Beta Bulge Loop

Motif Glossary

Beta Bulge Loop


A motif of five or six consecutive residues and two H-bonds in which there is:

Show 3D view
Beta Bulge Loop-5   Beta Bulge Loop-6


These encompass the following conformations for each of 5-residue and 6-residue variants:

Alternative visualization

It is sometimes helpful to present the β-bulge loop in a way that relates to the β-bulge, rather than the β-turn. This involves replacing the numbering from 1-to-5 by X, –1-to-2, and repositioning X:


Milner-White, Biochim. Biophys. Acta 911:261–265 (1987)


1. The previous definition (now superseded) implicitly restricted the motif to the 2,3-αRαR (illustrated) and 2,3-αLαL enantiomers, which are the only ones found in the 6-residue form.

2. The 5- and 6-residue variants are also termed 'type 1' and 'type 2'.

3. In the vast majority of cases additional H-bonds are present.


Beta Bulge Loops occur at the loop ends of beta-hairpins.