Notes to Secondary Structures of Globular Proteins

The following tendencies have been observed for secondary structures and their connections in globular proteins  

In solvent exposed a-helices, the plane of the peptide bond is often rotated so that the C=0 group points outward from the helical axis toward the solvent. The helical axis is often curved with the surface on the outside of the globular structure somewhat extended. 

P Pleated sheet structures are actually a special case of secondary structure because they cannot form without intervening secondary structure, which usually take the form of an a helix. The most conunon such structure is PaP crossover connection (Richardson, 1973), where the two connected P-strands are adjacent. 

Most parallel P-sheets are protected by an a-helix or other structure. In the antiparallel p-sheet, one side of the sheet is exposed to solvent and the other is buried in the protein interior. Thus a hydrophilic residue and hydrophobic residue appear alternately in the amino acid sequence. The parallel P-sheet seems to be less stable than the antiparallel P-sheet due to the non-linear hydrogen bonds. 

Types I, II and III reverse turns are closely related. From their dihedral angles it can be seen that the Type I turn begins roughly in the region of the right-hand a-helix and ends close to the region occupies by a P-strand, whereas Type III is actually a helical configuration (3.0io). 

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