Protein Chemistry. Structure, Properties, Research Methods - Shendryk A.N. 2022

Protein Structure
Spatial Structure of Proteins
Secondary Protein Structure - Beta-Sheet Structure

The pleated-sheet Structure is also stabilized by cooperative Hydrogen Bonds between the same dipoles. A completely novel structure is formed in this case through intermolecular hydrogen bonds between two or more chains (in Fibrous Proteins) or intramolecular hydrogen bonds between different regions of a single chain (in Globular proteins). Pauling and Corey postulated “flat” parallel and antiparallel ß-pleated sheets.

In a parallel pleated sheet, both peptide chains or two distinct segments of the same peptide chain run in the same spatial direction, i.e., they are parallel. In an antiparallel pleated sheet, the chains run in opposite directions, i.e., they are antiparallel. Both structures occur in natural PROTEINS AND Peptides, but the antiparallel arrangement is more stable and therefore more common. In this conformation, the C=O and N-H dipoles are aligned parallel or coaxially.

Class="center">

Fragments of the parallel and antiparallel pleated-sheet structures are shown in the figures.

The arrangement of R-groups in pleated sheets is as follows. When viewed along the polypeptide backbone, the R-groups project alternately above and below the plane of the sheet. Pleated structures are quite common. A most characteristic example is silk ß-Fibroin, which adopts an antiparallel pleated-sheet structure. Portions of the fibroin molecule feature a repeating structural motif: Gly-Ala-Gly-Ser-Gly-Ala. Upon Formation of the ß-antiparallel structure, the R-groups of Ala and Ser are positioned on one side, while those of Gly are on the other (resulting in a pile-like surface on one side). Because the sheets are held together solely by non-covalent interactions, silk is highly flexible.

Most pleated sheets are not strictly flat; instead, they exhibit a left-handed twist when viewed along the plane of the sheet perpendicular to its extended chains. In effect, the sheet can be described as a heavily stretched left-handed helix.

The length of a twisted sheet is unlimited. Silk ß-fibroin contains very long twisted ribbons of pleated sheet.

The a-helix and ß-Structure are represented in proteins by discrete, sometimes quite short, segments. A significant portion of the peptide chain consists of various types of loops that allow the chain to change direction. The most compact structural element enabling a peptide chain to reverse its direction by 180∘ is termed a ß-turn. ß-Turns are formed by four consecutively positioned amino acid residues stabilized by a 4→1 Hydrogen bond (forming a 10-atom ring). Two possible structures of ß-turns are shown below:

An analysis of chain reversals in globular proteins has shown that 65% of them are ß-turns (also referred to as ß-hairpins). The third amino acid in ß-hairpins is Glycine. Other types of turns generally lack hydrogen bonds. Occasionally, such non-standard bends contain a Cys-Pro sequence. Most reverse turns are located On the surface of the protein and predominantly contain hydrophilic residues.