Antiparallel 3-pleated sheet

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). 

D, short alpha-helical portions, region of antiparallel pleated sheets... 

Varies with source, fibrous-linear with cross-links crystalline regions contain antiparallel, pleated sheets... 

Oxytocin (OT) is a nonapeptide in which six amino acids form a ring closed by a disulfide bridge, while the ring itself forms an antiparallel pleated sheet. The tail portion of the peptide, composed of Pro-Leu-Gly-NHj, is also rigidly held in a folded conformation. Oxytocin causes the powerful contraction of some smooth muscles and plays a vital role in milk ejection (not to be confused with milk secretion, which is regulated by prolactin). It also has uterotonic action, contracting the muscles of the uterus, and is therefore used clinically to induce childbirth. 

Figure 2-18 Typical (3 bulges in antiparallel pleated sheets. The residues Rx, R2, and Rx identify the bulges. (A) A "classic" (3 bulge, in which < )[ and /j are nearly those of an a helix while other torsion angles are approximately those of regular (3 structures. (B) The G1 bulge in which the first residue is glycine with = 85°, /j = 0°. It is attached to a type II (3 turn of which the glycine (labeled 1) is the third residue.

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... 

An intramolecular bond goes between two parts of the same chain. An intermolecular bond goes between two different molecules. Intermolecular bonds are found in the (3-pleated sheet intramolecular bonds in the a-helix and in some antiparallel pleated sheets. 

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).

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.)...

Figure II-2 Major elements of secondary structure of proteins. Left, the a-helix right, representation of the antiparallel pleated sheet structures for polypeptides. (After Pauling, L., and R. B. Corey (1951). Proc Natl Acad Sci USA 37 729).

Fig. 19.4. Secondary-structure hydrogen bonding in ribonuclease Tj. Arrows (-+) point in the direction N-H- 0=C. Note regions of a-helix and of antiparallel / -pleated sheet. Also indicated are hydrogen-bonding contacts from main-chain atoms to side-chains and water molecules 1594]...

Fig. 6. The antiparallel pleated sheet structure of polypeptide chains. (Marsh et al, 1955.)...

The classical polypeptide conformations are the a-helix and the parallel and antiparallel -pleated sheets due to Pauling and colleagues [46,47]. These conformations can occur separately in fibrous proteins or they can often be beautifully combined, as in certain globular proteins such as triose phosphate isomerase [48] and carboxypeptidase A [49]. The interesting motifs constituting the anatomy of some globular proteins have been emphasized by the work of Richardson [50], and a combination of pleated sheet and a-helix was early proposed as a structural motif for dynamic voltage dependent channel formation [51,52]. Our concern here, however, is to characterize separately these conformations in order that a view of more complex combinations of these structures can be correctly obtained. 

Optical values for a-helix, antiparallel -pleated sheet and type II -turn... 

Figure 23. Simultaneous resolution of circular dichroism (A) and absorption (B) curves of poly-L-serine in the antiparallel -pleated sheet conformation. The critical values for the resolved bands are included in Table 2B. Reproduced, with permission, from [73],...

Figure 24. A, Stereo pair plot of poIy-L-alanine in the antiparallel -pleated sheet conformation of Pauling and Corey [47]. Sheet is tilted 10°. Stereo pairs arranged for cross-eye viewing. B, Stereo pair plots of poly-L-alanine in the parallel -pleated sheet conformation of Pauling and Corey [47]. Stereo pairs are arranged for cross-eye viewing.

Figure 33. Stereo pair plots of the Gramicidin A transmembrane channel. It is a single-stranded, left-handed j8-helix with approximately six residues per turn. A, Side view. Two molecules are hydrogen-bonded head to head (amino end to amino end) by means of six hydrogen bonds. The intermolecular hydrogen bonds have the pattern of an antiparallel -pleated sheet (see Figure 24A), whereas the intramolecular hydrogen bonding pattern is a parallel -pleated sheet (see Figure 24B). B, Channel view of a monomer. Reproduced, with permission from [96].

Figure 4. Elements of secondary structure. (A) The hydrogen-bonding pattern of an a-helix. Hydrogen bonds (denoted by dashed lines) form between residues four positions away from each other in the helix. Unmarked atoms are hydrogens. (B) Two-dimensional projections of the hydrogen-bonding patterns of parallel and antiparallel pleated sheets. Parallel and antiparallel pleated-sheets have different three-dimensional structures.

In general, in common solvent systems such as dimethylsulfoxide, methanol, and water, the /3-turn is characterized in terms of amide proton magnetic resonances by a high-field shifted resonance with a decreased temperature dependence for residue / - - 2. by a small CH—NH coupling constant for residue /, and by a large aCH—NH coupling constant for residue / -El. If the /3-turn is part of an antiparallel pleated sheet, then Ach will also be large, of the order of 7 cps, for the resonances of residues / — 1 and / -E 2 and the temperature dependence of chemical shift of the amide proton resonance of residue / — 1 will be lowered. 

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