Beta sheet

As β -sheet (English β - pleated sheet or β -sheet ) is referred to in biochemistry a common secondary structure element of a protein. Of the secondary structure of a protein is understood without regard to the side groups of the three-dimensional structure of the amino acid chain. The secondary structure of a protein is shown ( amino acid sequence) in the primary structure. Parent structure 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 ).

History

In the late 1930s began William Astbury crystal structure analyzes carried out on crystalline peptides. It was found that repeat certain spatial features on a regular basis, in which hydrogen bonds were assumed within the molecule. However, he was not yet known, the planarity of the peptide bond. The most common spatial structures were later called α -helix and β -sheet. Linus Pauling, Robert Corey and Herman Branson Brainard proposed in 1951 a model of the β -sheet. The β in " β -sheet " contains no scientific statement, but brings only expressed that the β -sheet after the α -helix was found. The name leaflet results from the three-dimensional structure of the β -sheet, the " zigzag shape " a regularly folded sheet of paper is similar.

Structure

Because of the side-chain β -pleated sheets are not flat, but concertina similar fluted (german pleated ). The polypeptide chains of a leaflet are referred to as β - strands. These strands can be linked together in different ways:

  • Anti-parallel link ( in opposite directions ): The carbonyl and amino groups of one strand are connected via hydrogen bonds with the amino and carbonyl group of the other strand
  • Parallel linkage ( same direction ): The NH group of one strand is linked to the CO group of the other strand via a hydrogen bond. The carbonyl group, however, forms a hydrogen bond with the amino group is removed from the partner on the other strand, two residues. Through this connection, each amino acid of said one strand is - connected to two amino acids of the other strand - with the exception of the amino acids at the ends. The strands extend in the same direction, so that the individual atoms are arranged in parallel.
  • Mixed operation: there are both parallel and antiparallel links In the mixed linkage.

The three-dimensional display is reminiscent of a folded sheet, said peptide groups lie in the areas and the intervening carbon atoms, on the edges of a multi-folded sheet. The peptide bonds of several chains interact with each other and form hydrogen bonds along the polypeptide backbone from. 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 the folder with 0.35 nm significantly greater than in the α -helix with 0.15 nm However, the side groups are in the β -strands very close together, so that bulky or charged residues equal to disturb the arrangement. Larger sheet structure areas are only possible if the remains are relatively small. To pass the protein chains of natural silk, which are arranged exclusively in the sheet structure, 86 percent of glycine, alanine and serine, that amino acids with small residues. Another example of proteins with predominantly sheet structure are immunoglobulins. Are mainly large amino acid residues ago, the polypeptide chain associates preferentially to as α - helix.

Representation

  • Band diagram: In protein structures β -sheets are often depicted as arrows. An arrow corresponds to a strand of the folded sheet. Several arrows linked by turns or loops, arrows form the sheet structure. The arrows indicate this in the direction of the carboxyl terminus, to reflect the structure of the folded sheet.
  • Space-filling and ball- and-stick model: often be omitted in the representation of the hydrogen atoms, as can the positions of these atoms is difficult to determine. In addition, would, under the representation of the hydrogen atoms clarity suffer.
  • Backbone model: Only the α - carbon atoms are usually represented, thereby better to see the course of the polypeptide chain as the dome or ball stick model. Elements of secondary structure are still not well recognized.

Other important structural elements

In addition to the α -helix and β -sheet secondary structure, there are other types of designs. The not belonging to a motive parts of the primary structure of a protein called random loops ( random coil structures). These structures are instrumental in the formation of the entire protein structure.

Other frequently occurring motifs are:

  • π -helix
  • 310- helix
  • Left-handed α - helix of collagens
  • β - loop
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