Beta sheet: Difference between revisions

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Karsten Theis

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 ==Structure, hydrogen bonding and composition==
 <StructureSection load='' size='350' side='right' caption='' scene='88/889825/Toxin_1f94/1'>
-The <scene name='88/889825/Toxin_1f94/1'>initial scene</scene> shows a protein with two sheets, one composed of four strands and the other of two. Beta strands are characterized by the extended conformation of the main chain (with phi and psi angles in the upper left quadrant of the [[Ramachandran plot]]) and hydrogen bonds to the neighboring strands. Strands are either in <scene name='88/889825/6l26_direction/1'>a parallel or an antiparallel arrangement</scene>, resulting in different <scene name='88/889825/6l26_direction/2'>hydrogen bonding patterns</scene> [https://www.ncbi.nlm.nih.gov/books/NBK22580/#_A328_]. Accordingly, beta sheets are classified as parallel, antiparallel or mixed. The antiparallel arrangement of strands is more prevalent <ref>DOI:10.1021/ci200027d</ref>.  The beta strands on the edge of a sheet will have hydrogen bond donors and acceptors that have no other strand to partner with unless the sheet forms a cylindrical structure called a beta barrel, such as in the [[green fluorescent protein]] structure.
+The <scene name='88/889825/Toxin_1f94/1'>initial scene</scene> shows a protein with two sheets, one composed of four strands and the other of two. Beta strands are characterized by the extended conformation of the main chain (with phi and psi angles in the upper left quadrant of the [[Ramachandran plot]]) and hydrogen bonds to the neighboring strands. The beta sheet is also called beta pleated sheet because the peptide planes follow a zig-zag pattern.
-The side chains of amino acids in a beta sheet point outward on either side of the sheet, with alternate side chains in a strand pointing in opposite directions. The side chains of beta sheets are often in contact with side chains of other secondary structure elements connecting the strands, forming a hydrophobic core on either side of the sheet. This explains why many amino acids in sheets are hydrophobic. Just as for alpha helices, proline and glycine residues are uncommon. Proline lacks the hydrogen donor present in the peptide group, and glycine has more conformational states than all the other amino acids. Different from alpha helices, beta sheets are able to accomodate beta-branched side chains such as valine and isoleucine, so these are more prevalent in beta sheets than in alpha helices.
+[[Image:339px-Beta-Faltblatt.svg.png]]
+For a strand in the middle of a sheet (as opposed to on its edge), all main chain hydrogen bond donors (the N-H groups) and acceptors (the C=O groups) are part of inter-strand hydrogen bonds. Strands are either in <scene name='88/889825/6l26_direction/1'>a parallel or an antiparallel arrangement</scene>, resulting in different <scene name='88/889825/6l26_purple_green/1'>hydrogen bonding patterns</scene> [https://www.ncbi.nlm.nih.gov/books/NBK22580/#_A328_]. Accordingly, beta sheets are classified as parallel, antiparallel or mixed. The antiparallel arrangement of strands is more prevalent <ref>DOI:10.1021/ci200027d</ref>.  The beta strands on the edge of a sheet will have hydrogen bond donors and acceptors that have no other strand to partner with unless the sheet forms a cylindrical structure called a beta barrel, such as in the [[green fluorescent protein]] structure.
+<scene name='88/889825/6l26_purple_green/3'>blabla</scene>
+The side chains of amino acids in a beta sheet <scene name='88/889825/6l26_direction/4'>point outward</scene> on either side of the sheet, with alternate side chains in a strand pointing in opposite directions. The side chains of beta sheets are often in contact with side chains of other secondary structure elements connecting the strands, forming a hydrophobic core on either side of the sheet. This explains why many amino acids in sheets are hydrophobic. Just as for alpha helices, proline and glycine residues are uncommon. Proline lacks the hydrogen donor present in the peptide group, and glycine has more conformational states than all the other amino acids. Different from alpha helices, beta sheets are able to accomodate beta-branched side chains such as valine and isoleucine, so these are more prevalent in beta sheets than in alpha helices.
 </StructureSection>
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 ===Beta sheets in amyloid fibrils===
 ==History==
-Alpha helices and beta sheets are named after two conformations of keratin, a fiber occuring in mammals (wool, hair, quills) <ref>PMID: 15240497</ref>. Alpha keratin is composed of [[coiled coil|colide coils]] of alpha helices, whereas hard stretching these fibers in water changes the conformation to beta sheets. The two conformations show different diffraction data under X-ray illumination.
+Alpha helices and beta sheets are named after two conformations of keratin, a fiber occuring in mammals (wool, hair, quills) <ref>PMID: 15240497</ref>. Alpha keratin is composed of [[coiled coil|coiled coils]] of alpha helices, whereas hard stretching these fibers in water changes the conformation to beta sheets. The two conformations show different diffraction data under X-ray illumination.
 ==Experimental evidence==
 Apart from the historical fiber diffraction data, various spectroscopic techniques may be used to show the presence of beta sheets. Circular dichroism (CD) or infrared (IR) spectroscopy allows an estimate of the beta sheet content of a protein sample. NMR spectroscopy, after resonance assignment, allows secondary structure assignment residue by residue based on chemical shifts of the alpha carbon and beta carbon resonances.