Stereochemistry of guanidine–metal interactions: Implications for L ‐arginine–metal interactions in protein structure and function

The geometries of 150 guanidine–metal ion interactions retrieved from crystal structures deposited in the Cambridge Structural Database have been analyzed. Metal ions exhibit a preference for anti coordination stereochemistry in the plane of the unprotonated guanidine group, usually in chelate complexes with a diguanidine moiety, but syn‐oriented interactions are occasionally found for single guanidine–metal interactions. Three L ‐arginine‐metal coordination interactions are found in metalloenzyme structures deposited in the Protein Data Bank: biotin synthase from E. coli , His‐67 → Arg human carbonic anhydrase I, and inactivated B. caldovelox arginase complexed with L ‐arginine. In these proteins, L ‐arginine‐metal coordination adopts syn/out‐of‐plane and anti/in‐plane coordination stereochemistry. The implications of these results for L ‐arginine‐metal interactions in protein structure and function are discussed. Although such interactions are rare, this analysis serves as a useful reference point for the growing interest in enzymes containing L ‐arginine residues that function as general bases or metal ligands. Proteins 2006. © 2006 Wiley‐Liss, Inc.