[Structure and family of proteins, a puzzle]

It now seems quite clear that the number of modes of three-dimensional arrangement of peptide chains of globular proteins is much smaller than the great sequential variability would suggest. Structural families and super families are formed with members which show little if any sequential identity (in the statistical sense) with older members of these families. In addition, it is evident that many proteins are formed like puzzles, by the juxtaposition of domains and sub-domains. Each brick usually has a well-defined intrinsic biological function. The components of these macro molecular puzzles have very different sizes, from a few amino acids to complete protein globules of several hundreds of amino acids, via intermediary local secondary ans super-secondary structures. This building system, the result of a long evolution after duplication or replication of genes, offers banks (sequences and three-dimensional structures) while limiting the complexity of the protein world.