When α-helices supercoil around one another, this structure is called a:

The correct answer is coiled coil. When α-helices supercoil around one another, they form a special structural motif known as a coiled coil. This structural formation is characterized by two or more α-helices twisted around each other in a left-handed manner, often stabilized by hydrophobic interactions between the side chains of the amino acids in the helices. This arrangement allows for increased stability and is commonly observed in various proteins, particularly those involved in structural or regulatory roles in the cell.

Coiled coils are often found in structural proteins such as keratin and in transcription factors where they can facilitate dimerization and interaction with other proteins. The specific arrangement and interactions help maintain the integrity of the protein structure and contribute to its function.

The other options present distinct structural motifs that do not involve the same arrangement of α-helices supercoiling around one another. For example, a helical bundle refers to a group of α-helices that may exist separately but do not wrap around each other, while the helix-loop-helix motif describes a structure where helices are connected by flexible loop regions, not helices being coiled around one another. Similarly, a beta fold refers to a different structure involving beta sheets rather than helical formations.