P-pleated sheets

Proteins or sections of proteins sometimes exist as random coils, an arrangement that lacks the regularity of the a helix or pleated p sheet... 

Pleated p sheet (Section 27.19) Type of protein secondary structure characterized by hydrogen bonds between NH and C=0 groups of adjacent parallel peptide chains. The individual chains are in an extended zigzag conformation. 

Figure 2.6 Two projections of an antiparaUel pleated P-sheet established by three sets of four pep-tidic groups in a protein. The left diagram is a projection on the plane of the H-bonded N-H - 0=C groups of the p-sheet and the right diagram is a projection of the same part of the protein rotated by about 90° along the T-axis of the figure. The first set of four peptides is shown starting at the N-terminal of the protein. The two other sets appear further along the succession of peptides of the protein. The sequence of peptides that link these three peptidic groups are shown as white circles.

The primary structure of a peptide is its amino acid sequence. We also speak of the secondary structure of a peptide, that is, the conformational relationship of nearest neighbor amino acids with respect to each other. On the basis of X-ray crystallographic studies and careful examination of molecular models, Linus Pauling and Robert B. Corey of the California Institute of Technology showed that certain peptide conformations were more stable than others. Two arrangements, the a. helix and the pleated p sheet, stand out as secondary stractural units that are both particularly stable and commonly encountered. Both of these incorporate two important features ... 

Secondary structure The conformation with respect to nearest neighbor amino acids in a peptide or protein. The a helix and the pleated P sheet are examples of protein secondary structures. 

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