Cooperativity of protein binding to vesicles.

Electrostatics role is studied in protein adsorption to phosphatidylcholine (PC) and PC/phosphatidylglycerol (PG) small unilamellar vesicles (SUVs). Protein interaction is monitored vs. PG content at low ionic strength. Adsorption of lysozyme, myoglobin and bovine serum albumin (BSA) isoelectric point (pI) is investigated in SUVs, along with changes in protein fluorescence emission spectra. Partition coefficients and cooperativity parameters are calculated. At pI, binding is maximum while at lower/higher pHs binding drops. In Gouy-Chapman model activity coefficient goes with square charge number, which deviations indicate asymmetric location of anionic lipid in the bilayer inner leaflet, in agreement with experiments and molecular dynamics simulations. Vesicles bind myoglobin anti-cooperatively and lysozyme/BSA cooperatively. Hill coefficient reflects subunit cooperativity of bi/tridomain proteins.