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Alpha sheet
Alpha sheet (also known as alpha pleated sheet or polar pleated sheet) is an atypical secondary structure in proteins, first proposed by Linus Pauling and Robert Corey in 1951.〔Pauling, L. & Corey, R. B. (1951). The pleated sheet, a new layer configuration of polypeptide chains. ''Proc. Natl. Acad. Sci. USA'' 37, 251–6. PMID 14834147〕〔Pauling, L. & Corey, R. B. (1951). The structure of feather rachis keratin. ''Proc. Natl. Acad. Sci. USA'' 37, 256–261. PMID 14834148〕〔Pauling, L. & Corey, R. B. (1951). Configurations of Polypeptide Chains With Favored Orientations Around Single Bonds: Two New Pleated Sheets. ''Proc. Natl. Acad. Sci. USA'' 37, 729–740. PMID 16578412〕 The hydrogen bonding pattern in an alpha sheet is similar to that of a beta sheet, but the orientation of the carbonyl and amino groups in the peptide bond units is distinctive; in a single ''alpha strand'', all of the carbonyl groups are oriented in the same direction on one side of the pleat, and all of the amino groups are oriented in the same direction on the opposite side of the sheet. Thus the alpha sheet accumulates an inherent separation of electrostatic charge, with one edge of the sheet exposing negatively charged carbonyl groups and the opposite edge exposing positively charged amino groups. Unlike the alpha helix and beta sheet, the alpha sheet configuration does not require all component amino acid residues to lie within a single region of dihedral angles; instead, the alpha sheet contains residues of alternating dihedrals in the traditional right-handed (αR) and left-handed (αL) helical regions of Ramachandran space. Although the alpha sheet is only rarely observed in natural protein structures, it has been speculated to play a role in amyloid disease〔Daggett V. (2006). Alpha-sheet: The toxic conformer in amyloid diseases? ''Acc Chem Res'' 39(9):594-602. PMID 16981675〕 and it was found to be a stable form for amyloidogenic proteins in one set of molecular dynamics simulations.〔Armen RS, DeMarco ML, Alonso DO, Daggett V. (2004). Pauling and Corey's alpha-pleated sheet structure may define the prefibrillar amyloidogenic intermediate in amyloid disease. ''Proc Natl Acad Sci USA'' 101(32):11622-7. PMID 15280548〕 Alpha sheets have also been observed in X-ray crystallography structures of designed peptides.〔
==Experimental evidence==
When Pauling and Corey first proposed the alpha sheet, they suggested that it agreed well with fiber diffraction results from beta-keratin fibers.〔 However, since the alpha sheet did not appear to be energetically favorable, they argued that beta sheets would occur more commonly among normal proteins,〔 and subsequent demonstration that beta-keratin is made of beta sheets consigned the alpha sheet proposal to obscurity. Recently the alpha strand conformation has been observed in isolated instances in native state proteins as solved by X-ray crystallography or protein NMR, although an extended alpha sheet was not identified in any known natural protein. Native proteins identified as containing alpha-strand regions or alpha-sheet-patterned hydrogen bonding include synaptotagmin, lysozyme, and potassium channels, where the alpha-strands line the ion-conducting pore.〔
Alpha-sheet conformations have been observed in crystal structures of short non-natural peptides, especially those containing a mixture of L and D amino acids. The first crystal structure containing an alpha sheet was observed in the capped tripeptide Boc–AlaL–a-IleD–IleL–OMe.〔Di Blasio B, Saviano M, Fattorusso R, Lombardi A, Pedone C, Valle V, Lorenzi GP. (1994). A crystal structure with features of an antiparallel alpha-pleated sheet. ''Biopolymers'' 34(11):1463-8. PMID 7827259〕 Other peptides that assume alpha-sheet structures include capped diphenyl-glycine-based dipeptides〔De Simone G, Lombardi A, Galdiero S, Nastri F, Di Costanzo L, Gohda S, Sano A, Yamada T, Pavone V. (2000). The crystal structure of a Dcp-containing peptide. ''Biopolymers'' 53(2):182-8. PMID 10679622〕 and tripeptides.〔Pavone V, Lombardi A, Saviano M, Nastri F, Zaccaro L, Maglio O, Pedone C, Omote Y, Yamanaka Y, Yamada T. (1998). Conformational behaviour of C(alpha,alpha)-diphenylglycine: folded vs. extended structures in DphiG-containing tripeptides. 4(1):21-32. PMID 9523753〕
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