Polypeptides parallel pleated-sheet structure

There are two ways in which proteins chains can form the pleated sheet structure. One is with the chains running in the same direction i.e. the -COOH or NH2 ends of the polypeptide chains lying all at the top or all at the bottom of the sheet. This is called parallel pleated-sheet structure. In another type, known as antiparallel p-pleated sheet structure, the polypeptide chains alternate in such a way that the -COOH end of the one polypeptide is next to the -NH2 end of the other i.e., polypeptide chains run in opposite directions. 

Hydrogen bonds (a) in a parallel -pleated sheet structure, in which all the polypeptide chains are oriented in the same direction, and (b) in an antiparallel fi-pleated sheet, in which adjacent polypeptide chains run in opposite directions. For color key, see Fig. 22.7. 

The p-pleated sheet structure occurs in fibrous as well as globular proteins and is formed by intermolecular hydrogen bonds between a carboxyl group oxygen of one amino acid and an amine hydrogen of an adjacent polypeptide chain. Parallel p-pleated sheets form when the adjacent polypeptide chains are oriented in one direction (from N-terminal to C-terminal end or vice versa). Antiparallel p-pleated... 

In a further exploration of the relationship between dye structure and wet fastness on silk, four novel monoazo J acid derivatives (3.169 X = Xx to X4), including 3.168 (X = X2) made from 2-aminobenzophenone, were synthesised. Silk was dyed at pH 4 and 85 °C and the dyeings tested for fastness to washing, perspiration and dry cleaning. The highest allround fastness was shown by the 4 aminobenzophenone derivative (X = X4), a structure that resembles the anti-parallel pleated sheet arrangement of polypeptide chains in silk [183]. 

Figure 3-1. a. Alpha-helix structure for a polypeptide or protein b. Pleated sheet structures, depicting parallel (1) and antiparallel (2) variants (Elias 1997, reprinted courtesy ofWiley-VCH.). 

Hair keratin has historically been associated with the a-helical structure. For that reason, it is termed a-keratin. And indeed the basic keratin polypeptides are a-helical except for their N- and C-terminal domains. These are believed to be involved in head-to-tail condensation to form keratin polymers. When hair keratin is stretched, the resulting secondary structure is the parallel pleated sheet (see Chapter 4). Stretched keratin is referred to as /3-keratin to emphasize its secondary structure. 

Pleated sheet structures are parallel or antiparallel. In the local minimum in the Ramachandran qt/y/ plot (Fig. 19.3) of y3-pleated sheet structures, two configurations are possible, with parallel and antiparallel orientation of the polypeptide strands (Fig. 19.6). The strands are linked by mferchain N-H 0=C hydrogen bonds, which run both ways between the strands and produce a characteristically different pattern in parallel and antiparallel sheets. It is a particular stereochemical feature of the /7-pleated sheets that amino acid side-chains point alternately up and down, and adjacent side-chains interact sterically to produce a right-handed twist [597, 5981 (see Fig. 19.7 a). The regular pattern of a /7-sheet can be interrupted locally by insertion of an extra amino acid, giving rise to a so-called /7-bulge [599]. 

P structure is the structure where the polypeptide chain is elongated. The structure can be the type where all the molecular chain run in the same direction and form a parallel pleated sheet or the type where the molecular chain run in the alternate direction and form anti parallel chain pleated sheet. In the case of P-structure, there are three important features, namely the period that is repeating period of the polypeptide chain, the spacing of the molecular chain in the sheet and the distance between the sheets [76]. 

Repeating sequences of amino acids with small, compact R-groups (e.g., glycine, alanine) tend to form the (3, or pleated sheet, structure, which consists of parallel (Fig. 2-3a) or antiparallel (Fig. 2-36) polypeptide chains linked by interchain hydrogen bonds. Silk is an example of the antiparallel sheet. 

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... 

Hydrogen bonds stabilize secondary structures. These can be within a chain (as in an ot-helix) or between different chains (as in a / -pleated sheet). Figure 6.6 illustrates how hydrogen bonds stabilize four different helical structures. When hydrogen bonds stabilize adjacent polypeptide chains in / -sheet structures, the adjacent chains can be oriented parallel or antiparallel to each other. When both chains are parallel, they have the same amino to carboxyl orientation. When they are antiparallel, the two chains have opposite amino to carboxyl orientations. 

