Ion-binding to phospholipids. Interaction of calcium with phosphatidylserine

Abstract

The binding of Ca2+ to monolayers and bilayers of phosphatidylserine has been investigated as a function of pH, ionic strength (NaCl concentration) and Ca2+ concentration using surface and colloid chemical techniques. The molar ratio of lipid to bound calcium decreases to 2 as the Ca2+ concentration is increased to about 0.1 mM. At [Ca2+] greater than 0.1 mM a 1:1 complex is formed. The apparent binding constant Ka ranges from about approximately 10(6) - 10(4) l/mol depending on the Ca2+ concentration. After allowing for electrostatic effects and neighbour group interactions, the intrinsic binding constant Ki of the phosphorylserine polar group at pH 7 (I = 0.01 M), where it carries a net negative charge of one, is approximately 10(4) l/mol; consistent values for Ki were obtained using several independent approaches. Ka for Ca2+ binding decreases with increasing NaCl concentration because the monovalent cations compete with Ca2+ for the same binding site. Na+ and K+ are equally effective in displacing 45Ca2+ adsorbed to monolayers of phosphatidylserine, both with respect to the kinetics and the equilibrium of the displacement. Ka for the reaction between phosphatidylserine and monovalent cations is about 10(3)-fold smaller than that of Ca2+. An investigation of the binding of Mn2+ to phosphatidylserine by both surface chemical and nuclear magnetic resonance methods shows that this cation has a similar binding constant to that of Ca2+. The Ca2+-binding capabilities of monolayers containing only carboxyl groups (i.e. arachidic acid) and phosphodiester groups (i.e. dicetyl phosphate) have also been determined; the apparent pK for the - COOH group in monolayers is larger than or equal to 9 and that for the phosphodiester group is less than 4. Since these groups do not retain the same pK values when they are in close proximity in the phosphorylserine group, the relative contributions of the two groups to the binding of Ca2+ to phosphatidylserine is not obvious.