Proteins have a limited number of structural motifs. This is the HELIX-LOOP-HELIX motif.

This helix-loop-helix motif is called the "EF HAND" and represents the CALCIUM-binding motif in many calcium-binding proteins (based on 5cpv.pdb, Swain, Kretsinger & Amma J. Biol. Chem 264 16620 (1989)). The calcium ion is bound to three Asp side chains.

This is a HAIRPIN BETA SHEET motif.  Note that the beta strands are anti-parallel (direction of the arrows).

This is the BETA-ALPHA-BETA motif.  Note that the beta strands are parallel (direction of the arrows).