Helix-Loop-Helix

"Helix-Loop-Helix" is characterized by two a helices connected by a loop. It should not be confused with "Helix-Turn-Helix".  In fact, these two structures are quite different, but the Helix-Loop-Helix looks like leucine zipper.  Example: myoblast determination proteins (MyoD).  

(a)

(b)

Figure 4-F-6. The transcription factor MyoD.  (a) The domain organization.  (b) The structure of the helix-loop-helix motif.  PDB ID = 1MDY.

Review Article:

Helix-Loop-Helix Proteins: Regulators of Transcription in Eucaryotic Organisms - Molecular and Cellular Biology, 2000.