Five coplanar anion binding sites on one face of phospholipase A(2). Relationship to interface binding

We report the structures of the crystallographic dimer of porcine pancreati c IB phospholipase A(2) (PLA2) with either five sulfate or phosphate anions bound. In each structure, one molecule of a tetrahedral mimic MJ33 [1-hexa decyl-3-(trifluoroethyl)-sn-glycero-2-phosphomethanol] and the five anions are shared between the two subunits of the dimer, The sn-2-phosphate of MJ3 3 is bound in the active site of one subunit (A), and the alkyl chain exten ds into the active site slot of the second subunit (B) across the subunit-s ubunit interface. The two subunits are packed together with a large hydroph obic and desolvated surface buried between them along with the five anions that define a plane. The anions bind by direct contact with two cationic re sidues (R6 and K10) per subunit and through closer-range H-bonding interact ions with other polarizable ligands. These features of the "dimer" suggest that the binding of PLA2 to the anionic groups at the anionic interface may be dominated by coordination through H-bonding with only a partial charge compensation needed. Remarkably, the plane defined by the contact surface i s similar to the i-face of the enzyme [Ramirez, F., and Jain, M. K. (1991) Proteins: Struct., Funct., Genet. 9, 229-239], which has been proposed to m ake contact with the substrate interface for the interfacial catalytic turn over. Additionally, these structures not only offer a view of the active PL A2 complexed to an anionic interface but also provide insight into the envi ronment of the tetrahedral intermediate in the rate-limiting chemical step of the turnover cycle. Taken together, our results offer an atomic-resoluti on structural view of the i-face interactions of the active form of PLA2 as sociated to an anionic interface.