Structural insights and binding of a natural ligand, succinic acid with serine and cysteine proteases.

In the age of growing infectious diseases, there is a great demand for new inhibitors which can exhibit minimum side effects. Owing to the importance of proteases in life cycle and invasion, they have been projected as attractive targets for structure based drug designing against microbes including viruses. Here we report the inhibitory activity of a well known natural compound succinic acid against both serine and cysteine proteases. The ligand is found co-crystallized with Bovine pancreatic trypsin in one of our crystallization trials and the diffraction data up to1.9 Å reveal its interactions with the catalytic triad residues Histidine 57 and Serine 195. Binding of the ligand with these proteases have been validated using caseinolysis inhibition. With trypsin, ITC analysis showed tight binding of the ligand, resulting in change in Gibb's free energy (ΔG) by -20.31 kJ/mol. To understand the existence of succinic acid at the active site, molecular docking was performed and it revealed binding of it with trypsin and papain at corresponding active sites. This dual inhibitory activity of natural ligand, succinic acid can be accounted for the recent reports on anti-viral property of plant extracts where dicarboxilic fatty acids are normally abundant.