Beta Bulge Loop
Definition
A motif of five or six consecutive residues and two H-bonds in which there is:
- H-bond between main-chain NH of residue-1 and main-chain CO of the ultimate residue (i.e. -5 or -6)
- H-bond between main-chain CO of residue-1 and main-chain NH of the penultimate residue (i.e. -4 or -5)
Sub-categories
These encompass the following conformations for each of 5-residue and 6-residue variants:- 2,3-αRαR (aka: type I or R-handed)
- 2,3-βRαL (aka: type II or R-handed flipped)
- 2,3-αLαL (aka: type I' or L-handed)
- 2,3-βLαR (aka: type II' or L-handed flipped)
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:
Reference
Milner-White, Biochim. Biophys. Acta 911:261–265 (1987)
Comments
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 The previous definition (now superseded) implicitly restricted the motif to the 2,3-αRαR (illustrated) and 2,3-αLαL enantiomers, which are the only ones found in the 6-residue form.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.
Occurrence
Beta Bulge Loops occur at the loop ends of beta-hairpins.