http://www.cathdb.info/

CATH is a protein structure classification database. The database is organized by Class, Architecture, Topology, and Homologous super-family.

For any given structure classified in the database, CATH gives you information on the structure and function of that protein. The evolutionary relationships involving the structure of interest and other proteins in the database can also be determined.

CATH also gives an overall view of the known protein structure universe to date. You can find which folds and superfamilies are the most populated, for example, and which structures are rare in nature.

http://www.cathdb.info/wiki/doku.php?id=faq