Computational studies of the activation of lipases and the effect of a hydrophobic environment.

We have investigated the activation pathway of three wild type lipases and three mutants using molecular dynamics techniques combined with a constrained mechanical protocol. The activation of these lipases involves a rigid body hinge-type motion of a single helix, which is displaced during activation to expose the active site and give access to the substrate. Our results suggest that the activation of lipases is enhanced in a hydrophobic environment as is generally observed in experiments. The energy gain upon activation varies between the different lipases and depends strongly on the distribution of the charged residues in the activating loop region. In a low dielectric constant medium (such as a lipid environment), the electrostatic interactions between the residues located in the vicinity of the activating loop (lipid contact zone) are dominant and determine the activation of the lipases. Calculations of the pKas qualitatively indicate that some titratable residues experience significant pK shifts upon activation. These calculations may provide sufficient details for an understanding of the origin and magnitude of a given electrostatic effect and may provide an avenue for exploring the activation pathway of lipases.