Phosphatidylinositol-specific phospholipase C forms different complexes with monodisperse and micellar phosphatidylcholine.

Phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus forms a premicellar complex E(#) with monodisperse diheptanoylphosphatidylcholine (DC(7)PC) that is distinguishable from the E complex formed with micelles. Results are interpreted with the assumption that in both cases amphiphiles bind to the interfacial binding surface (i-face) of PI-PLC but not to the active site. Isothermal calorimetry and fluorescence titration results for the binding of monodisperse DC(7)PC give an apparent dissociation constant of K(2) = 0.2 mM with Hill coefficient of 2. The gel-permeation, spectroscopic, and probe partitioning behaviors of E(#) are distinct from those of the E complex. The aggregation and partitioning behaviors suggest that the acyl chains in E(#) but not in E remain exposed to the aqueous phase. The free (E) and complexed (E(#) and E) forms of PI-PLC, each with distinct spectroscopic signatures, readily equilibrate with changing DC(7)PC concentration. The underlying equilibria are modeled and their significance for the states of the PI-PLC under monomer kinetic conditions is discussed to suggest that the Michaelis-Menten complex formed with monodisperse DC(7)PC is likely to be E(#)S or its aggregate rather than the classical monodisperse ES complex.