Beta-Helix
 
A b-helix is a type of protein motif or domain found in a very few protein structures. It is characterized by an unusual parallel b-sheet topology formed from three parallel b-sheets wound together into a right-handed helical structure (Fig. 1). The b-strands in each b-sheet are short, having only 2 to 5 residues. Each coil of the b-helix has the same three-dimensional arrangement of a group of secondary structure elements and is thus similar to other coiled repeating structures, such as the b-roll, composed of two parallel b-sheets forming a b-helix, and the leucine-rich repeat of ribonuclease inhibitor that is composed of repeating a-helices and b-strands forming a horseshoe-shaped structure. In the b-helix, the side chains of repeating residues are packed into the center of the helix and interact with one another to form, for example, “asparagine ladders,” “serine stacks,” and/or “aromatic stacks.” 

Figure 1. Orthogonal views of the b-helix of pectate lyase E (1), showing the three parallel b-sheets. For clarity, only the b-strand secondary structure (as purple arrows) and loops connecting the strands (yellow) are shown. Short helical regions and the N- and C-terminal loops have been removed. This figure was generated using Molscript (2) and Raster3D (3, 4).
References
1.M. D. Yoder, S. E. Lietzke, and F. Jurnak (1993) Structure 1, 241–251. 
2.P. J. Kraulis (1991) J. Appl. Crystallogr. 24, 946–950. 
3.E. A. Merritt and M. E. P. Murphy (1994) Acta Crystallogr. D50, 869–873. 
4. D. J. Bacon and W. F. Anderson (1988) J. Mol. Graphics 6, 219–222.