Beta-bulges within loops as recurring features of protein structure.

Beta-bulges have been described as common irregularities in antiparallel beta-sheets. The two commonest kinds are described as 'classic' and 'G1'. The G1-type is often observed to occur at the loop end of beta-hairpins. I confirm this but also find that the characteristic G1 beta-bulge features (hydrogen bonds and dihedral angles) are found, in some loops, in the absence of any adjacent antiparallel beta-hairpin, so that the beta-bulge is effectively acting as a new sort of turn. Also I show that G1 beta-bulge loops occur in a number of different structural forms, some of which have not been described in detail before. G1 beta-bulge loops are common features; on average, a half of all proteins have one.