Editing Beta sheet (section)

==Structural topology==
The '''topology''' of a β-sheet describes the order of [[hydrogen bond|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 bond]]s 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-barrel|β-barrel]]s'' (such as the [[TIM barrel]]). [[beta-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|β-propeller domain]] or [[immunoglobulin fold]]) or edge-to-edge, forming one big β-sheet.