Supersecondary structures of antiparallel β sheets. (a) The β hairpin motif is an antiparallel β sheet composed of two β strands adjacent in the primary structure; the strands are often connected by β or γ turns. (b) The β strands of antiparallel (shown here) and parallel β sheets tend to have a right-handed twist. (c) The β barrel motif is formed by the twist in a β sheet with a connection between the first and last strands. This example is a single domain of hemolysin (a pore-forming toxin that kills a cell by creating a hole in its membrane) from the bacterium Staphylococcus aureus. The diagram below each structure shows the folding topology of the polypeptide chain. The β strands in a β barrel can have several different chain topologies; shown in (c) is an “up-and-down barrel,” reflecting the chain topology. In a simple β barrel, the strands would be connected as shown in the topology diagram (bottom), but the bottom strands of the particular β barrel shown here (top) are not connected, because the barrel is associated with additional domains (not shown).