Understanding the Structural Role of Metal-Ligand Bond in Metalloprotein Using Single Molecule Atomic Force Microscopy

Metalloprotein constitutes more than thirty percent of all proteins in nature in which the metal plays critical functional and structural roles. The forming of metal-ligand bond, which is usually between metal and residues from protein matrix, can support protein three dimensional structures and facilitate protein folding. Thus, understanding the metal-ligand bond strength in protein can provide important information of its structural role. Here we used single molecule Atomic Force Microscopy  to directly determine the mechanical strength of ferric-thiolate bonds in a simple iron-sulfur protein rubredoxin. We observed that the ferric-thiolate bond ruptured at surprisingly low forces of ~200 pN, one order of magnitude lower than that of typical covalent bonds. And the mechanical strength of Fe-thiolate bonds is observed to correlate with the covalency of the bonds. Our results shed new lights on the nature of Fe-thiolate bonds, and raise interesting question about the stability contribution from these metal-ligand bonds for the protein.