Question

Define the heptad repeat unit in keratin intermediate filaments and discuss its importance to the structure of hair.

Answer 1

Each seventh (That's why it is known as heptad) residue in each propeller is leucine.

The two helicoids (helix are linked in a criss-cross manner by weak interactions, such as van der Waals forces. The manner in which the repetition of the leucine residues is arranged is important because such repeating allows the propellers to interact with one another through complementary repetitions.

Thus, for example, the repeating residues can be hydrophobic, establishing Van der Waals forces among them or they can have opposite electrical charges allowing ionic interactions. In addition, the two helices can be joined by disulfide bridges formed by neighboring residues of cysteine: these unions account for the physical properties of hair:

It is extensible and can be lengthened up to twice its length due to the elasticity of the a-helices, by breaking the weak interactions between neighboring helix.

However, the covalent disulfide bridges resist breaking and the fiber returns to its original state when it is released from the traction force.