Last modified 14th April '95 © Birkbeck College 1995
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Protein folds which consist of almost entirely beta sheets
exhibit a completely or mostly antiparallel arrangement. Many of these antiparallel domains
consist of two sheets packed against each other, with
hydrophobic side chains forming the interface. Bearing in mind that side chains
of a beta-strand point alternately to opposite sides of a sheet, this means
that such structures will tend to have a sequence of alternating hydrophobic and polar
Beta Sandwiches and Beta Barrels
The immunoglobulin fold has already been introduced, in the section on
mosaic proteins. In this fold,
the strands form two sheets packed against each other, forming a "beta
sandwich". Also look at the
fibronectin type 3 domain in the same section.
Aligned and Orthogonal Beta Sandwiches
In the immunoglobulin and fibronectin type-3 folds, the
two sheets are approximately aligned.
In fact the mean angle between the 2 sheets is approximately 30°
(designated -30° because the uppermost sheet is rotated
clockwise with respect to the lower). The two sheets are usually independent
in that the linking residues between them are not in beta sheet conformation.
The angle between the sheets is determined by their right-handed twist. The
observed angle varies between -20° and -50°; this is due to variation in
the twist. Also side-chains are not always ideally aligned at the interface. See
Orthogonal beta sheet packings consist of beta sheets folded on themselves; the two sheets
make an angle of -90°. The strands at one corner or 2 diagonally opposite corners go
uninterrupted from one layer to the other. Local coiling at the corner or a beta bulge facilitates
the right-angled bend. These bends are right-handed, due to permitted phi
and psi angles. The figure
below illustrates this model.
Only along one diagonal do the two sheets make contact. Large
side-chains in loops usually fill the spaces between the splayed corners.
Click here for a diagram
of this beta sheet arrangement in
the lipocalin intestinal
fatty acid-binding protein, is just one member of the
Lipocalin family, which bind small
molecules between the sheets of the sandwich.
Some antiparallel beta sheet domains are better described as beta
barrels rather than beta sandwiches,
for example streptavadin. See also
the diagrams of porin in the section on
membrane proteins in the previous chapter. Note that some structures are
intermediate between the extreme barrel and sandwich arrangements.
Up-and-down Antiparallel Beta Sheets
The simplest topology for an antiparallel beta sheet involves
loops connecting adjacent strands, as shown:
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. An example is given below.
Here is the crystal
structure. Notice that this has a mixed sheet- there are two parallel
pairs of strands.
Gamma-crystallin has two domains each of which is an eight-
stranded beta barrel-type structure composed of two Greek
In fact, the structure is more accurately described as consisting
of two beta sheets, one consisting of strands 2,1,4,7 (white) and the
other of strands 6,5,8,3 (red) as indicated in the diagram. This can
be seen by examining the
structure of gamma-crystallin. Sequence homology has been found between the
two Greek key motifs within each domain, and also between the two domains
themselves. The latter homology is higher than the former; this implies that the
structure evolved from a single Greek key fold by means of a gene duplication to
produce a domain of two Greek keys, followed by a second duplication resulting in
two similar domains. This is supported by the fact that in some crystallins each
Greek key motif is coded by a different exon, with introns between them.
The Jellyroll Topology
Richardson(1981) describes the jellyroll fold as being
formed by the addition of an extra "swirl" to a Greek key:
Click here for a diagram
illustrating this fold in
the coat protein of
satellite tobacco necrosis virus.
Click here to examine the crystal structure of
one subunit (about 470 residues) of neuraminidase. One molecule is
composed of four of these subunits. Each is a superbarrel of six
four-stranded antiparallel sheets. The whole structure has a basically
up-down topology as shown:
This fold is called a beta propellor; there are two other examples,
and Gal oxidase
. Here is a picture of the latter.
This fold has an approximately 3-fold axis of symmetry. Examine the crystal
erythrina trypsin inhibitor
This dramatically unusual fold was discovered quite recently. The beta strands
wind round the structure describing a helical topology, as can be seen in this
structure is also available for interactive examination.
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J. Walshaw & Alan Mills