Fi-pleated sheets

Fig. 3. Ramachandran plot showing the allowed angles for poly-L-alanine (grey regions), a, (p-y/values that produce the right-handed a-helix ji, the antiparallel fi-pleated sheet /3, the parallel (5-pleated sheet C, the collagen helix.

Both theory (CASP6 blind prediction, Fig. 5.3a) and experiment (carried out within CASP6 as well, Fig. 5.3b) give the target molecule containing five a-helices and two fi-pleated sheets (wide arrows). These secondary structure elements interact and form the unique (native) tertiary structure, which is able to perform its biological function. Both structures in atomic resolution differ by the r.m.s. equal to 2.9 A, which is a rather small deviation. 

In contrast, the j8-strand is a much more extended structure. Rather than hydrogen bonds forming within the secondary structural luiit itself, stabilization occms through bonding with one or more adjacent )8-strands. The overall structure formed through the interaction of these individual )8-strands is known as a fi-pleated sheet. These sheets can be parallel or antiparallel, depending on the orientation of the N-and C-terminal ends of each component )8-strand. A variant of the )8-sheet is the )8-turn in this structure the polypeptide chain makes a sharp, hairpin bend, producing an antiparallel j8-sheet in the process. 

X, conformation of the a-helix fi, pleated sheet structure letters in brackets indicate the conformation on drawing IO3 or 3, other helices h, helix-forming r, helix-breaking 0, indifferent... 

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. 

A second form of keratin, known as fi-keratin, is produced by stretching the a-keratin in hair to about twice its original length. The X-ray diffraction pattern of -keratin is similar to that of silk fibroin (page 56) and its structure, like that of fibroin, is based on the /5-pleated sheet. There is a difference, however, in that polypeptide chains are in a parallel (Figure 5.2a) and not an antiparallel arrangement. 

Figure 1.8 Parallel (top) and antiparallel (bottom) fi sheet viewed from an angle to illustrate how the geometry at the alpha carbon causes the sheet to be pleated. The amide H-atoms have been omitted for clarity.

Peptide chains (or two parts of the same chain) connected with each other through multiple hydrogen bonds form fi-sheets. The participating chains can be parallel or antiparallel to each other. An antiparallel sheet is shown here including a view along the general plane of the sheet to indicate its pleated character ... 

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