Pleated sheets alignment

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. 

Ultrastructural patterns that arise when amino acids or small peptides interact with mineral surfaces have been studied in some detail99,100). A poly-L-alanine solution evaporated at 40 °C on a rhombohedral plane of R-quartz deposits the peptide principally in a-conformation. Chain-folded helices are aligned in the form of lamellae which exhibit a sharp phase boundary at the organic-mineral contact zone (Fig. 9). Frequently the lamellae are split along the direction of their fold axis ("zipper effect ). Insertion of 0-pleated sheets running perpendicular to the long axis of the lamellae act as dispersion forces and cause the formation of cross-0-... 

Also, increase in water temperature favors -conformation. Inasmuch as the conformation of CP probably determines the tertiary structure of MM, slight changes in CP conformation introduced by cellular or environmental effects may alter MM conformation. This in turn is reflected in the mineral form and structural pattern of the inorganic phases. Perhaps, nacreous layers in molluscs represent an almost ideal situation where MM and CP are aligned in a symmetrical way. In fish otoliths, the fibrous organic matrix is a mixture of helices and 0-pleated sheets. It is tentatively concluded that the morphology of shell structures is a macroscopic expression of the molecular interactions between MM and CP which are controlled in part by cellular activities and in part by the environment. 

The second common secondary structure in proteins resembles the pleated folds of drapery and is known as p-pleated sheet (Figure 19.8). All of the carbonyl oxygens and amide hydrogens in a p-pleated sheet are involved in hydrogen bonds, and the polypeptide chain is nearly completely extended. The polypeptide chains in a P-pleated sheet can have two orientations. If the N-termini are head to head, the structure is known as a parallel p-pleated sheet. And if the N-terminus of one chain is aligned with the C-terminus of a second chain (head to tail), the structure is known as an antiparallel p-pleated sheet. 

A parallel p-pleated sheet is one in which the hydrogen bonded peptide chains have their amino-termini aligned head-to-head. 

A structure such as the six-stranded coenzyme binding domain in the dehydrogenases would be disrupted by insertions or deletions of amino acids (see Fig. 7 for elaboration). Hence, sequence comparisons of parallel pleated sheet regions are particularly reliable. Structural methods of alignment of sheet areas have been discussed in Section II. The corresponding amino acid comparisons are made in Table IV. For tbe purpose of this chapter, the present LDH numbering scheme (4) will be used as the generalized reference system. 

The arrangement of atoms in the /3-pleated sheet conformation differs markedly from that in the a-helix. The peptide backbone in the j8-sheetis almost completely extended. Hydrogen bonds can be formed between different parts of a single chain that is doubled back on itself (intrachain bonds) or between different chains (interchain bonds). If the peptide chains run in the same direction (i.e., if they are aU aligned in terms of their N-terminal and G-terminal ends), a parallel... 

Polypeptide chains are almost completely extended in the jS-pleated-sheet conformation (Fig. 20-2). There is a side-by-side alignment of adjacent polypeptide chains. Hydrogen bonding occurs between adjacent polypeptide chains—more precisely, between the C=0 oxygen of one chain and the N—H hydrogen of an adjacent chain. Each polypeptide chain is hydrogen-bonded to adjacent chains on each side of it, except for the chains at the two ends of the sheet. The sheets are pleated... 

If the only structural characteristics of proteins were their amino acid sequences (primary structures), all protein molecules would consist of long chains arranged in random fashion. However, protein chains fold and become aligned in such a way that certain orderly patterns result. These orderly patterns, referred to as secondary structures, result from hydrogen bonding and include the a-helix (alpha-helix) and the P-pleated (beta-pleated) sheet. 

Another important secondary strucmre is the -pleated sheet, which is again stabilized by hydrogen bonds between peptide linkages. A parallel /1-pleated sheet forms when the chains are aligned in the same direction when the chains alternate in direction, an antiparaUel pleated sheet is formed (Fig. 2). 

The pleated sheet can also be observed with optical polarization microscopy [119, 133]. Fig. 15 demonstrates the origin of the various banded patterns in the polarizing microscope, which are seen on a longitudinal section of a fiber. In the dark areas the chains are aligned parallel to the analyzer or polarizer direction. Such a banded texture is often observed in films and fibers of lyotropic and thermotropic polymers. 

Pleated sheet A type of secondary structure in which two sections of polypeptide chain are aligned parallel or antiparallel to one another. 

Pleated sheet (Section 27.6B) A type of polypeptide secondary structure in which sections of polypeptide chains are aligned parallel or antiparaUel to one another. 

The complex morphology of hair essentially consists of four components of different functionality (i) The cortex that gives the hair its mechanical properties consists of elongated, spin-shaped cells aligned in the direction of the fiber axis. The keratinized protein in the form of microfibrils resides in these cells, (ii) The medulla is located in the center of some thicker fibers and it consists of a loosely packed porous cellular structure (it does not contribute to the mechanical properties of the hair), (iii) Cell membrane complex which cements the various cells of the cuticula and the cortex and it consists of several layers, (iv) Cuticle, a multilayered structure which consists of flat cuticle cells and the most outer layer, i.e. the epicuticle (which is about 2.5 nm thick) is the most important part for deposition of surfactants and polymers in the shampoo formulation. This consists of 25 % lipids and 75 % protein, the latter having an ordered possibly p-pleated sheet structure with 12% cystine. The cystine groups are acylated by fatty acids which form the hydrophobic surface region. A schematic representation of the epicuticle is shown in Fig. 1.46. 

Another popular crystalline structure for protein is the structure formed by extended protein molecules. A pleated sheet structure is formed by hydrogen bonds between adjacent extended molecules aligned in parallel. Piezoelectricily in B-form protein was first observed by Fukada in 1956 for silk fibroin [3]. Since there is no intrinsic pt -zation in the B structure, pyroelectricity and ten piezoelectricity are not observed. However, the piezoelectricity due to shear is observed. If the shear is applied such as to cause slip between oriented molecules, polarizatioo is induced in the direction perpendicular to the plane of the shear. Shear piezoelectricity is observed for most natural biopolymers, not only keratin but abo trumy other proteins. 

Antiparallel P-pleated sheets are more stable then parallel P-pleated sheets because of geometry. The N-H and C=0 of one chain are directly aligned with the N-H and C=0 of an adjacent chain in the antiparallel P-pleated sheet, whereas they are not in the parallel P-pleated sheet. This makes the latter set of hydrogen bonds weaker. 

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