Identification of a potential metal cation–π binding site in the structure of a thermophilic Bacillus stearothermophilus triosephosphate isomerase mutant

A potential metal cation–π interaction between a sodium cation (Na⁺) and the indole ring of a tryptophan residue was detected in the crystallographic structure (2 Å) of the thermophilic Bacillus stearothermophilus triosephosphate isomerase H12N/K13G mutant (bTIMmut). The cation–π binding site is located near the surface of the protein, the alkali metal ion facing the benzo ring of Trp9. The presence of Phe21 and Glu17 close to Trp9 could indicate an additional role for those residues in the stability of the sodium–indole interaction. The sodium cation lies in a position that is occupied by CE and NZ of Lys13 in the wild-type structure.