P-pleated structures

Another important natural fibrous material is wool. Wool is mainly made of proteins called keratins. Not a single kind, but several different keratins are involved acidic and basic keratins and keratin-associated proteins. Keratins are related to silk fibroin mentioned earlier. Both a-helix and P-pleated structures of keratin are involved. The keratin-associated proteins contain high level of the sulfur-containing amino acid, cysteine. The sulfhydryl (-SH) group of cysteine can readily be oxidized and combine with another sulfhydryl sulfur atom of another cysteine residue on another polypeptide. The result is the formation of sulfur-sulfur... 

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

The gene-encoded primary structure of a polypeptide is the sequence of its amino acids. Its secondary structure results from folding of polypeptides into hydrogen-bonded motifs such as the a helix, the P-pleated sheet, P bends, and loops. Combinations of these motifs can form supersecondary motifs. 

X-ray diffraction analysis reveals the three-dimensional structure of both IL-l molecules to be quite similar. Both are globular proteins, composed of six strands of antiparallel P pleated sheet forming a barrel that is closed at one end by a further series of P sheets. 

Monomeric TNF is biologically inactive the active form is a homotrimer in which the three monomers associate non-covalently about a threefold axis of symmetry, forming a compact bellshaped structure. X-ray crystallographic studies reveal that each monomer is elongated and characterized by a large content of antiparallel P pleated sheet, which closely resembles subunit proteins of many viral caspids (Figure 9.4). 

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 second structural element to be proposed by Pauling and Corey was the P-pleated sheet (Figure 4.7). These sheets are made up of P-strands, typically from 5 to 10 residues long, in an almost fully extended conformation, aligned alongside one another with hydrogen bonds formed between the C=0 bonds of one strand and the NH of the other, and vice versa. The P-sheets are pleated (i.e. they undulate) with the Ca atoms alternatively a little above, or a little below the plane of the P-sheet, which means that the side chains project alternatively above and below the plane. P-Strands can interact to form two types of pleated sheets. 

Two common examples are the a-helix and the P-pleated sheet. These shapes are reinforced by hydrogen bonds. An individual protein may contain both types of secondary structures. Some proteins, like collagen, contain neither but have their own more characteristic secondary structures. 

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. 

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)... 

The 3-sheet is another form of secondary structure in which all of the peptide bond components are involved in hydrogen bonding (Figure 2.7A). The surfaces of 3-sheets appear "pleated," and these structures are, therefore, often called "P-pleated sheets." When illus trations are made of protein structure, 3-strands are often visualized as broad arrows (Figure 2.7B). 

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. 

A protein s primary structure is its amino acid sequence. Its secondary structure is the orientation of segments of the protein chain into a regular pattern, such as an a-helix or a P-pleated sheet. Its tertiary structure is the three-dimensional shape into which the entire protein molecule is coiled. 

The secondary level of structure in a protein is the regular folding of regions of the polypeptide chain. The two most common types of protein fold are the a-helix and the P-pleated sheet. In the rod-like a-helix, the amino acids arrange... 

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