When there are only two antiparallel β-strands, like in the Figure on the right, the structural motif is called a β-hairpin. The loop between the two strands is called a β-turn.
Short turns and longer loops are essential in protein 3D structures, connecting strands to strands, strands to α-helices, or helices to helices. The amino acid sequences in loop regions are often highly variable within a protein family. Nevertheless, sometimes, when a loop has some specific function, for example, interaction with another protein, the sequence may be conserved. Loop length in organisms living at elevated temperatures (thermophilic organisms) is usually shorter than in proteins from lower-temperature family members. It presumably gives a protein additional stability at high temperatures, preventing its unfolding and denaturation. During sequence alignment and modeling, when it is essential to have an accurate sequence alignment, the highly variable length of loops justifies the localization of insertions and deletions in the amino acid sequence to loop regions.
The following section will examine how secondary structure elements connect, forming common motifs, folds, and domains