Adsorption of vitamin K-dependent blood coagulation proteins to spread phospholipid monolayers as determined from combined measurements of the surface pressure and surface protein concentration.

Spread phospholipid monolayers are particularly useful as model membranes in that changes in surface pressure (Deltapi) can be monitored in response to protein adsorption to the monolayer, thus providing a unique manner of assessing protein-membrane contact. In the present study, spread monolayers below their collapse pressures have been utilized to evaluate Ca2+-specific adsorption of several vitamin K-dependent coagulation proteins to monolayers that contain negatively charged phospholipid. From combined measurements of Deltapi and Gamma (the surface excess protein concentration), values of dGamma/dpi have been evaluated for different proteins with varying lipid composition of the monolayers. Using mixed, liquid-expanded monolayers at equivalent initial surface pressures (pii) and which contain different amounts of phosphatidylserine, phosphatidylcholine, and phosphatidylethanolamine, the dGamma/dpi of bovine prothrombin was shown to decrease monotonically with increasing protein affinity for the monolayer. For example, KD values of 7, 20, and 60 nM produced dGamma/dpi values of 14, 17, and 21 nmol m-1 mN-1, respectively. However, the trend in dGamma/dpi appears to originate from characteristics of the monolayer and not from those of the protein, since a much different adsorbate (i.e., a positively charged pyrene derivative) exhibited a similar trend in dGamma/dpi with monolayer composition. On the other hand, dGamma/dpi values of bovine prothrombin, human factor IX, human protein S, bovine protein C, and human protein C, determined using liquid-expanded phosphatidylserine monolayers, were essentially equivalent. Therefore, the five vitamin K-dependent proteins that were examined were equivalent in terms of the manner in which the gamma-carboxyglutamic acid (Gla) domain of each protein perturbed the surface pressure. This study shows that Ca2+-specific membrane contact sites in the Gla domain of the five proteins tested are similar despite the naturally occurring differences in the normal Gla domain sequence of these proteins.