On the supersecondary structure of acid proteases.

It was found that polypeptide chains of porcine pepsin molecules and mold acid proteases consist of four topologically equivalent structural units. Each pair of units forms domain. The symmetrical packing of two units within each domain is the important structural feature of acid proteases. Although the primary structures of the four structural units are in general not homologous there are close similarities of the sequences of some topologically equivalent elements. These data are considered as the development of the idea on gene duplications and the subsequent fusion during evolution of acid proteases.