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).
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).
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.
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).
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.