Sheets in Proteins: Difference between revisions

<Structure load='Silk 64.mol' size='500' frame='true' align='right' caption='Structures of Sheets' scene='Sheets_in_Proteins/Syn_sheet1/2' />

A β-pleated sheet contains multiple peptide strands that are positioned adjacent to one another as the one shown on the right<ref>This model was hand constructed using HyperChem and setting all phi and psi values to -139&deg; and 135&deg;, respectively.</ref> (<scene name='Sheets_in_Proteins/Syn_sheet1/2'> (Initial scene)</scene>).  The planes of the <scene name='Sheets_in_Proteins/Syn_sheet2/2'>pleats</scene> are formed by the planes of the peptide bond.  The alpha carbons of the peptide chain are at the valleys and peaks of the pleats.  The peptides are <scene name='Sheets_in_Proteins/Syn_sheet3/3'>rainbow colored</scene> (blue amino end changing to red carboxy end) to show that the adjacent peptides are running in opposite directions making the sheet antiparallel.  Another way of detecting the <scene name='43/437754/1/1'>antiparallel sheet</scene> is by displaying as cartoon.  The adjacent chains align so that <scene name='Sheets_in_Proteins/Syn_sheet4/1'>hydrogen bonds</scene> are formed between the imino hydrogens of one chain and the carbonyl oxygens of an adjacent chain.  These hydrogen bonds provide the major attractive force which maintains the sheet structure.  Phi and psi values that permit this alignment in antiparallel sheets have <scene name='Sheets_in_Proteins/Syn_sheet5/1'>median values</scene> of -139&deg; and +135&deg;, respectively.  The median values for a parallel sheet are -119&deg; and +113&deg;.   

Real sheets (rather than the simple models shown above) as found in globular proteins are <scene name='43/437754/6ssc_antiparallel/1'>twisted</scene>. Unlike the <scene name='Sheets_in_Proteins/Syn_sheet6/2'>above two types</scene> of sheets, the valleys and the peaks of a <scene name='Sheets_in_Proteins/Gly_phosyl2/2'>twisted sheet</scene><ref>This sheet is part of the structure of domain 2 of glycogen phosphorylase (PDB code [[1abb]]).</ref> do not fall on parallel lines.  Observe that the sheet is <scene name='Sheets_in_Proteins/Gly_phosyl/4'>parallel</scene>, and showing <scene name='Sheets_in_Proteins/Gly_phosyl3/1'>hydrogen bonds</scene>.  Notice that in contrast to the hydrogen bonds of the antiparallel sheet shown above where the bonds were parallel here the bonds are diagonal to each other.  Show <scene name='Sheets_in_Proteins/Gly_phosyl5/2'>phi and psi values</scene> for randomly chosen residues.  There are a wide range of values for both phi and psi, -108&deg; to -142&deg; and +96&deg; to +148&deg;, respectively.  The above sheet shown in the context of <scene name='Sheets_in_Proteins/Gly_phosyl/5'>domain 2</scene> of glycogen phosphorylase. An examples of <scene name='Sheets_in_Proteins/1dtg1/2'>antiparallel twisted sheet</scene><ref>These sheets are part of the structure of human transferrin n-lobe mutant (PDB code [[1dtg]]).</ref>.  Showing only the <scene name='Sheets_in_Proteins/1dtg2/2'>sheets</scene>, and with <scene name='Sheets_in_Proteins/1dtg3/1'>hydrogen bonds</scene>.  <scene name='Sheets_in_Proteins/1dtg4/3'> Values</scene> of phi and psi for randomly chosen residues; these values range from -98&deg; to -178&deg; and from +90&deg; to +167&deg;, respectively.  These ranges overlap the values for the twisted parallel showing that there is no differences in the ranges for the two types of twisted sheets.  Median values for phi and psi are -125&deg; and +140&deg;, respectively.

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