Parallel /3-pleated sheet, structure

Figure 4.11 Ramachandran plot showing permissible conformational regions. aR and aL indicate positions of the right- and left-handed a helices /3A and /3P indicate the positions of the anti-parallel and parallel pleated sheet structures, respectively. C indicates collagen.

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. 

A classic parallel pleated sheet structure is exhibited in the crystal by Gly-LPhe-Gly, as shown in Fig. 26 (Marsh and Glusker, 1961), where NH 0=C hydrogen bonds are formed between the adjacent molecules. The phenylalanine side-chain group is extended away from the polar moieties of the peptide chain. An example of an antiparallel pleated sheet is shown by the LAla-LAla-LAla molecules (Fig. 27) (Fawcett et al, 1975). 

Hydrogen bonds (a) in a parallel -pleated sheet structure, in which all the polypeptide chains are oriented in the same direction, and (b) in an antiparallel fi-pleated sheet, in which adjacent polypeptide chains run in opposite directions. For color key, see Fig. 22.7. 

Fig. 2.28 X-ray crystal structures of parallel sheet-forming and all-un//fce-/F -peptides 116 and 117 [10, 191]. Views along the parallel amide planes and crystal packing diagram show the parallel pleated sheet arrangement (view perpendicular to the amide planes).

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... 

Arnott, S., Dover, S. D., and Elliott, A. (1967). Structure of / -poly-L-alanine Refined atomic co-ordinates for an anti-parallel /(-pleated sheet./. Mol. Biol. 30, 201-208. 

In a further exploration of the relationship between dye structure and wet fastness on silk, four novel monoazo J acid derivatives (3.169 X = Xx to X4), including 3.168 (X = X2) made from 2-aminobenzophenone, were synthesised. Silk was dyed at pH 4 and 85 °C and the dyeings tested for fastness to washing, perspiration and dry cleaning. The highest allround fastness was shown by the 4 aminobenzophenone derivative (X = X4), a structure that resembles the anti-parallel pleated sheet arrangement of polypeptide chains in silk [183]. 

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)... 

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.). 

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.)...

Hair keratin has historically been associated with the a-helical structure. For that reason, it is termed a-keratin. And indeed the basic keratin polypeptides are a-helical except for their N- and C-terminal domains. These are believed to be involved in head-to-tail condensation to form keratin polymers. When hair keratin is stretched, the resulting secondary structure is the parallel pleated sheet (see Chapter 4). Stretched keratin is referred to as /3-keratin to emphasize its secondary structure. 

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]. 

P structure is the structure where the polypeptide chain is elongated. The structure can be the type where all the molecular chain run in the same direction and form a parallel pleated sheet or the type where the molecular chain run in the alternate direction and form anti parallel chain pleated sheet. In the case of P-structure, there are three important features, namely the period that is repeating period of the polypeptide chain, the spacing of the molecular chain in the sheet and the distance between the sheets [76]. 

FIGURE 10-7 Pleated sheet structures proposed for /8-keratin, (a) The parallel-chain pleated sheet, (b) The anti parallel-chain pleated sheet. [From Pauling and Corey, Proc. NatL Acad. Sci. U.S. 37, 729-40 (1951).]... 

Repeating sequences of amino acids with small, compact R-groups (e.g., glycine, alanine) tend to form the (3, or pleated sheet, structure, which consists of parallel (Fig. 2-3a) or antiparallel (Fig. 2-36) polypeptide chains linked by interchain hydrogen bonds. Silk is an example of the antiparallel sheet. 

Hydrogen-bonding pattern of parallel (a) and antiparallel (b) /i-pleated sheet structures. 

P-Pleated sheets form when two or more polypeptide chain segments line up side by side (Figure 5.19). Each individual segment is referred to as a ji-strand. Rather than being coiled, each /1-strand is fully extended. /J-Pleated sheets are stabilized by hydrogen bonds that form between the polypeptide backbone N—H and carbonyl groups of adjacent chains. There are two /Fplcatcd sheets parallel and antiparallel. In parallel /kpleated sheet structures, the polypeptide chains... 

Members of tbe amyloid family bave no sequence homology with each other, but fibers formed from proteins in tiiis group share a similar secondary structure. Two characteristic reflections (at 10 and 4.8 A) are produced when fibers formed from any of the amyloid proteins are analyzed by x-ray diffraction (Sunde et al, 1997). This pattern defines the cross P-pleated sheet structure in which P sheets lie parallel to the long axis of the fiber, with tbe strands of each sheet oriented perpendicular to the fiber axis (Sunde et al, 1997). Fibers formed from tbe NM region of Sup35 share tbis structure (Serio etal, 2000) thus, Sup35 can be defined as an amyloidogenic protein. 

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. 

There are three hydrophobic regions in this domain. One is involved in the subunit interaction and is discussed in Section II,C,3,c. The other two are important for the folding of this domain since they form hydro-phobic cores between the helices and the parallel pleated sheet (116). Table V lists the residues involved. The importance of these hydrophobic cores for the proper folding is realized from the fact that almost all residues in LDH and GAPDH which are structurally equivalent to those of LADH listed in Table V are also hydrophobic. 

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