Q2(b). Secondary structure of proteins: α-helix and β-pleated sheet

Introduction. The secondary structure of a protein is the regular local conformation of its polypeptide backbone, held by hydrogen bonds between backbone C=O and N–H groups (not side chains). The peptide bond is planar (partial C=N character), so only rotation about the N–C_α (φ) and C_α–C (ψ) bonds is possible; the two regular, H-bond-saturated arrangements are the α-helix and the β-pleated sheet (Pauling and Corey).

Figure: Left: one chain coils; H-bonds (dashed) run along the axis. Right: extended strands side by side; H-bonds across.

α-Helix

1. One chain coils into a right-handed helix (L-amino acids).
2. 3.6 residues per turn; rise 1.5 Å per residue; pitch 5.4 Å per turn.
3. Every C=O of residue i is H-bonded to the N–H of residue i+4; the H-bonds are intrachain and nearly parallel to the helix axis.
4. Side chains point outwards from the helix. Proline (no N–H, rigid ring) breaks a helix; glycine (too flexible) and clusters of like charges or bulky β-branched residues destabilise it.
5. Examples: α-keratin of hair and wool (coiled coils); the helices of myoglobin and haemoglobin.

β-Pleated sheet

1. The chain is almost fully extended (about 3.5 Å per residue); two or more strands lie side by side.
2. H-bonds are between strands (interchain or between distant parts of one chain), roughly perpendicular to the strands.
3. Strands are antiparallel (opposite N→C directions, linear and stronger H-bonds) or parallel (same direction, bent H-bonds).
4. The tetrahedral C_α atoms give the sheet a zig-zag pleat; side chains alternate above and below the sheet. Small residues (Gly, Ala) allow close packing.
5. Example: silk fibroin (Gly–Ala–Gly–Ser repeats): strong but not stretchable.

Conclusion. Secondary structure is the H-bonded regular folding of the backbone. The α-helix is a right-handed coil of one chain (3.6 residues per turn, pitch 5.4 Å, C=O(i)…H–N(i+4)); the β-sheet is made of extended strands, parallel or antiparallel, H-bonded side by side, with side chains alternating above and below.

Application. Conversion of α-helix to β-sheet in prion proteins and amyloid-β produces the insoluble fibrils of mad-cow disease and Alzheimer's disease; perming hair rearranges the S–S links between keratin helices.