Identifying key electrostatic interactions in Rhizomucor miehei lipase:: the influence of solvent dielectric

The conformational change associated with the interfacial activation of Rhizomucor miehei lipase involves the displacement of an m-helical lid (residues 82-96) away from the active site on moving from water (high dielectric) to lipid (low dielectric). The presence of two media of very different dielectric properties suggests that electrostatic inter-actions play an important role in this process. We have used linearized Poisson-Boltzmann calculations to examine the key electrostatic interactions which contribute to lid stability in the closed and open states. It is the two charged residues of the lid, Arg86 and Asp91, that form the strongest electrostatic interactions with the rest of the protein. We identify key residues whose interactions with the lid are significantly perturbed by the change in the dielectric of the medium: Asp61, Arg80, Lys109, Glu117 and the active-site residues Asp203 and Asp256, all of which lie within approximately 20 Angstrom of the lid. We suggest that these residues are good candidates for site-specific mutation studies, which could help elucidate their role in the lipase activation mechanism.