These domains are in some instances better described as beta
barrels rather than beta sandwiches,
for example 1STP; there
have a hydrophobic core in the centre of the barrel.
Up-and-down Antiparallel Beta Sheets
The simplest topology for an antiparallel beta sheet involves
loops connecting adjacent strands, as shown:
Barrel structures composed entirely of up-and-down sheet are
relatively uncommon
e.g. soybean trypsin inhibitor, the N-terminal domain of papain,
rubredoxin. azurins? plastocyanins?
The Greek Key Topology
The Greek Key topology, named after a pattern that was
common on Greek pottery, is shown below. Three up-and-down beta
strands connected by hairpins are followed by a longer connection
to the fourth strand, which lies adjacent to the first.
Folds including the Greek key topology have been found to have
5-13 strands. Examples are given below.
1STP
staphylococcal nuclease
Gamma-crystallin
Gamma-crystallin has two domains each of which is an eight-
stranded beta barrel-type structure composed of two Greek
keys:
In fact, the structure is more accurately described as consisting
of two beta sheets, one consisting of strands 2,1,4,7 and the
other of strands 6,5,8,3, as indicated in the diagram. This can
be seen by examining the
crystal
structure of gamma-crystallin.
The Jellyroll Topology
Richardson(1979) describes the jellyroll fold as being
formed by the addition of an extra "swirl" to a Greek key: