Grouping of residues based on their contact interactions.

Based on the concept of energy landscape a grouping method of residues for reducing the sequence complexity in proteins is presented. For the Miyazawa and Jernigan matrix, rational groupings of 20 kinds of residues with minimal mismatches, under the consideration of local minima and statistics on correlation between the residues, are studied. A hierarchical tree of groupings relating to different numbers of groups N is obtained, and a plateau around N=8-10 is found, which may represent the basic degree of freedom of the sequence complexity in proteins.