Beta trefoil fold

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Beta trefoil fold is a protein shape built from six beta hairpins (each hairpin has two beta strands). These form a barrel with a triangular cap, giving roughly three-fold symmetry. The hairpins arrange in three repeating β-β-β-loop-β segments, each creating a Y-shaped trefoil. The first and fourth strands make one hairpin; the second and third make the other. Each hairpin is one arm of the Y, with a long loop acting as the trunk. The barrel is about 16 angstroms across and packed with amino acid side chains. This fold is found in several proteins, including plant Kunitz inhibitors from soybean, Erythrina afra, and wheat; some interleukin-1 family proteins (IL-1α, IL-1β, and IL-1 receptor antagonist); and fibroblast growth factors 1 and 2.