How do lipases and esterases work: the electrostatic contribution.

This work explores the role of one of the factors explaining lipase/esterase activity: the contribution of electrostatic interactions to lipase/esterase activity. The electrostatic potential distribution on the molecular surface of an enzyme as a function of pH determines, to a large extent, the enzyme's pH activity profile. Other important factors include the presence and distribution of polar and hydrophobic residues in the active cleft. We have mapped the electrostatic potential distribution as a function of pH on the molecular surface of nine lipases/esterases for which the 3D structure is experimentally known. A comparison of these potential maps at different pH values with the corresponding pH-activity profile, pH optimum or pH range where the activity displayed by the enzyme is maximum, has revealed a considerable correlation. A negative potential in the active site appears correlated with maximum activity towards triglycerides, which has prompted us to propose a model for product release ('The electrostatic catapult model') after cleavage of an ester bond. At the same time as the bottom of the active site cleft becomes negatively charged, other nearby regions also titrate and become negatively charged when pH becomes more alkaline, for some of the studied lipases. If such lipases also show phospholipase activity (such as guinea pig lipase-related proteins 2 chimera) we raise the hypothesis that such other titratable regions after becoming negatively charged might stabilise the positive charge present in the polar head of phospholipids, such as phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine. The distribution of polar, weak polar and non-polar residues on the molecular surface of each studied lipase, in particular the active site region, was compared for all the lipases studied. The combination of graphical visualisation of the electrostatic potential maps and the polarity maps combined with knowledge about the location of key residues on the protein surface allows us to envision atomic models for lipolytic activity.