Basic Elements of Protein Structure

The a-helix is the classic element of protein structure. A single a-helix can order as many as 35 residues whereas the longest strands include only about 15 residues, and one helix can have more influence on the stability and organization of a protein than any other individual structure element. a-Helices have had an immense influence on our understanding of protein structure because their regularity makes them the only feature readily amenable to theoretical analysis. 

With 3.6 residues per turn, side chains protrude from the a-helix at about every 100° in azimuth. Since the commonest location for a helix is along the outside of the protein, there is a tendency for side chains to change from hydrophobic to hydrophilic with a periodicity of three to four residues (Schiffer and Edmundson, 1967). This trend can sometimes be seen in the sequence, but it is not strong enough for reliable prediction by itself. Different residues have weak but definite preferences either for or against being in a-helix Ala, Glu, Leu, 

The only other principal helical species besides the a-helix which occurs to any great extent in globular protein structure is the 310-helix 

A short segment of 3l0 helix from carbonic anhydrase (residues 159-164). Main chain carbonyl oxygens are shown as open circles. 

Another frequent feature of the C-termini of helices is a residue (usually glycine) in left-handed a conformation with its NH making a hydrogen bond to the CO of residue n - 5 (see Schellman, 1980) this often follows a residue with the 3I0 or a bonding described above. 

---