The Heptad Repeat of The Coiled-coil Structure

A coiled-coil protein consists of two identical strands of amino acid sequences that wrap around each other as shown in Figure-1.

Figure-1 Coiled-coil protein structure

The amino acids in a coiled-coil structure reside on seven different structural positions on the coil, forming a heptad repeat, as illustrated in Figure-2.

Figure-2 The seven positions of the coiled-coil structure

Each strand of a coiled-coil protein may be viewed as a repeated consensus substrings of the form (a-b-c-d-e-f-g)n, where a, b, c,...,g are the seven different structural positions on the coil. The first and fourth position (a and d) are generally apolar or hydrophobic amino acids. Discontinuities in the heptad pattern, such as stutters are quite frequent. The mapping of the amino acids in the strand to the seven positions is hence not always continuous. When the two strands coil around each other positions a and d are internalized, stabilizing the structure, while positions b, c, e, f, g are exposed on the surface of the protein.