Question

What forces stabilise this structure?

• Describe the structure of the antiparallel and parallel beta pleated sheet.

• In relation to the backbone where are the side chains of the amino acids?

Answer 1

The beta pleated sheet is the more extended secondary conformation of polypeptide chains where the distance between adjacent amino acids is 3.5 Angstrom. In the beta conformation, the backbone of the polypeptide chain is extended into a zigzag manner. The arrangements of several segments connected laterally by at least two or three backbone hydrogen bonds are called beta sheet. The adjacent strands in a sheet can run in the same direction with regard to amino and carboxyl orientations or in opposite direction, and thus can be parallel and anti-parallel beta sheet respectively. The beta pleated sheet is stabilized by hydrogen bonds between NH and CO groups of neighboring polypeptide segments.

In anti-parallel arrangement, the C-terminus end of one segment is on the same side as the N-terminus end of the other segment. This arrangement produces the strongest interstrand stability because the interstrand hydrogen bonds are essentially in line in the anti-parallel beta sheet. The peptide backbone dihedral angles are about -140o and 135o in anti-parallel sheets. In this case, the two adjacent carbon atoms of two hydrogen bonded beta strands may form two mutual backbone hydrogen bonds to their respective peptide groups flanked to each other, forming a close pair of hydrogen bonds. The anti-parallel beta pleated sheet forms by the reversal of the polypeptide chain which can occur in the presence of two consecutive proline residues.

In parallel arrangement, the C-terminus and the N-terminus end are on the same sides for both segments. This arrangement is less stable as compared to the anti-parallel, beacuse the interstrand H-bonding is distorted or nonplanar. The dihedral bond angles are about -120o and 115o in parallel sheets. This particular arrangement of parallel sheet is fundamentally difficult to form as the C and N terminus are distant apart in sequence, so a smaller number of strands may produce unstability. In parallel beta pleated sheet, if two carbon atoms are adjacent in two hydrogen bonded beta strands, then they do not form H-bond with each other, instead one residue forms hydrogen bonds to the residues that flanked the other, forming a wide pair of H-bonds.

The individual segments that form a beta sheet are usually nearby on the polypeptide chain but can also be at a fair distant from each other. The side chains or the R groups of adjacent amino acids protrude from the zigzag structure in opposite directions, creating an alternating pattern. The side chain is connected to the alpha carbon atom of the amide backbone. Larger aromatic residues and beta branched amino acids are found in the middle of beta sheets of beta strands. Different types of residues like proline may be found at the edge strands of beta sheets.