Which of the following motifs is characterized by a repeated coiled structure?

The coiled coil motif is characterized by a repetitive helical structure formed by the intertwining of two or more alpha helices. This motif is prevalent in fibrous proteins and plays a critical role in providing structural stability and facilitating protein-protein interactions. The coiled coil structure is specifically defined by the arrangement of hydrophobic and charged residues along the helices, which promote the stability of the intertwined formation.

In contrast, the beta alpha beta unit contains alternating strands of beta sheets and alpha helices, forming a different type of interaction pattern. The hairpin motif consists of a short section of beta-sheet structure that folds back on itself, while the Greek key motif involves a series of beta strands that form a distinctive knot-like shape. Each of these motifs represents unique structural arrangements that contribute differently to protein architecture, but the coiled coil specifically focuses on the repeated helical nature.