Proteins
Proteins

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Proteins

1.3.2 β pleated sheets

Another common secondary structure is the β pleated sheet, which contains extended stretches of polypeptide chain with hydrogen bonds between neighbouring strands. In parallel β pleated sheet, polypeptide strands run in the same direction (i.e. from N- to C-terminus) whereas in antiparallel β pleated sheet, neighbouring strands extend in opposite directions (Figure 10).

Figure 10 Hydrogen bonding between neighbouring stretches of polypeptide in (a) antiparallel and (b) parallel β pleated sheet structures. Note a difference in the hydrogen bonds, which are offset in the parallel sheet. The side-chains have been omitted for clarity.

Strands are not fully extended but have a zig-zag shape, which gives the sheet formation, in both parallel and antiparallel structures, a pleated appearance when viewed edge-on (Figure 11). The Cα atoms of successive residues are at, alternately, the top and bottom of each pleat, with the side-chains pointing away from the sheet. Thus there is a two-residue repeating unit, as indicated in Figure 11a, which spans 7 Å. For simplicity, β pleated sheets are often represented as ribbons with arrowheads pointing in the direction of the C-terminus (Figure 11b).

Figure 11 (a) Side-chains of residues in parallel or antiparallel β pleated sheet point away from the sheet, with successive side-chains alternately above and below. The two-residue repeating unit indicated spans 7 Å. (b) A three-stranded antiparallel β pleated sheet, showing the polypeptide backbone only, drawn to emphasise its pleated appearance.

In globular proteins, antiparallel pleated sheets can contain from two to 15 polypeptide strands, with the average being six strands. A sheet containing six strands is approximately 25 Å wide. Each individual strand can contain up to 15 amino acids, with the average being six.

  • What would be the length of a β pleated sheet in which the strands contained six residues?

  • The length of the sheet would be 21 Å (3 × 7 Å).

Parallel β sheets appear to be less stable that antiparallel sheets and rarely contain fewer than five chains. The relative instability of parallel β sheets may be due to the offset in hydrogen-bonding groups between neighbouring strands (Figure 10b). This offset causes some distortion, and hence weakening, in the hydrogen bonds compared to those between antiparallel strands. Mixed parallel and antiparallel β sheets also occur.

Within the context of the entire peptide chain, regions of β sheet are connected by linking peptide. Figure 12 illustrates the different kinds of connection that can occur between adjacent strands in pleated sheets. In antiparallel β pleated sheet, a simple hairpin turn links successive strands (Figure 12a). There are two options for linking neighbouring parallel strands: the connection can be either a right-handed crossover or a left-handed crossover (Figure 12b and c respectively), but the latter rarely occurs. The connections between strands in β pleated sheet can be very long and can themselves contain elements of secondary structure, such as helices.

Figure 12 Types of linkage between adjacent β sheet strands. (a) Simple hairpins in antiparallel structures. (b) and (c) Crossover connections in parallel β pleated sheets.

When viewed along its length, a polypeptide strand in a pleated sheet can be seen to also have a slight helical twist to the right. This twist arises from the conflict between conformational stability within chains and that derived from hydrogen bonds between chains. As a consequence, the sheet as a whole is seen to have a right-handed twist. These twisted sheet structures often form the core of globular proteins (Figure 13).

Figure 13 The polypeptide backbones of (a) bovine carboxypeptidase A (pdb file 2ctb) and (b) chicken triose phosphate isomerase (pdb file 1tim). Strands of the β sheets are represented as ribbon arrows, pointing in the direction of the C-terminus, and α helices are shown as cylinders. Note the pronounced right-handed twist of the β sheets in these two enzyme structures. Note also the location of the β sheet structures at the core of the proteins.
  • Looking at the structures in Figure 13, are the β pleated sheets parallel, antiparallel or mixed?

  • Carboxypeptidase A contains a mixed parallel and antiparallel β pleated sheet structure and triose phosphate isomerase contains a parallel β pleated sheet structure.

S377_2

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