Beta barrel

A beta barrel is a large beta-sheet that twists and coils to form a closed structure in which the first strand is hydrogen bonded to the last. Beta-strands in beta-barrels are typically arranged in an antiparallel fashion. Barrel structures are commonly found in porins and other proteins that span cell membranes and in proteins that bind hydrophobic ligands in the barrel center, as in lipocalins. Porin-like barrel structures are encoded by as many as 2–3% of the genes in Gram-negative bacteria.

In many cases the strands contain alternating polar and hydrophobic amino acids, so that the hydrophobic residues are oriented into the interior of the barrel to form a hydrophobic core and the polar residues are oriented toward the outside of the barrel on the solvent-exposed surface. Porins and other membrane proteins containing beta barrels reverse this pattern, with hydrophobic residues oriented toward the exterior where they contact the surrounding lipids, and hydrophilic residues oriented toward the interior pore.