Effect of protein aggregation in the aqueous phase on the binding of membrane proteins to membranes

Analysis of the binding of hydrophobic peptides or proteins to membranes ge nerally assumes that the solute is monomeric in both the aqueous phase and the membrane. Simulations were performed to examine the effect of solute se lf-association in the aqueous phase on the binding of monomeric solute to l ipid vesicles. Aggregation lowered the initial concentration of monomeric s olute, which was then maintained at a relatively constant value at the expe nse of the aggregated solute, as the lipid concentration was increased. The resultant binding isotherm has a more linear initial portion rather than t he classic hyperbolic shape, Although this shape is diagnostic of solute se lf-association in the aqueous phase, various combinations of values for the membrane partition coefficient and the solute self-association constant wi ll generate similar isotherms. Data for cytochrome b(5) were analyzed and, when the self-association constant was estimated by gel filtration, a uniqu e value for the membrane partition coefficient was obtained. Thus, to obtai n a true partition coefficient the state of the solute in the aqueous phase must be known. If the concentration of the monomeric solute species in the aqueous phase can be independently determined, then, even with heterogeneo us aggregates, the true partition coefficient can be obtained.