Pleated sheets.

While a fully extended polyglycine chain is possible, the side chains of other amino acids cannot be accommodated without some distortion of the structure. Thus, the peptide chains in silk fibroin have a repeat distance of 0.70 nm compared with the 0.72 nm for the fully extended chain. Pauling and Corey108 showed that this shortening of the chain could result from rotation of angle φ by ∼40° (to –140°) and rotation of ψ in the opposite direction by ∼45° (to +135°) to give a slightly puckered chain. The resulting multichain structure is known as a pleated sheet. As in this figure, both parallel and antiparallel strands are often present in a single β sheet within a protein.

Straight (left) and twisted (right) peptide chains in extended β conformations. From Chothia.

(A) Stereoscopic view of the nucleotide binding domain of glyceraldehyde phosphate dehydrogenase. The enzyme is from Bacillus stearothermophilus but is homologous to the enzyme from animal sources. Residues are numbered 0–148. In this wire model all of the main chain C, O, and N atoms are shown but side chains have been omitted. The large central twisted β sheet, with strands roughly perpendicular to the page, is seen clearly; hydrogen bonds are indicated by dashed lines. Helices are visible on both sides of the sheet. The coenzyme NAD+ is bound at the end of the β sheet toward the viewer. Note that the two phosphate groups in the center of the NAD+ are H-bonded to the N terminus of the helix beginning with R10. From Skarzynski et al. (B) Structural formula for NAD+.