Heptad repeat
The heptad repeat is an example of a structural motif that consists of a repeating pattern of seven amino acids:
a b c d e f g
H P P H C P C
where H represents hydrophobic residues, C represents, typically, charged residues, and P represents polar residues. The positions of the heptad repeat are commonly denoted by the lowercase letters a through g.
These motifs are the basis for most coiled coils and, in particular, leucine zippers, which have predominantly leucine in the d position of the heptad repeat.
