Why are the same protein folds used to perform different functions?

A small number of folding patterns describe in outline most of the known protein globules, the same folds being found in non-homologous proteins with different functions. We show that the 'popular' folding patterns are those which, due to some thermodynamic advantages of their structure, can be stabilized by a lot of random sequences. In contrast, the folds which are rarely or never observed in natural globular proteins can be stabilized only by a tiny number of random sequences. The advantageous folds are few, they tolerate various primary structures, and therefore they can and ought to perform different functions. A connection between the inherent 'weak points' of protein folding patterns and positions of active sites are discussed.