Contiguous binding of decylsulfate on the interface-binding surface of pancreatic phospholipase A2

Pig pancreatic IB phospholipase A 2 (PLA2) forms three distinguishable premicellar E i (#) ( i = 1, 2, and 3) complexes at successively higher decylsulfate concentrations. The Hill coefficient for E 1 (#) is n 1 = 1.6, and n 2 and n 3 for E 2 (#) and E 3 (#) are about 8 each. Saturation-transfer difference nuclear magnetic resonance (NMR) and other complementary results with PLA2 show that decylsulfate molecules in E 2 (#) and E 3 (#) are contiguously and cooperatively clustered on the interface-binding surface or i-face that makes contact with the substrate interface. In these complexes, the saturation-transfer difference NMR signatures of (1)H in decylsulfate are different. The decylsulfate epitope for the successive E i (#) complexes increasingly resembles the micellar complex formed by the binding of PLA2 to preformed micelles. Contiguous cooperative amphiphile binding is predominantly driven by the hydrophobic effect with a modest electrostatic shielding of the sulfate head group in contact with PLA2. The formation of the complexes is also associated with structural change in the enzyme. Calcium affinity of E 2 (#) appears to be modestly lower than that of the free enzyme and E 1 (#). Binding of decylsulfate to the i-face does not require the catalytic calcium required for the substrate binding to the active site and for the chemical step. These results show that E i (#) complexes are useful to structurally characterize the cooperative sequential and contiguous binding of amphiphiles on the i-face. We suggest that the allosteric changes associated with the formation of discrete E i (#) complexes are surrogates for the catalytic and allosteric states of the interface activated PLA2.