Beta Sheet - Structural Topology

Structural Topology

The topology of a β sheet describes the order of hydrogen-bonded β strands along the backbone. For example, the flavodoxin fold has a five-stranded, parallel β sheet with topology 21345; thus, the edge strands are β strand 2 and β strand 5 along the backbone. Spelled out explicitly, β strand 2 is H-bonded to β strand 1, which is H-bonded to β strand 3, which is H-bonded to β strand 4, which is H-bonded to β strand 5, the other edge strand. In the same system, the Greek key motif described above has a 4123 topology. The secondary structure of a β sheet can be described roughly by giving the number of strands, their topology, and whether their hydrogen bonds are parallel or antiparallel.

β sheets can be open, meaning that they have two edge strands (as in the flavodoxin fold or the immunoglobulin fold) or they can be closed beta barrels (such as the TIM barrel). β-Barrels are often described by their stagger or shear. Some open β sheets are very curved and fold over on themselves (as in the SH3 domain) or form horseshoe shapes (as in the ribonuclease inhibitor). Open β sheets can assemble face-to-face (such as the beta-propeller domain or immunoglobulin fold) or edge-to-edge, forming one big β sheet.