Ca²⁺ switches the effect of PS-containing membranes on Factor Xa from activating to inhibiting: implications for initiation of blood coagulation

Calcium (Ca²⁺) plays a pivotal role in cellular and organismal physiology. The Ca²⁺ ion has an intermediate protein-binding affinity and thus it can serve as an on/off switch in the regulation of different biochemical processes. The serum level of ionized Ca²⁺ is regulated with normal ionized Ca²⁺ being in the range 1.10–1.3 mM. Hypocalcaemia (free Ca²⁺<1.1 mM) in critically ill patients is commonly accompanied by haemostatic abnormalities, ranging from isolated thrombocytopenia to complex defects such as disseminated intravascular coagulation, commonly thought to be due to insufficient functioning of anticoagulation pathways. A small amount of fXa (Factor Xa) produced by Factor VIIa and exposed tissue factor is key to initiating blood coagulation by producing enough thrombin to induce the later stages of coagulation. fXa must bind to PS (phosphatidylserine)-containing membranes to produce thrombin at a physiologically significant rate. In the present study, we show that overall fXa activity on PS-containing membranes is sharply regulated by a ‘Ca²⁺ switch’ centred at 1.16 mM, below which fXa is active and above which fXa forms inactive dimers on PS-exposing membranes. Our data lead to a mathematical model that predicts the variation of fXa activity as a function of both Ca²⁺ and membrane concentrations. Because the critical Ca²⁺ concentration is at the lower end of the normal plasma ionized Ca²⁺ concentration range, we propose a new regulatory mechanism by which local Ca²⁺ concentration switches fXa from an intrinsically active form to a form requiring its cofactor [fVa (Factor Va)] to achieve significant activity.