BP06 The a-helix and b-sheet

The two most important secondary structures had already been suggested in 1951 by Linus Pauling and Robert Carey: the -helix and the -sheet.

-helix (left) the polypeptide chain is rolled up like a spiral with 3.6 residues per turn. The helix is right-handed and is stabilised by intramolecular forces, i.e. hydrogen bonds (within the chain). These exist between the C=O group and N-H group that is found 4 residues further. The

The number of -helix domains that exist in the secondary structure is determined by the nature of the R-side-chains that point outwards. For example a proline residue cannot participate in an -helix structure, and the consecutive positioning of isoleucine residues (which are large R-side-groups) would give rise to sterical problems and destabilise the formation of an -helix.

In the -sheet (right) every residue is turned 180° with respect to the preceding one. A number of neighbouring chains (in the figure just two have been drawn) fold themselves in an accordion-like fashion, and intermolecular (between-the-chains) hydrogen bonds hold the whole thing together. Schematically the
-sheet is represented by an arrow. The direction of the arrow indicates the N è C direction of the polypeptide chain. If neighbouring chains run in the same direction then we speak of a parallel -sheet. If the neighbouring chains run in opposite directions (as in the illustration) then we talk of an anti-parallel -sheet.