As a β-sheet (engl. Β-pleated sheet or β-sheet ) is in biochemistry a common secondary structure element of a protein designated. The secondary structure of a protein is understood to mean the spatial structure of the amino acid chain without taking the side groups into account. The secondary structure of a protein is derived from its primary structure ( amino acid sequence ). Superordinate structural levels are the tertiary structure and the quaternary structure . The three-dimensional structure of a protein is crucial for its selective function (see protein structure ).
In the late 1930s, William Astbury began to perform crystal structure analyzes on crystalline peptides . It was found that certain spatial features repeat regularly, where hydrogen bonds within the molecule were suspected. However, he was not yet aware of the planarity of the peptide bond . The most common spatial structures were later called α-helix and β-sheet. Linus Pauling , Robert Brainard Corey and Herman Branson proposed a model of the β-sheet in 1951. The β in "β-sheet" does not contain any scientific statement, but only expresses that the β-sheet was found after the α-helix. The name Faltblatt results from the three-dimensional structure of the β-pleated sheet, whose “zigzag shape” resembles a regularly folded sheet of paper.
Because of the side chains, β- pleated sheets are not flat, but fluted like an accordion ( pleated ). The polypeptide chains of a sheet are called β-strands. These strands can be linked in different ways:
- Antiparallel linkage (in opposite directions): The carbonyl and amino groups of one strand are each connected to the amino and carbonyl group of the other strand via hydrogen bonds
- Parallel connection (in the same direction): The NH group of one strand is linked to the CO group of the other strand via a hydrogen bridge. The carbonyl group, on the other hand, forms a hydrogen bond with the amino group, which is two residues away from the respective partner on the other strand. Through this connection, each amino acid on one strand - with the exception of the amino acids at the ends - is linked to two amino acids on the other strand. The strands run in the same direction, so that the individual atoms are arranged parallel to one another.
- Mixed link: In the mixed link, there are both parallel and anti-parallel links.
The three-dimensional representation is reminiscent of a folded sheet, with the peptide groups in the surfaces and the carbon atoms in between lie on the edges of a sheet that has been folded several times. The peptide bonds of several chains interact with each other and form hydrogen bonds along the polypeptide backbone. These occur in pairs of two at a distance of 7.0 Å and ensure the stability of the structure. The distance between vicinal amino acids is significantly larger in the folded sheet with 0.35 nm than with the α-helix with 0.15 nm.
However, the side groups in the β-sheet are very close together, so that bulky or similarly charged residues disturb the arrangement. Larger structural areas of the leaflet only come about if the remains are relatively small. The protein chains of natural silk , which are arranged exclusively in the sheet structure, consist of 86 percent glycine , alanine and serine , i.e. amino acids with small residues. Another example of proteins with a predominantly sheet structure are immunoglobulins . If large amino acid residues are predominantly present, the polypeptide chain is preferably arranged as an α-helix.
- Ribbon diagram: β-sheets are often represented as arrows in protein structures . One arrow corresponds to one strand of the leaflet. Several arrows linked by turns or loops form the structure of the leaflet. The arrows point in the direction of the carboxyl terminus in order to reproduce the structure of the folded sheet.
- Space-filling representation and ball-and-stick model: The hydrogen atoms are often left out of the representation, as the positions of these atoms are difficult to determine. In addition, the representation of the hydrogen atoms would impair the clarity.
- Backbone model: Usually only the α-C atoms are shown, which means that the course of the polypeptide chain can be seen better than in the spherical or spherical rod model. However, elements of the secondary structure are not very clearly recognizable.
Other important structural elements
In addition to the α-helix and the β-sheet, there are other types of secondary structure motifs. The parts of the primary structure of a protein that do not belong to a motif are called random loops ( random coil structures ). These structures play a key role in the formation of the entire protein structure .
Other common motifs are:
- Jeremy M. Berg: Stryer Biochemistry. Spektrum Akademie Verlag, Munich 2007, ISBN 978-3-8274-1800-5
- William T. Astbury, S. Dickinson, K. Bailey: The X-ray interpretation of denaturation and the structure of the seed globulins. In: The Biochemical journal. Volume 29, Number 10, October 1935, pp. 2351-2360.1, PMID 16745914 . PMC 1266766 (free full text).
- William T. Astbury: The structural proteins of the cell. In: The Biochemical journal. Volume 39, Number 5, 1945, p. Lvi, PMID 21020817 .
- WT Astbury, R. Reed, LC Spark: An X-ray and electron microscope study of tropomyosin. In: The Biochemical journal. Volume 43, Number 2, 1948, pp. 282-287, PMID 16748402 . PMC 1274681 (free full text).
- Linus Pauling, Robert Brainard Corey, Herman R. Branson: The structure of proteins; two hydrogen-bonded helical configurations of the polypeptide chain. In: Proceedings of the National Academy of Sciences . Volume 37, Number 4, April 1951, pp. 205-211, PMID 14816373 . PMC 1063337 (free full text).
- D. Eisenberg: The discovery of the alpha-helix and beta-sheet, the principal structural features of proteins. In: Proceedings of the National Academy of Sciences . Volume 100, Number 20, September 2003, pp. 11207-11210, doi : 10.1073 / pnas.2034522100 . PMID 12966187 . PMC 208735 (free full text).
- JM Scholtz, RL Baldwin: The mechanism of alpha-helix formation by peptides. In: Annual review of biophysics and biomolecular structure. Volume 21, 1992, pp. 95-118, doi : 10.1146 / annurev.bb.21.060192.000523 . PMID 1525475 . PDF .