Structure, function and interfacial allosterism in phospholipase A2: insight from the anion-assisted dimer.

Enzymes that function on membrane surfaces offer many challenges to understanding structural and functional details due to the difficulties of obtaining relevant information of the protein in a physiological environment. Focusing on this aspect of structural biology, it is important to develop conditions that mimic the interaction of membrane proteins with their binding surface and ultimately the mechanisms of action. This approach has been used to characterize the allosteric nature of secreted phospholipase A2 (PLA2) to its substrate interface. The breakthrough here was to crystallize the pancreatic group-IB PLA2 in an anion-assisted dimer with five coplanar phosphate anions bound. In the anion-assisted dimer structure one molecule of a tetrahedral mimic inhibitor and five anions are shared between the two subunits of the dimer. The sn-2-phosphate of the inhibitor, which mimics the tetrahedral intermediate of the esterolysis reaction, is bound in the active site of one subunit, and the alkyl chain extends into the active site slot of the second subunit across the subunit-subunit interface. This interface-bound structural mimic provided insight into the active site environment and specific anionic interactions to the i-face of the protein. The presence or absence of a single critical active site water, corresponds to the difference between the activated or inactivated form of the enzyme. The anion-assisted dimer structure supports a calcium coordinated nucleophilic water mechanism, with its pK(a) modulated by this assisting water. This working model has been further strengthened with an enzyme-product complex structure solved with the hydrolysis products of the substrate PAF also bound to the anion-assisted dimer form of PLA2. Additional confirmation of the assisting-water mechanism comes from a structure of the inactive zymogen proPLA2 also crystallized in an anion-assisted dimer. Remarkably, the assisting water present in the activated complex is absent in this proPLA2 structure.