Molecular recognition between bacterial phosphorothioate DNA and sulfur-binding domain (SBD): competition between the water cage and chalcogen-hydrophobic packet.

Bacterial DNA phosphorothioation (PT) physiologically and stereo-specifically replaces a non-bridging oxygen in a phosphate link with a sulfur atom, which can be recognized by a highly conserved sulfur-binding domain (SBD). Here we conducted thermodynamic integration (TI), molecular dynamics simulation, and quantum chemical calculations to decipher the specific molecular interactions between PT-DNA and SBD in type IV restriction enzyme ScoMcrA. The TI-calculated binding affinity of (5'-CCGGCCGG-3') is larger than that of (5'-CCGGCCGG-3') by about 7.4-7.7 kcal mol. The binding difference dominantly stems from hydration energy of non-phosphorothioate DNA (9.8-10.6 kcal mol) in aqueous solution, despite the persistent preference of 2.6-3.2 kcal mol in the DNA-SBD MD simulations. Furthermore, the quantum chemical calculations reveal an unusual non-covalent interaction in the phosphorothioate-binding scenario, where the S⋯N chalcogen bond prevails the S⋯HC vdW interactions from the adjacent residues H116-R117-Y164-P165-A168. Thus, the chalcogen-hydrophobic interaction pulls PT-DNA into the SBD binding pocket while the water cage pulls a normal DNA molecule out. The synergetic mechanism suggests the special roles of the proline pyrrolidine group in the SBD proteins, consistent with the experimental observations in the X-ray crystallography and structural bioinformatics analysis.