These represent the sheetlike arrangement of the polypeptide chains. The hydrogen bonds are found between the adjacent polypeptide chains. The polypeptide chains involved in the pleated sheet structure can be either parallel or antiparallel. Hydrogen bonds stabilize the p-pleated sheet (see Figure 28-7). 

Fig. 10.4. Representation of (a) the parallel and (b) the antiparallel pleated-sheet structures for polypeptides (Pauling and Corey, 1951). (From Mahler and Cordes, 1966.)...

About a year ago, in the course of the consideration of configurations of polypeptide chains with favored orientations around single bonds, we described two pleated-sheet structures. These structures are suited to polypeptide chains constructed entirely of l amino-acid residues or of d amino-acid residues. In one pleated sheet alternate polypeptide chains are antiparallel, and in the other they are parallel. The amide groups have the trans configuration. 

In contrast to the a-helical structure of the a-K. discussed above, the -K. have -pleated sheet structure. The most prominent representative of this class is silk fibroin (iff, 365,000, 2 subunits). Here the chains run antiparallel rather than parallel, and form a zig-zag structure. The formation of hydrogen bonds between the -CH(=0) and -NH- groups of neighboring chains stabilizes the pleated sheet structure. Together with weak hydrophobic interactions, the hydrogen bonds link pairs of polypeptides into a three-dimensional protein complex. These are additionally stabilized, in silk, by a water-soluble protein, sericin. The resultant fiber is very resistant and flexible, but only slightly elastic. The amino acid sequence which repeats over long stretches of the chain is, for silk fibroin, (Gly-Ser-Gly-Ala-Gly-Ala-) . 

Collagen (see) has a specialized structure containing interchain, hydrogen-bonded, left-handed helices. Otherwise, all known P. helices are right-handed, but the possibility remains that left-handed helices might be found in P. which have not yet been analysed by X-ray diffraction, e.g. membrane P. In the pleated sheet structure, the polypeptide chain is more or less stretched out, and neighboring lengths of chain can be parallel or antiparallel (with respect to their N- and C-termini), e.g. the pleated sheet structure of silk fibroin (Fig. 8) consists of antiparallel polypeptide chains. 

Now the polypeptide chain depicted above is in the extended or /3-form and in protein films (and possibly in the protein mono-layers of cell membranes) this form can be stabilised by hydrogen bonding between the carbonyl (> C = O) and imide (HN <) groups of the backbones of parallel orientated polypeptide chains to give a pleated-sheet structure (Fig. 3.3(A)). This is the structure of several fibrous proteins, e.g. j8-keratin. From the primary polypeptide unit of structure, a secondary structure has arisen. 

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. 

There are two stable arrangements of nearly completely extended polypeptide chains forming hydrogen bonds with neighboring chains.114 They are the parallel-chain pleated sheet (Fig. 12-19) and the anti-parallel-chain pleated sheet (Fig. 12-20). The identity distance in the direction of the chains is found to be different for the two structures when the requirement that the N—H---0 bonds be linear is imposed ... 

Fibroin, the fibrous protein found in silk, has a secondary structure called a beta- (/8-) pleated sheet, in which a polypeptide chain doubles back on itself after a hairpin bend. The two sections of the chain on either side of the bend line up in a parallel arrangement held together by hydrogen bonds (Figure 24.8). Although not as common as the a-helix, small pleated-sheet regions are often found in proteins. 

Much remains to be done in characterizing the vibrational spectra of known polypeptide chain structures. Although some preliminary studies were done on the parallel-chain pleated sheet (Krimm and Abe, 1972 Moore and Krimm, 1975), a full analysis of this structure found in... 

The folding of polypeptide chains into ordered structures maintained by repetitive hydrogen bonding is called secondary structure. The chemical nature and structures of proteins were first described by Linus Pauling and Robert Corey who used both fundamental chemical principles and experimental observations to elucidate the secondary structures. The most common types of secondary structure are the right-handed cx-helix, parallel and antiparallel /3-pleated sheets, and (3-turns. The absence of repetitive hydrogen-bonded regions (sometimes erroneously called random coil ) may also be part of secondary structure. A protein may possess predominantly one kind of secondary structure (a-keratin of hair and fibroin of silk contain... 

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