Heptad repeat

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The heptad repeat is an example of a structural motif which consists of a repeating pattern of amino acids:

 a b c d e f g
 H P P H P P P

where H represents hydrophobic residues and P represents polar residues. The positions of the heptad repeat are commonly denoted by the lower case letters a through g.

These motifs are the basis for most coiled coils and in particular, leucine zippers, which predominantly have leucine in the d position of the heptad repeat.