 sheet, in which the amino-terminal to carboxyl-terminal orientation of adjacent chains (arrows) is inverse, (b) Parallel f) sheet. 

Antiparallel /3-pleated sheet (/3 sheet). A hydrogen-bonded secondary structure formed between two or more extended polypeptide chains. 

The second common secondary structure in proteins resembles the pleated folds of drapery and is known as p-pleated sheet (Figure 19.8). All of the carbonyl oxygens and amide hydrogens in a p-pleated sheet are involved in hydrogen bonds, and the polypeptide chain is nearly completely extended. The polypeptide chains in a P-pleated sheet can have two orientations. If the N-termini are head to head, the structure is known as a parallel p-pleated sheet. And if the N-terminus of one chain is aligned with the C-terminus of a second chain (head to tail), the structure is known as an antiparallel p-pleated sheet. 

Some fibrous proteins are composed of p-pleated sheets. For example, the silkworm produces silk fibroin, a protein whose structure is an antiparallel p-pleated sheet (Figure 19.9). The polypeptide chains of a p-pleated sheet are almost completely extended, and silk does not stretch easily. Glycine accounts for nearly half of the amino acids of silk fibroin. Alanine and serine account for most of the others. The methyl groups of alanines and the hydroxymethyl groups of serines lie on opposite sides of the sheet. Thus the stacked sheets nestle comfortably, like sheets of corrugated cardboard, because the R groups are small enough to allow the stacked-sheet superstructure. 

The second type of secondary structure is a j8-pleated sheet. In a j8-pleated sheet, the polypeptide backbone is extended in a zigzag structure resembling a series of pleats. The hydrogen bonding in a j8-pleated sheet occurs between neighboring peptide chains, and these chains can run in the same direction or in opposite directions—called a parallel jS-pleated sheet and an antiparallel jS-pleated sheet (Figure 22.9). A j8-pleated sheet is almost fully extended the average two amino acid repeat distance is 7.0 A. 

P-Pleated sheets form when two or more polypeptide chain segments line up side by side (Figure 5.19). Each individual segment is referred to as a ji-strand. Rather than being coiled, each /1-strand is fully extended. /J-Pleated sheets are stabilized by hydrogen bonds that form between the polypeptide backbone N—H and carbonyl groups of adjacent chains. There are two /Fplcatcd sheets parallel and antiparallel. In parallel /kpleated sheet structures, the polypeptide chains... 

The /S-pleated structure is markedly different from the a helix in that it is like a sheet rather than a rod. The polypeptide chain is almost fully extended, and each chain forms many intermolecular hydrogen bonds with adjacent chains. Figure 25.13 shows the two different types of /S-pleated structures, called parallel and antiparallel. Silk molecules possess the /S structure. Because its polypeptide chains are already in extended form, silk lacks elasticity and extensibility, but it is quite strong due to the many intermolecular hydrogen bonds. 

The p Sheet Another type of secondary structure, the p sheet, consists of laterally packed p strands. Each p strand is a short (5- to 8-residue), nearly fully extended polypeptide segment. Hydrogen bonding between backbone atoms in adjacent p strands, within either the same polypeptide chain or between different polypeptide chains, forms a p sheet (Figure 3-4a). The planarity of the peptide bond forces a p sheet to be pleated hence this structure is also called a 3 pleated sheet, or simply a pleated sheet. Like a helices, p strands have a directionality defined by the orientation of the peptide bond. Therefore, in a pleated sheet, adjacent p strands can be oriented in the same (parallel) or opposite (antiparallel) directions with respect to each other. In both arrangements, the side chains project from both faces of the sheet (Figure 3-4b). In some proteins, p sheets form the floor of a binding pocket the hydrophobic core of other proteins contains multiple P sheets. 

In contrast, the j8-strand is a much more extended structure. Rather than hydrogen bonds forming within the secondary structural luiit itself, stabilization occms through bonding with one or more adjacent )8-strands. The overall structure formed through the interaction of these individual )8-strands is known as a fi-pleated sheet. These sheets can be parallel or antiparallel, depending on the orientation of the N-and C-terminal ends of each component )8-strand. A variant of the )8-sheet is the )8-turn in this structure the polypeptide chain makes a sharp, hairpin bend, producing an antiparallel j8-sheet in the process. 

A classic parallel pleated sheet structure is exhibited in the crystal by Gly-LPhe-Gly, as shown in Fig. 26 (Marsh and Glusker, 1961), where NH 0=C hydrogen bonds are formed between the adjacent molecules. The phenylalanine side-chain group is extended away from the polar moieties of the peptide chain. An example of an antiparallel pleated sheet is shown by the LAla-LAla-LAla molecules (Fig. 27) (Fawcett et al, 1975). 

From WAXS and SAED data of both ProNectin F lyophilized powder and sprayed fibrils, the current model indicates that ProNectin F crystallizes into a chain folded pleated sheet of beta strands (Anderson et a/. 1994). The strands are oriented antiparallel. The beta strands are not fully extended, but have a more compressed crankshaft conformation. This conformation agrees with the predicted conformation of unoriented silk fibroin protein, the Silk I structure (Lotz and Keith 1971). The crystal dimension in the c direction (along the peptide backbone) is consistent with a theoretical length of 11.6 nm for nine SEP blocks (54 amino acids) in this conformation. This predicts that the width of the ProNectin F tile is controlled at least in part by the number of amino acids in the silklike block domains. 

The /3-pleated-sheet secondary structure is formed by hydrogen bonds between two adjacent protein chains. In silk fibroin the adjacent protein chains run in opposite directions, called an antiparallel arrangement (Figure 22.4). The pleated sheets can then stack on each other, like the pages of a book. The backbones of these protein chains are already extended. As a result, silk is not elastic. If you try to stretch silk, it will tear. 

The other structures that Pauling discovered by model-building are the j8-structures (pronounced beta), also known as the pleated sheet, from its appearance when many lengths of chain associate in parallel. They are formed by side-to-side association of almost fully extended chains, and the reason that two forms are possible is that adjacent chains may run in parallel or antiparallel (i.e. the same or opposite) directions, as shown here ... 

P-Structures are somewhat less common regular secondary structures of proteins, also known as p-pleated sheets or just P-sheets (Figure 2.19). These are characterised by long extended polypeptide chains in contrast to the compact hehces. These structures are formed by combining parallel or antiparallel oriented polypeptide chains (P-strands) that hne up and form bridges of extramolecular hydrogen bonds (unhke the hehces, which... 

Another form of the ordered structure is the /3 sheet. In this structure, an extended chain makes interstrand rather than intrastrand H-bondings with one or two of the neighboring parallel or antiparallel strands. The peptide backbone in the /3 sheets has a geometry that approaches the most extended chain conformation allowed by normal bond lengths and angles. The displacement of each amino acid residue in the pleated /3 sheet is 3.47 A. 

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