Beta bulge
Encyclopedia
A beta bulge is a localized disruption of the regular hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

ing of a beta sheet
Beta sheet
The β sheet is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet...

, usually by inserting a residue with helical
Alpha helix
A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...

 dihedral angle
Dihedral angle
In geometry, a dihedral or torsion angle is the angle between two planes.The dihedral angle of two planes can be seen by looking at the planes "edge on", i.e., along their line of intersection...

s into one or both H-bonded
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

 β-strands.

Types

β-bulges can be grouped according to their length of the disruption, the number of residues inserted into each strand, whether the disrupted β-strands are parallel or antiparallel and by their dihedral angle
Dihedral angle
In geometry, a dihedral or torsion angle is the angle between two planes.The dihedral angle of two planes can be seen by looking at the planes "edge on", i.e., along their line of intersection...

s (which controls the placement of their side chains).

Effects on structure

At the level of the backbone structure, β-bulges can cause a simple aneurysm
Aneurysm
An aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...

 of the β-sheet, e.g., the bulge in the long β-hairpin of ribonuclease A
Ribonuclease A
Ribonuclease A is a pancreatic ribonuclease that cleaves single-stranded RNA. Bovine pancreatic RNase A is one of the classic model systems of protein science.-History:...

 (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 angle
Dihedral angle
In geometry, a dihedral or torsion angle is the angle between two planes.The dihedral angle of two planes can be seen by looking at the planes "edge on", i.e., along their line of intersection...

s are inserted opposite to each other in a β-hairpin, as occurs at Met9 and Asn16 in pseudoazurin (PDB accession code 1PAZ).

Effect on Functionality of Proteins

Bulges play an important role in affecting the functionality of proteins in which they are conserved. The most basic function of Bulges is to accommodate an extra residue, that has been added due to mutation etc., while still maintaining the bonding pattern and thus the overall architecture of the protein. In specific cases like the Immunoglobulin-family proteins, these Bulges are conserved and help in dimerization of the Ig domains. They have found to be of functional importance in the protein DHFR(Di-Hydro Folate Reductase) and SOD (Super Oxide Dismutase) where the loops, containing the bulges, help to enclose the active site.
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