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The most common secondary structures are alpha helices and beta sheets. Other helices, such as the 3 10 helix and π helix , are calculated to have energetically favorable hydrogen-bonding patterns but are rarely observed in natural proteins except at the ends of α helices due to unfavorable backbone packing in the center of the helix.
When Pauling and Corey first proposed the alpha sheet, they suggested that it agreed well with fiber diffraction results from beta-keratin fibers. [2] However, since the alpha sheet did not appear to be energetically favorable, they argued that beta sheets would occur more commonly among normal proteins, [3] and subsequent demonstration that beta-keratin is made of beta sheets consigned the ...
Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet. A β-strand is a stretch of polypeptide chain typically 3 to 10 amino acids long with backbone in an extended conformation .
The alpha helix spiral formation An anti-parallel beta pleated sheet displaying hydrogen bonding within the backbone. Formation of a secondary structure is the first step in the folding process that a protein takes to assume its native structure.
The α-helices and β-pleated-sheets are folded into a compact globular structure. The folding is driven by the non-specific hydrophobic interactions , the burial of hydrophobic residues from water , but the structure is stable only when the parts of a protein domain are locked into place by specific tertiary interactions, such as salt bridges ...
The beta strands are parallel, and the helix is also almost parallel to the strands. This structure can be seen in almost all proteins with parallel strands. The loops connecting the beta strands and alpha helix can vary in length and often binds ligands. Beta-alpha-beta helices can be either left-handed or right-handed.
All-β proteins are a class of structural domains in which the secondary structure is composed entirely of β-sheets, with the possible exception of a few isolated α-helices on the periphery. Common examples include the SH3 domain, the beta-propeller domain, the immunoglobulin fold and B3 DNA binding domain.
Under high tension, alpha-keratin can even change into beta-keratin, a stronger keratin formation that has a secondary structure of beta-pleated sheets. [11] Alpha-keratin tissues also show signs of viscoelasticity, allowing them to both be able to stretch and absorb impact to a degree, though they are not impervious to fracture.