Members of this family adopt a structure consisting of two layers of parallel alpha-helices, five in the inner layer and four in the outer, arranged in an antiparallel manner, with perpendicular loops containing short helical segments on top. They are required for the formation of a deep cleft harbouring the catalytic Zn2+ site in Leukotriene A4 hydrolase [1].

1: Crystal structure of human leukotriene A(4) hydrolase, a bifunctional enzyme in inflammation. Thunnissen MM, Nordlund P, Haeggstrom JZ; Nat Struct Biol. 2001;8:131-135. PMID:11175901