Beta barrel

In protein structures, a beta barrel (β barrel) is a beta sheet (β sheet) composed of tandem repeats that twists and coils to form a closed toroidal structure in which the first strand is bonded to the last strand (hydrogen bond).

Most of them are water-soluble outer membrane proteins and frequently bind hydrophobic ligands in the barrel center, as in lipocalins.

Adjacent strands along the sequence alternate between the two sheets, such that they are "wrapped" in three dimensions to form a barrel shape.

Sixteen- or eighteen-stranded up-and-down beta barrel structures occur in porins, which function as transporters for ions and small molecules that cannot diffuse across a cellular membrane.

Beta barrels also function within endosymbiont derived organelles such as mitochondria and chloroplasts to transport proteins.

[8] Within the mitochondrion two complexes exist with beta barrels serving as the pore forming subunit, Tom40 of the Translocase of the outer membrane, and Sam50 of the Sorting and assembly machinery.

Lipocalins are typically eight-stranded up-and-down beta barrel proteins that are secreted into the extracellular environment.

A distinctive feature is their ability to bind and transport small hydrophobic molecules in the barrel calyx.

The determination of shear number requires the assumption that each amino acid in one strand of a beta sheet is adjacent to just one amino acid in the neighboring strand (this assumption may not hold if, for example, a beta bulge is present).