Polypeptides sheet-like structures

The j8-pleated structure gives the protein a sheet-like structure rather than the rodlike structure of the a heUx. The polypeptide chain is almost fuUy extended, and each chain forms many intermolecular hydrogen bonds with adjacent chains. Figure 16.22 shows the two different types of iQ SuctEDSlledB332lt dll0 1 3 ra//e/. Silk molecules possess the antiparallel structure. Because its polypeptide chains are already in an extended form, silk lacks elasticity and extensibility, but it is quite strong due to the many intermolecular hydrogen bonds. 

In a P-pleated sheet, hydrogen bonds are formed between —NH and—CO groups in different polypeptide chains or different areas of the same polypeptide chain. Figure 28.9 shows the P-pleated sheet in the structural protein, silk. A fairly flat sheet-like structure is formed. 

Pauling and Corey also proposed a second ordered structure, the p-pleated sheet for polypeptide. This structure is a resiilt of intermolecular hydrogen bonding between the polypeptide chains to form a sheet like arrangement (Fig. 5.3). 

In real space, by critically examining the electron density map, we can also evaluate quality. The questions we ask should test the physical reality of the map. For example, are rotational symmetry axes free of density Are the boundaries of individual molecules clearly marked by low-density solvent regions Can we follow segments of the polypeptide chain Is the direction of chain evident from visible carbonyl oxygen positions Can we identify characteristic secondary structural features such as alpha helices and beta sheets like those in Figures 10.12 and 10.13 Can individual amino acid side chains like those in Figure 10.14 be recognized ... 

The p Sheet Another type of secondary structure, the p sheet, consists of laterally packed p strands. Each p strand is a short (5- to 8-residue), nearly fully extended polypeptide segment. Hydrogen bonding between backbone atoms in adjacent p strands, within either the same polypeptide chain or between different polypeptide chains, forms a p sheet (Figure 3-4a). The planarity of the peptide bond forces a p sheet to be pleated hence this structure is also called a 3 pleated sheet, or simply a pleated sheet. Like a helices, p strands have a directionality defined by the orientation of the peptide bond. Therefore, in a pleated sheet, adjacent p strands can be oriented in the same (parallel) or opposite (antiparallel) directions with respect to each other. In both arrangements, the side chains project from both faces of the sheet (Figure 3-4b). In some proteins, p sheets form the floor of a binding pocket the hydrophobic core of other proteins contains multiple P sheets. 

Not all proteins, however, form helical structures. If the substituent groups on the amino acids are small, as found in silk fibroin, then the polypeptide chains can line up side by side and form sheet-like arrangements. The chains tend to contract to acconunodate hydrogen bonding and form pleated sheets. This is called a -arrangement. Such an arrangement can be parallel and antiparallel. The identity period of the parallel one is 6.5 A and that of the antiparallel, 7.0 A. 

An essential difference however is that where the a-helix gives a long thin rod-like molecule, this extended structure is sheet-like and stabilized by hydrogen bonds perpendicular to the chains (Figure 4.12a). p-sheets may be parallel or antiparallel (as in fibroin) and are formed by appropriate folding of the polypeptide chain (Figure 4.12b). 

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