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Super-secondary motifs

As the number of solved structures began to increase it became apparent that different folds often possessed similar arrangements of a two to four consecutive secondary structures. These recurring units are called super-secondary structures[Rao & G., 1973] or motifs. Possibly the most common super-secondary motif is the $\beta\alpha\beta$ unit[Sternberg & Thornton, 1976], found in folds with parallel $\beta$-sheets. The $\beta$-hairpin (two $\beta$-strands connected by a turn) and the four-strand ``Greek key'' motif[Richardson, 1981,Richardson, 1986] are other common super-secondary structures.