Estimation of a stronger heparin binding locus in fibronectin domain III using thermodynamics and molecular dynamics.

The HEP II (Heparin-binding site II) region of fibronectin (FN) containing domain III plays a crucial role in cell adhesion and migration through heparin-binding on the cell surface. There are two such fibronectin heparin interacting peptide (FHIP I and FHIP II) sequences present in HEP II. However, the molecular principles by which these sites orchestrate heparin-binding processes are poorly understood. Such knowledge would have great implications in the therapeutic targeting of FN. With this aim, we have explored the binding studies of FHIP I and FHIP II with heparin using various biophysical methods. A fluorescence melting study specifically revealed the preference of heparin for domain III in FN, indicating the key contribution of FHIP I and FHIP II in heparin binding. In isothermal titration calorimetry (ITC), the higher binding affinity observed for FHIP II (∼10 mol) compared to FHIP I (∼10 mol) is expected due to the presence of a superior cluster of Arg and Lys residues in FHIP II, which can facilitate specific H-bonding interactions with heparin. Based on heat capacity changes, the key role of H-bonding, electrostatic and hydrophobic interactions was demonstrated in binding. Finally, the molecular docking and MD simulation results reinforced that the interaction of heparin (dodecasaccharide) is stronger and stable with the FHIP II peptide. The results described here suggest that these peptides provide all the structural and thermodynamic elements necessary for heparin-binding of HEP II of FN. Subsequently, it can be concluded that FHIP II could be a better location for therapeutic intervention in cell adhesion activity by FN.