Beta-hairpin families in globular proteins

Beta-hairpins, one of the simplest supersecondary structures, are widespread in globular proteins, and have often been suggested as possible sites for nucleation. Here we consider the conformation and sequences of the loop regions of beta-hairpins by analysing proteins of known structure. We find that the 'tight' beta-hairpins, classified by the length and conformations of their loop regions, form distinct families and that the loop regions of the family members have sequences which are characteristic of that family. The two-residue hairpin loops include almost entirely I' or II' beta-turns, in contrast to the general preference for type I and type II turns. These findings are being used to help define templates or consensus sequences to be incorporated into our existing supersecondary structure prediction algorithm. This information can also be used in model-building homologous proteins.