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Reference.org
Heptad repeat
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<p>The heptad repeat is an example of a <a href="/facts/Structural_motif/g8EBDBGW">structural motif</a> that consists of a repeating pattern of seven <a href="/facts/Amino_acid/WDxYwBny">amino acids</a>: </p> <i>a b c d e f g</i> H P P H C P C <p>where H represents <a href="/facts/Hydrophobic/Ifya0CXQ">hydrophobic</a> residues, C represents, typically, charged residues, and P represents <a href="/facts/Chemical_polarity/KlIJ0zjB">polar</a> (and, therefore, <a href="/facts/Hydrophilic/k8p95WYu">hydrophilic</a>) residues. The positions of the heptad repeat are commonly denoted by the lowercase letters <i>a</i> through <i>g</i>. </p><p>These motifs are the basis for most <a href="/facts/Coiled_coil/U0YEzcB4">coiled coils</a> and, in particular, <a href="/facts/Leucine_zipper/tlAMv6Ul">leucine zippers</a>, which have predominantly <a href="/facts/Leucine/X7YrkvA5">leucine</a> in the <i>d</i> position of the heptad repeat. </p><p>A conformational change in a heptad repeat in the <a href="/facts/SARS-CoV-2/DxF8rDc1">SARS-CoV-2</a> <a href="/facts/Coronavirus_spike_protein/JTcbvUn2">spike protein</a> facilitates entry of the virus into the host cell membrane. </p>