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A polyproline helix is a type of protein secondary structure which occurs in proteins comprising repeating proline residues. [1] A left-handed polyproline II helix (PPII, poly-Pro II, κ-helix [2]) is formed when sequential residues all adopt (φ,ψ) backbone dihedral angles of roughly (-75°, 150°) and have trans isomers of their peptide bonds.
Multiple prolines and/or hydroxyprolines in a row can create a polyproline helix, the predominant secondary structure in collagen. The hydroxylation of proline by prolyl hydroxylase (or other additions of electron-withdrawing substituents such as fluorine ) increases the conformational stability of collagen significantly. [ 20 ]
The collagen triple helix is made of three collagen peptides, each of which forms its own left-handed polyproline helix. [5] When the three chains combine, the triple helix adopts a right-handed orientation. The collagen peptide is composed of repeats of Gly-X-Y, with the second residue (X) usually being Pro and the third (Y) being ...
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In addition to the protein domains, there are unusual transmembrane elements formed by peptides. A typical example is gramicidin A, a peptide that forms a dimeric transmembrane β-helix. [8] This peptide is secreted by gram-positive bacteria as an antibiotic. A transmembrane polyproline-II helix has not been reported in natural proteins ...
In such rings the polypeptide has the conformation of beta sheet or of type II polyproline helix (PPII). A number of glutamines and asparagines help bind short peptides (with the PPII conformation) in the groove of class II MHC ( Major Histocompatibility Complex ) proteins [ 2 ] by forming these motifs.
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Resilin, however, has an absence of an alpha-helix leading to a randomly coiled structure and a disordered structure. [10] This is primarily due to the significantly high proline content in resilin. Proline is a bulky amino acid that has the ability to cause a kink the peptide chain and due to the sterically hindered side chains, it is not able ...