Pleated-sheet structure

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... 

The thermal stability of these silicones is very high and even at 200° C over months no racemization of the chiral groups can be detected. We attribute this phenomenon to the formation of a specific pleated-sheet structure in an apolar environment, which stabilizes the L-configuration and prevents inversion at the as-symmetric carbon. 

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). 

The fact that a denatured protein can spontaneously return to its native conformation was demonstrated for the first time with ribonuclease, a digestive enzyme (see p. 266) consisting of 124 amino acids. In the native form (top right), there are extensive pleated sheet structures and three a helices. The eight cysteine residues of the protein are forming four disulfide bonds. Residues His-12, Lys-41 and His-119 (pink) are particularly important for catalysis. Together with additional amino acids, they form the enzyme s active center. 

Secondary structures are regions of the peptide chain with a defined conformation (see p. 68) that are stabilized by H-bonds. In insulin (2), the a-helical areas are predominant, making up 57% of the molecule 6% consists of p-pleated-sheet structures, and 10% of p-turns, while the remainder (27%) cannot be assigned to any of the secondary structures. 

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. 

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)... 

There are a number of MHC class I and class II polymorphic molecules, all with substantially the same basic structure. The MHC class I molecule is a dimer consisting of a glycoslylated transmembrane peptide of molecular weight 45 kDa covalently linked to a 12 kDa peptide it is found on the surface of most nucleated cells within the body. It is believed that the polypeptide backbones fold in such as way as to form a platform of -pleated sheet structures to support a peptide binding cleft in which the antigen fragments are held and presented to the T cells. 

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... 

The p pleated sheet structure occurs commonly in insoluble structural proteins and only to a limited extent in soluble proteins. It is characterised by hydrogen-bonding between polypeptide chains lying side by side, as illustrated in Fig. 5.A3b. 

Plate 15 A selection of common types of computer graphics models, all showing the same three strands of pleated-sheet structure from cytochrome b5 (PDB 3b5c). (a) Wireframe (b) ball and stick (c) space filling (d) ribbon backbone with ball-and-stick side chains. (For discussion, see Chapter 11.) Image SPV/POV-Ray. (Continues)... 

Fig. 25. Energy contours for poly-L-alanine helices, in kcal mole-1. The symbols R and L indicate the positions of the right- and left-handed a-helices the symbols fix and /Ja designate the positions of the parallel and antiparallel pleated-sheet structures (Ooi et al., 1967).

Thus, we feel that the a-helical and the extended" helical structure are well established in VCD, and that there exists a simple method for the interpretation of the data. The VCD features of the B-pleated sheet structure appear reasonably well established, too, although its interpretation is much more difficult. Since the data are mono-signate, the DECO model is not appropriate (it always predicts conservative couplets). Nafie and coworkers explained such monosignate VCD in terms of a model similar to one described earlier by Schellman [24], with nearly co-linear (and antiparallel) electric and magnetic dipole transition moments [30]. 

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.). 

Be familiar with the various secondary structures of proteins and their dimensions the a helix, /3 turns, pleated sheet structures, and collagen and what dictates the assumption of such... 

Figure 4.8 Parallel and antiparallel pleated sheet structures. Dotted lines indicate hydrogen bonds. (Reproduced with permission from Bezkorovainy A. Basic Protein Chemistry. Springfield, IL Thomas, p. 114, 1970.)...

Ramachandran plots serve to answer the question of why the a-helical or the pleated sheet structures have the properties that they do however, the plots do not serve to predict whether a given polypeptide chain will assume the a-helical, the pleated sheet, or a random conformation. Anfinsen and his colleagues have proposed that it is the amino acid composition and sequence in a given peptide chain that determine the conformation the chain assumes. Ideally, we should be able to look at an amino acid sequence of a protein and then... 

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