The b-sheet structure:
Pauling and Corey derived a model for the conformation of fibrous proteins known as b-keratins. In this conformation the polypeptide does not form a coil. Instead, it zig-zags in a more extended conformation than the a-helix. Amino acid residues in the b-conformation have negative F angles and the Y angles are positive. Typical values are F = -140 degrees and Y = 130 degrees. In contrast, a-helical residues have both F and Y negative. A section of polypeptide with residues in the b-conformation is referred to as a b-strand and these strands can associate by main chain hydrogen bonding interactions to form a sheet.
In a beta-sheet two or more polypeptide chains run alongside each other and are linked in a regular manner by hydrogen bonds between the main chain C=O and N-H groups. Therefore all hydrogen bonds in a a-sheet are between different segments of polypeptide. This contrasts with the a-helix where all hydrogen bonds involve the same element of secondary structure. The R-groups (side chains) of neighbouring residues in a b-strand point in opposite directions.
Imagining two strands parallel to this, one above the plane of the screen and one behind, it is possible to grasp how the pleated appearance of the b-sheet arises. Note that peptide groups of adjacent residues point in opposite directions whereas with a-helices the peptide bonds all point one way:
The axial distance between adjacent residues is 3.5 Å. There are two residues per repeat unit which gives the b-strand a 7 Å pitch. This compares with the a-helix where the axial distance between adjacent residues is only 1.5 Å. Clearly, polypeptides in the b-conformation are far more extended than those in the a-helical conformation.
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