Editing Coiled coil (section)

==Molecular structure==

Coiled coils usually contain a repeated pattern, ''hxxhcxc'', of hydrophobic (''h'') and charged (''c'') [[amino-acid]] residues, referred to as a [[heptad repeat]].<ref name="mason2004">
{{cite journal | vauthors = Mason JM, Arndt KM | title = Coiled coil domains: stability, specificity, and biological implications | journal = ChemBioChem | volume = 5 | issue = 2 | pages = 170–176 | date = February 2004 | pmid = 14760737 | doi = 10.1002/cbic.200300781 | s2cid = 39252601 }}</ref>
The positions in the heptad repeat are usually labeled ''abcdefg'', where ''a'' and ''d'' are the hydrophobic positions, often being occupied by [[isoleucine]], [[leucine]], or [[valine]]. Folding a sequence with this repeating pattern into an [[alpha-helix|alpha-helical]] [[secondary structure]] causes the hydrophobic residues to be presented as a 'stripe' that coils gently around the helix in left-handed fashion, forming an [[amphiphile|amphipathic]] structure. The most favorable way for two such helices to arrange themselves in the water-filled environment of the [[cytoplasm]] is to wrap the hydrophobic strands against each other sandwiched between the [[hydrophilic]] amino acids. Thus, it is the burial of hydrophobic surfaces that provides the [[thermodynamic]] driving force for the oligomerization. The packing in a coiled-coil interface is exceptionally tight, with almost complete [[van der Waals force|van der Waals]] contact between the [[Substituent|side-chains]] of the ''a'' and ''d'' residues. This tight packing was originally predicted by [[Francis Crick]] in 1952<ref name="crick52"/> and is referred to as [[knobs into holes packing]].

The [[alpha-helix|α-helices]] may be parallel or anti-parallel, and usually adopt a ''left-handed'' super-coil (Figure 1).  Although disfavored, a few ''right-handed'' coiled coils have also been observed in nature and in designed proteins.<ref name="harbury1998">{{cite journal | vauthors = Harbury PB, Plecs JJ, Tidor B, Alber T, Kim PS | title = High-resolution protein design with backbone freedom | journal = Science | volume = 282 | issue = 5393 | pages = 1462–1467 | date = November 1998 | pmid = 9822371 | doi = 10.1126/science.282.5393.1462 }}</ref>