Alpha Helix

The alpha helix is the most common secondary structure in proteins: a right-handed coil of the polypeptide backbone held in shape by hydrogen bonds. Linus Pauling and his colleagues predicted it in 1951, before it was seen in a protein.

The helix makes one full turn for every 3.6 amino acid residues, advancing about 0.54 nanometers per turn, with the side chains pointing outward from the coil. The backbone carbonyl oxygen of each residue forms a hydrogen bond with the amide hydrogen of the residue four positions later, so the bonds run parallel to the axis and stabilize the whole turn.

Alanine, leucine and glutamate favor helices, while proline, whose ring locks the backbone angle and lacks an amide hydrogen, usually breaks them. Because every peptide bond has a small dipole, helices have a net dipole too. Helices in membrane proteins are made of hydrophobic residues that cross the lipid bilayer, as in many receptors and channels.

Alpha helices form the rigid rods of keratin in hair and nails, the transmembrane segments of receptors and the DNA-binding motifs of transcription factors. Their partner among secondary structures is the beta sheet. Structures of both were worked out with X-ray crystallography, and they are built from amino acids joined by peptide bonds.

• Beta Sheet — the other main secondary structure
• Proteins — the molecules it builds
• Hydrogen Bond — the bond that holds it