Editing Alpha helix (section)

== Coiled coils ==
Coiled-coil α helices are highly stable forms in which two or more helices wrap around each other in a "supercoil" structure. [[Coiled coil]]s contain a highly characteristic [[sequence motif]] known as a '''[[heptad repeat]]''', in which the motif repeats itself every seven residues along the sequence (''amino acid'' residues, not DNA base-pairs). The first and especially the fourth residues (known as the ''a'' and ''d'' positions) are almost always [[hydrophobic]]; the fourth residue is typically [[leucine]]{{snd}} this gives rise to the name of the [[structural motif]] called a ''[[leucine zipper]]'', which is a type of coiled-coil. These hydrophobic residues pack together in the interior of the helix bundle. In general, the fifth and seventh residues (the ''e'' and ''g'' positions) have opposing charges and form a salt bridge stabilized by [[electrostatic]] interactions. [[Fibrous protein]]s such as [[keratin]] or the "stalks" of [[myosin]] or [[kinesin]] often adopt coiled-coil structures, as do several [[protein dimer|dimerizing]] proteins. A pair of coiled-coils{{snd}} a four-[[helix bundle]]{{snd}} is a very common structural motif in proteins. For example, it occurs in human [[growth hormone]] and several varieties of [[cytochrome]]. The [[Rop protein]], which promotes plasmid replication in bacteria, is an interesting case in which a single polypeptide forms a coiled-coil and two monomers assemble to form a four-helix bundle.