Pleated sheet structure parallel-chain

Figure 7.34. Stereo perspectives of antiparallel (A) and parallel (B) P-pleated sheet structures (three chains each), with the lefthand pair for cross-eye viewing and the righthand pair for walleye viewing. Open arrows indieate chain direction, antiparallel above and parallel below. Note that all peptide NH...

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... 

Crystal structure(s) of ACTH-(1-39) or 1-24 are not known. Suitable crystals for X-ray diffraction experiments could be obtained however, for the heptapeptide 4-10 (54, 55) and the smaller tetrapeptide 4-7 (54, 56). In the former case, an anti--parallel p-pleated sheet structure of the backbone was found with clustering of hydrophobic (Met, PheandTrp) and hydrophilic (Glu, His, Arg) side-chains as remarkable features. ACTH-(4-7)... 

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]. 

FIGURE 10-7 Pleated sheet structures proposed for /8-keratin, (a) The parallel-chain pleated sheet, (b) The anti parallel-chain pleated sheet. [From Pauling and Corey, Proc. NatL Acad. Sci. U.S. 37, 729-40 (1951).]... 

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. 

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. 

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). 

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). 

Figure 9. Drawing representing the anti-parallel-chain pleated-sheet structure.

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. 

The Parallel-Chain Rippled Sheet.— The parallel-chain rippled sheet, shown in figure 2, is closely similar to the parallel-chain pleated sheet, and has the same diagrammatic representation, showm in figure 5 of the previous paper. The unit of structure was found by measurement of a model to have lateral identity distance Oo = 9.60 A, and identity distance along the fiber axis 6o = 6.50 A. Atomic coordinates are given in table 2. 

Pleated sheet structures ifi structures) have little stretchability, but high tensile strength. In pleated sheets the peptide chains lie in a plane, either parallel to each other as in j -keratin of bird feathers or antiparallel as in the more highly crystalline silks. 

Figure 25.12 Hydrogen bonds (a) in a parallel p-pleated sheet structure, in which all the polypeptide chains are oriented in the same direction, and p) in an antiparaiiei fS-pleated sheet, in which adpcent polypeptide chains run in opposite directions. For color key, see Fig. 25.9.

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. 

Diagrammatic representation of the antiparallel-chain pleated-sheet structure (left) and the parallel-chain pleated-sheet structure (right). 

A 3.25 A resolution electron-density map of the semiquinone form shows that flavodoxin is a smorgasbord of secondary structures . The chain folding contains 35% helix, a central region of pleated sheet (one parallel pair and two pairs antiparallel), and a number of 3io bends. The exact orientation of the FMN prosthetic group appears to be in some doubt. However, Jensen et al, have recently computed a 2.5 A electron-density map based on one samarium derivative for a closely related flavodoxin which shows the same general chain folding and indicates clearly the orientation of the FMN group. 

The lack of a center of symmetry in the monomer repeat imit structure of PA6 imparts a directionality to the PA6 chains. Consequently, PA6 can form the a-crystalline, planar hydrogen bonded sheet structure only between antiparallel chains. Parallel chains, on the other hand, form a 7-crystalline, pleated sheet structure in which the chains assume a twisted helix conformation to permit lull H-bonding. Due to these symmetry and entropic restrictions, PA6 is somewhat less crystalline and less ordered, usually containing a mixture of a and 7-crystalline forms in the injection molded parts. As a consequence of these crystallinity and morphology differences, PA6 is intrinsically more ductile than PA66, since it is easier to deform a less ordered polymer phase. 

The pleated sheet structure applies to proteins of the silk fibroin-/3-keratin group. For silk fibroin itself it may be assumed that the structure has been proved correct by Pauling s calculations (antiparallel chains). A pleated sheet structure with parallel chains is proposed for stretched hair (/3-keratin). Investigations on some other natural representatives have not yet been concluded. Possibly, amorphous or differently arranged sections occur in combination with pleated-sheet areas. 

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