title: Heptad repeat
text: # Heptad repeat
  
  The heptad repeat is an example of a structural motif that consists of a repeating pattern of seven amino acids:
  
  where H represents hydrophobic residues, C represents, typically, charged residues, and P represents polar (and, therefore, hydrophilic) 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.
  
  A conformational change in a heptad repeat in the SARS-CoV-2 spike protein facilitates entry of the virus into the host cell membrane.
  
  