Coiled coils (CC) are protein structural domains made out of alpha-helices packed together through tight packing of side-chains also known as knob-into-hole packing. We usually think of coiled-coils as a pair of intertwined alpha-helices, however, Moutevelis E & Woolfson DN performed a systematic analysis and classification of coiled-coil structures in the PDB, and entail a more complex story. This analysis is summarized in the “periodic table of coiled-coils“.
Periodic table of coiled-coil structure - adapted from: http://coiledcoils.chm.bris.ac.uk/ccplus/search/
As it turns, dimeric coiled coils only populate 73.8% of all found CC architectures, whereas the other 26% are more complex. The columns of the table represent the number of helices participating in the CC motif, while the rows of the table represent the complexity of relationship between these motifs.
The analysis identified some CC architechtures that have not been assigned as such before, but more importantly, the table and these new classes of coiled-coil assemblies highlight the structural space currently occupied by coiled-coils and possibly predict viable assemblies yet to be seen.
E MOUTEVELIS, D WOOLFSON (2009). A Periodic Table of Coiled-Coil Protein Structures Journal of Molecular Biology, 385 (3), 726-732 DOI: 10.1016/j.jmb.2008.11.028