Last modified 14th April '95 © Birkbeck College 1995

All-Beta Topologies

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 residues.

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 Chothia (1984).

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.

Beta barrels

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.

plastocyanin. Here is the crystal structure. Notice that this has a mixed sheet- there are two parallel pairs of strands.

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 (white) and the other of strands 6,5,8,3 (red) as indicated in the diagram. This can be seen by examining the crystal 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.

Beta Propellors

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, sialidase and Gal oxidase . Here is a picture of the latter.

Beta Trefoils

This fold has an approximately 3-fold axis of symmetry. Examine the crystal structure of erythrina trypsin inhibitor

Beta Helix

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 image. The structure is also available for interactive examination.

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J. Walshaw & Alan Mills