Structural evidences for a secondary gold binding site in the hydrophobic box of lysozyme.

A new crystal structure is reported here for the adduct formed in the reaction between NH4 [Au(Sac)2], AuSac2, a cytotoxic homoleptic gold(I) complex with the saccharinate ligand, and the model protein hen egg white lysozyme. To produce this adduct, AuSac2 breaks down and releases both saccharinate ligands. The resulting Au(I) ions bind the protein to ND1 and NE2 atoms of His15 but also to SD atom of the zero-solvent accessible Met105 side chain, which is located in the protein hydrophobic box. The unexpected existence of this secondary gold(I) binding site is confirmed by spectroscopic and spectrometric measurements in solution.