Knobs into holes packing

Knobs into holes packing is a protein folding motif that occurs mainly in alpha helix or coiled coil domains. One such example is fibrinogen fibril formation.

This motif occurs in coiled coil motifs to maintain and reach a 20 degree "left hand crossing angle that is characteristic of coiled coils". This motif looks much like two springs slammed together. (See also Helix bundle)

Alpha helix is little tighter packed in knobs into holes packing, having on average 3.5 amino acid residues per turn. Side chains are packed in the gaps of the complementary alpha helix and these helices are parallel to each other.