β-Sheet

The β-sheet is another form of secondary structure in which all of the peptide bond components are involved in hydrogen bonding (Fig. 1A).
Because the surfaces of β-sheets appear “pleated,” they are often called β-pleated sheets. [Note: Pleating results from successive α-carbons being slightly above or below the plane of the sheet.] Illustrations of protein structure often show β-strands as broad arrows (Fig. 2B).

Figure 1 A. Structure of a β-sheet. B. An antiparallel β-sheet with the β-strands represented as broad arrows. C. A parallel β-sheet formed from a single polypeptide chain folding back on itself.

1. Formation: A β-sheet is formed by two or more peptide chains (β-strands) aligned laterally and stabilized by hydrogen bonds between the carboxyl and amino groups of amino acids that either are far apart in a single polypeptide (intrachain bonds) or are in different polypeptide chains (interchain bonds). The adjacent β-strands are arranged either antiparallel to each other (with the N-termini alternating as shown in Fig.1B) or parallel to each other (with the N-termini together as shown in Fig. 1C). On each β-strand, the R groups of adjacent amino acids extend in opposite directions, above and below the plane of the β-sheet. [Note: β-sheets are not flat and have a right-handed curl (twist) when viewed along the polypeptide backbone.]
2. Comparing α-helices and β-sheets: In β-sheets, the β-strands are almost fully extended and the hydrogen bonds between the strands are perpendicular to the polypeptide backbone (see Fig. 1A). In contrast, in α-helices, the polypeptide is coiled and the hydrogen bonds are parallel to the backbone .