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.