Parallel beta-pleated sheet

P (beta)-pleated sheet A secondary level of protein structure that consists of hydrogen bonds between peptide links in parallel polypeptide chains. 

Fibroin, the fibrous protein found in silk, has a secondary structure called a beta- (/8-) pleated sheet, in which a polypeptide chain doubles back on itself after a hairpin bend. The two sections of the chain on either side of the bend line up in a parallel arrangement held together by hydrogen bonds (Figure 24.8). Although not as common as the a-helix, small pleated-sheet regions are often found in proteins. 

The other structures that Pauling discovered by model-building are the j8-structures (pronounced beta), also known as the pleated sheet, from its appearance when many lengths of chain associate in parallel. They are formed by side-to-side association of almost fully extended chains, and the reason that two forms are possible is that adjacent chains may run in parallel or antiparallel (i.e. the same or opposite) directions, as shown here ... 

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