LPleated Sheet

Extended polypeptide chains can associate by hydrogen bonding to form sheetlike structures in parallel /-sheets, with all /-strands pointed in the same direction, and in antiparallel /-pleated sheets, with alternate strands pointed in opposite direction. Individual /-strands or /-pleated sheets have a structure generated by a regular sequence of dihedral angles (<p = —120° and \|/ = +140°) that orient the NH and C = O bonds nearly perpendicular to the long axis of the polypeptide chain. Also, the idealized ¡3-structure based on 2.0 residues per turn originally proposed was a flat pleated sheet structure; actual /-pleated sheets found in proteins usually have a left-handed twist. Globular proteins contain about 15% sheet structures. The average length of /-structure is about 6 residues, which is equal to 20 A in length, corresponding approximately to the diameters of domains.

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