Pleated sheet structure antiparallel-chain

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

Drawing representing the antiparallel-chain pleated-sheet structure. 

The influence of chain length and side-chain modifications of ACTH-derived peptides on active avoidance behaviour in rats will be discussed. H-Met(02)-Glu-His--Phe-D-Lys-Phe-OH (Org 2766) emerged from these studies as an orally active peptide with an increased potency and selectivity of action. Physico-chemical data (from the literature) on the reference peptide ACTH--(4-10) did not point to a preferred conformation in solution, whereas in the crystalline state an antiparallel 3-pleated sheet structure was found. At the receptor site we suggested an a-helical conformation in which the Phe and Met residues are close together. Additional support for this suggestion came from the behavioural activity of [des-Tyr", Met ]enkephalin and of cyclo--(-Phe-Met-cAhx-), eAhx merely serving as a spacer. 

The (3-strand sequences are stretched out conformations of these polypeptide sections and are typically stabilized by inter-strand hydrogen bonds between keto (C = 0) oxygens and peptide bond NHs, the strands being arrayed in an antiparallel fashion. This type of secondary structure is favoured by amino acid residues with small R groups (such as Gly, Ala and Ser) that minimize steric overlap between chains. Thus a well-known protein having this type of secondary structure is silk fibroin that has a high proportion of repeated sequences involving Gly, Ala and Ser and an extensive antiparallel (3-pleated sheet structure. The macroscopic properties of silk fibroin (flexibility but lack of stretchability) reflect this type of secondary structure at the molecular level. 

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. 

The main spectroscopic observation that required explanation was the 60-cm splitting in the infrared-active amide 1 (mainly CO s) modes of antiparallel-chain pleated sheet (APPS) polypeptides. Miyazawa (1960a) proposed that such splittings must be a consequence of the interactions between similar oscillators within the repeat unit of the structure, namely, the four peptide groups in the present case. He showed by a perturbation treatment that the frequencies for the four possible coupled modes would depend on the relative phases of the vibrations and the magnitudes of the interactions between peptide groups according to the relation... 

Two structures have been proposed for (Gly) I an antiparallel-chain pleated sheet (APPS) and a similar rippled sheet (APRS) (see Section III,B,1). These structures have different symmetries the APPS, with D2 symmetry, has twofold screw axes parallel to the a axis [C (a)] and the b axis [C (b)], and a twofold rotation axis parallel to the c axis [62(0)] the APRS, with C2h symmetry, has a twofold screw axis parallel to the b axis ( 2(6)], an inversion center, i, and a glide plane parallel to the ac plane, o-Sj. Once these symmetry elements are known, together with the number of atoms in the repeat, it is possible to determine a number of characteristics of the normal modes the symmetry classes, or species, to which they belong, depending on their behavior (character) with respect to the symmetry operations the numbers of normal modes in each symmetry species, both internal and lattice vibrations their IR and Raman activity and their dichroism in the IR. These are given in Table VII for both structures. 

Structural Parameters of CrystalUne Antiparallel-Chain Pleated Sheet PolylL-alaninef... 

All the ferrichromes whose conformations have been studied are des-scribedin Fig. 1. This structural model is based on the X-ray crystallographic study of ferrichrome A by Zalkin et al. (27, 28) and in most respects, it is confirmed by PMR solution studies (29, 30). The structure exhibited by the model in Fig. 1 is globular with the three substituted ornithyl side chains embracing the metal ion in octahedral coordination while the cyclohexapeptide backbone assumes a distorted antiparallel /3-pleated sheet structure. 

In the formulation of positional parameters for all of the atoms within this pseudo unit cell, we have assumed that the basic structural component is the antiparallel-chain pleated sheet (Pauling Corey, 1953). The reasons for this choice have been discussed in eonneetion with Bombyx mori, they are founded principally on the eonfidence we place in our knowledge of the geometry of polypeptide chains and hydrogen bonds. In particular, the values for the a- and 6-axis identity distances— 9-44 and 6-95 A— are almost exactly those calculated for the antiparallel-chain pleated sheet—9-5 and 7-0 A (Pauling Corey, 1953). 

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. 

After some of the disulfide bonds are broken hair can be stretched to a little over twice its normal length. It then gives a pleated-sheet x-ray pattern. The structure seems to be that of the parallel-chain pleated sheet (length per residue along the chain axis 325 pm, 2.17 times that for the a-helix), rather than of the antiparallel-chain pleated sheet (length per residue 350 pm, 2.33 times that for the a-helix). 

Figure 26.6 (a) The /3-pleated sheet secondary structure of proteins is stabilized by hydrogen bonds between parallel or antiparallel chains, (b) The structure of concanavalin A, a protein with extensive regions of antiparallel / sheets, shown as flat ribbons. 

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... 

Silk is produced from the spun threads from silkworms (the larvae of the moth Bombyx mori and related species). The main protein in silk, fibroin, consists of antiparallel pleated sheet structures arranged one on top of the other in numerous layers (1). Since the amino acid side chains in pleated sheets point either straight up or straight down (see p. 68), only compact side chains fit between the layers. In fact, more than 80% of fibroin consists of glycine, alanine, and serine, the three amino acids with the shortest side chains. A typical repetitive amino acid sequence is (Gly-Ala-Gly-Ala-Gly-Ser). The individual pleated sheet layers in fibroin are found to lie alternately 0.35 nm and 0.57 nm apart. In the first case, only glycine residues (R = H) are opposed to one another. The slightly greater distance of 0.57 nm results from repulsion forces between the side chains of alanine and serine residues (2). 

Hormones related to oxytocin and vasopressin occur in most vertebrates, the compound vasotocin shown in Fig. 30-4 being the most common. Substitution of phenylalanine for isoleucine at position 3 gives arginine vasopressin, the vasopressin found in our bodies. Structure of oxytocin and related hormones82 are also shown in Fig. 30-4. Like somatostatin, vasopressin and oxytocin may also form antiparallel pleated sheet structures with P turns. The structural requirements for hormone activity have been studied intensively. Both the macrocyclic hexapeptide ring and the tripeptide side chains are necessary for maximal activity.83... 

Figure 25-13 Hydrogen-bonded structure of silk fibroin. Notice that the peptides run in different directions in alternate chains. This structure is called an antiparallel /3-pleated sheet.

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

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]. 

Secondary Structure. The silkworm cocoon and spider dragline silks are characterized as an antiparallel p-pleated sheet wherein the polymer chain axis is parallel to the fiber axis. Other silks are known to form a-helical (bees, wasps, ants) or cross- p-sheet (many insects) structures. 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 structures during processing from soluble protein in the glands to insoluble spun fibers. 

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