I have two protein A and B. I want to see an interaction between this two protein using protein-protein docking in clus pro 2.0. When I docked Phosphorylated A protein with protein B unphosphorylated. The phosphate group from Protein A getting removed. Protein A is a receptor, Protein B is Ligand. I added the phosphate group to a serine residue of minimized protein A using Discovery Studio 4.0. Can anyone suggest me how to keep phosphate attached to protein A in protein complex?