Structural motifs are closed into cycles in proteins

β-Hairpins, triple-strand β-sheets and βαβ-units represent simple structural motifs closed into cycles by systems of hydrogen bonds. Secondary closing of these simple motifs into large cycles by means of different superhelices, split β-hairpins or SS-bridges results in the formation of more complex structural motifs having unique overall folds and unique handedness such as abcd-units, φφ-motifs, five- and seven-segment α/β-motifs. Apparently, the complex structural motifs are more cooperative and stable and this may be one of the main reasons of high frequencies of occurrence of the motifs in proteins.

Research highlights
► β-Hairpins and βαβ-units represent simple motifs closed into cycles by H-bonds.
► Secondary closing of the simple motifs results in unique structural motifs.
► One of the two forms of β-hairpins and triple-strand β-sheets is selected.
► Apparently, secondary closing results in more cooperative and stable motifs.