Interaction of monodisperse anionic amphiphiles with the i-face of secreted phospholipase A2.

Pancreatic IB phospholipase A(2) (PLA2) forms aggregates of defined size with monodisperse alkyl sulfates in the premicellar concentration range. As an extension of the interfacial kinetic paradigm, results are interpreted in terms of a model in which several amphiphile molecules bind along their polar headgroup to the interface binding region (i-face) of PLA2. The resulting complex, E(#), has a half-micellar structure, and it acts as an "amphiphile" in the aqueous phase. E(#) not only self-aggregates but also binds hydrophobic probes and interacts with hydrophobic surfaces. As expected, resonance energy transfer from the tryptophan donor in PLA2 to an acceptor probe partitioned in E(#) shows a biphasic dependence as the probe coexisting with PLA2 is diluted at higher alkyl sulfate concentrations. The gel-permeation behavior of PLA2 at premicellar alkyl sulfate concentrations is also biphasic. For example, above 1.2 mM decyl sulfate (CMC = 3.5 mM) PLA2 elutes as a single sharp peak, presumably the self-aggregate of E(#) with apparent molecular mass of 120-150 kDa. At 0.4-1 mM decyl sulfate the retention volume is even larger than that for the 14 kDa PLA2. This anomalous retention is attributed to the interaction of the hydrophobic region of E(#) with the hydrophobic patches on the gel-permeation matrix. Elution behavior of the self-aggregated E(#) form of site-directed mutants in dodecyl sulfate suggests that certain substitutions in the conserved hydrogen-bonding network have a significant effect on the aggregate size. These results suggest a role for the network in the amphiphile binding along the i-face of PLA2, presumably through a change in the anion coordination ligands.