It has been noted that natural proteins adapt only a limited number of folds . The question why and how nature selected these small number of folds has intrigued several investigators.
Structures differ drastically in terms of their designability.
A small number of highly designable structures emerge with a number of associated sequences much larger than the average. These highly designable structures possess proteinlike secondary structures, motifs, and even tertiary symmetries. In addition, they are thermodynamically more stable and fold faster than other structures. These results suggest that protein structures are selected in nature because they are readily designed and stable against mutations, and that such a selection simultaneously leads to thermodynamic stability.