The β-α-β motif. (a) The β-α-β motif contains two parallel β strands connected by an α helix. (b) Four β-α-β motifs, interconnected by α helices, underlie the α/β barrel structure. This α/β barrel is from triosephosphate isomerase of Trypanosoma brucei. The topology diagram in (b) shows the general arrangement of β strands and α helices; the actual structure of this α/β barrel domain contains eight β strands connected by α helices.