Index to Course Material
Index to Section 9
Index to Super Secondary Structure
Beta-corners are observed to have a right-handed twist when viewed from the concave side.
A helix hairpin or alphaalpha-hairpin refers to the loop connecting two antiparallel alpha-helical segments. Clearly, the longer the length of the loop the greater the number of possible conformations. However, for short connections there are a limited number of conformations and for the shortest loops of two or three residues, there is only one allowed conformation. Antiparallel alpha-helices will interact generally by hydrophobic interactions between side chains at the interface. Therefore, hydrophobic amino acids have to be appropriately positioned in the amino acid sequence (one per turn of each helix) to generate a hydrophobic core. Efimov has analysed the conformations of alpha-alpha-hairpins and some of his results are summarised below.
The shortest alpha-helical connections involve two residues which are oriented approximately perpendicular to the axes of the helices. Analysis of known structures reveals that the first of these two residues adopts phi and psi angles in the bridging or alpha-helical regions of the Ramachandran plot. The second residue is always glycine and is in a region of the Ramachandran plot with positive phi which is not available to other amino acids. Here is an example structure file to look at with RasMol.
Three residue loops are also observed to have conformational preferences. The first residue occupies the bridging region of the Ramachandran plot, the second adopts the left-handed helical conformation and the last residue is in a beta-strand conformation.
Four-residue loops adopt one of two possible conformations. One is
similar to the three residue loop conformation described above except
that there is an additional residue in the beta-strand conformation at
the fourth position. The other conformation involves the four residues
adopting bridging, beta, bridging, and beta conformations,
Last updated 31st May '96