The β-sheet, also known as the β-pleated sheet due to its pleated appearance, is a secondary structure formed by hydrogen bonding between the peptide bond components. It consists of two or more peptide chains called β-strands, which are aligned laterally and stabilized by hydrogen bonds. These bonds can occur within a single polypeptide chain (intrachain bonds) or between different polypeptide chains (interchain bonds). The adjacent β-strands can be arranged either antiparallel or parallel to each other. The R groups of amino acids on each β-strand extend in opposite directions above and below the plane of the β-sheet, giving it a right-handed curl when viewed along the polypeptide backbone. 

In comparison to α-helices, β-sheets have almost fully extended β-strands with perpendicular hydrogen bonds between the strands. In α-helices, the polypeptide is coiled and the hydrogen bonds are parallel to the backbone. Both α-helices and β-sheets can have polar and nonpolar sides, making them amphipathic. 

β-Bends, also known as reverse turns or β-turns, reverse the direction of a polypeptide chain and contribute to the compact, globular shape of proteins. They are commonly found on the protein surface and often contain charged residues. β-Bends usually consist of four amino acids, including proline which causes a kink in the chain, and glycine which has a small R group. The formation of hydrogen bonds between the first and last residues in the bend stabilizes β-bends. 

Approximately half of a globular protein is made up of repetitive structures such as α-helices and β-sheets, while the remaining portion has a less regular structure described as loop or coil conformation. These nonrepetitive secondary structures are not random but have a more disordered structure compared to α-helices and β-sheets. Supersecondary structures, also known as motifs, are specific geometric patterns formed by combining secondary structural elements like α-helices, β-sheets, and coils. These motifs primarily exist in the core region of the protein and are connected by loop regions such as β-bends on the protein surface. Supersecondary structures are often formed by the close packing of side chains from adjacent secondary structural elements. Some motifs are associated with particular functions, such as the helix-loop-helix motif found in transcription factor proteins. 

Tertiary structure is the next level of protein structure beyond secondary structure.
The β-sheet is another form of secondary structure in which all of the peptide bond components are involved in hydrogen bonding (Fig. 2.7A). Because the surfaces of β-sheets appear “pleated,” they are often called βpleated 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. 2.7B).
1.
Formation: A β-sheet is formed by two or more peptide chains (βstrands) 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. 2.7B) or parallel to each other (with the N-termini together as shown in Fig. 2.7C). 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. 2.7A). In contrast, in α-helices, the polypeptide is coiled and the hydrogen bonds are parallel to the backbone (see Fig. 2.6).
The orientation of the R groups of the amino acid residues in both the αhelix and the β-sheet can result in formation of polar and nonpolar sides in these secondary structures, thereby making them amphipathic.
C. β-Bends (reverse turns, β-turns) β-Bends reverse the direction of a polypeptide chain, helping it form a compact, globular shape. They are usually found on the surface of protein molecules and often include charged residues. [Note: β-Bends were given this name because they often connect successive strands of antiparallel βsheets.] β-Bends are generally composed of four amino acids, one of which may be proline, the amino acid that causes a kink in the polypeptide chain. Glycine, the amino acid with the smallest R group, is also frequently found in β-bends. β-Bends are stabilized by the formation of hydrogen bonds between the first and last residues in the bend.
D. Nonrepetitive secondary structure
Approximately one half of an average globular protein is organized into repetitive structures, such as the α-helix and β-sheet. The remainder of the polypeptide chain is described as having a loop or coil conformation. These nonrepetitive secondary structures are not random but rather simply have a less regular structure than those described above. [Note: The term “random coil” refers to the disordered structure obtained when proteins are denatured (see p. 20).]
E. Supersecondary structures (motifs)
Globular proteins are constructed by combining secondary structural elements (that is, α-helices, β-sheets, and coils), producing specific geometric patterns, or motifs. These form primarily the core (interior) region of the molecule. They are connected by loop regions (for example, β-bends) at the surface of the protein. Supersecondary structures are usually produced by the close packing of side chains from adjacent secondary structural elements. For example, α-helices and β-sheets that are adjacent in the amino acid sequence are also usually (but not always) adjacent in the final, folded protein. Some of the more common motifs are illustrated in Figure 2.8.
Motifs may be associated with particular functions. Proteins that bind to DNA contain a limited number of motifs. The helix–loop–helix motif is an example found in a number of proteins that function as transcription factors (see p. 438).
IV. TERTIARY STRUCTURE