The other type is the G1 beta bulge, of which there are two common sorts, both mainly occurring in association with antiparallel sheet; one residue has the αL conformation and is usually a glycine.
At the level of the backbone structure, classic β-bulges can cause a simple aneurysm of the β-sheet, e.g., the bulge in the long β-hairpin of ribonuclease A (residues 88–91).
A β-bulge can also cause a β-sheet to fold over and cross itself, e.g., when two residues with left-handed and right-handed α-helical dihedral angles are inserted opposite to each other in a β-hairpin, as occurs at Met9 and Asn16 in pseudoazurin (PDB accession code 1PAZ).
The most basic function of bulges is to accommodate an extra residue added due to mutation etc., while maintaining the bonding pattern and thus the overall protein architecture.
They are also of functional importance in the proteins DHFR (Dihydrofolate Reductase) and SOD (Superoxide Dismutase), where loops containing bulges surround the active site.