Surface exclusion and molecular mobility may explain Vroman effects in protein adsorption

Data on protein adsorption usually show that for increasing surface coverage the adsorption velocity decreases much faster than linearly. This contrasts to the classical Langmuir model with an adsorption velocity proportional to the number of unoccupied binding sites. It has been shown that this non-linearity may explain phenomena like transient adsorption of different proteins from a protein mixture or dilution-dependent changes in binding properties, collectively called Vroman effects. However, the molecular mechanisms explaining this non-linear behavior remain to be established. A Monte Carlo simulation model is presented that incorporates steric hindrance, lateral mobility and mutual interactions of adsorbed molecules. Experimental data on the adsorption kinetics of prothrombin and annexin V, a recently discovered anticoagulant protein, at phospholipid bilayers are analyzed with this model. A major conclusion is that the steep decline in adsorption rates for increasing surface coverage can be explained, without assuming repulsive forces between adsorbed molecules, as a surface exclusion effect combined with lateral mobility of adsorbed molecules. The fact that annexin V shows this effect to a much lesser degree than prothrombin is tentatively explained by clustering of adsorbed annexin V molecules. A qualitative effect of lateral mobility on the adsorption characteristics, predicted by the model, is confirmed in experiments in which the fluidity of the bilayers was manipulated.