Membrane modulates affinity for calcium ion to create an apparent cooperative binding response by annexin a5

摘要: Isothermal titration calorimetry was used to characterize the binding of calcium ion (Ca2+) and phospholipid peripheral membrane-binding protein annexin a5. The a binary mixture neutral an acidic phospholipid, specifically phosphatidylcholine phosphatidylserine in form large unilamellar vesicles. To stringently define mode binding, global fit data collected presence absence membrane concentrations exceeding saturation performed. A partition function defined contribution all heat-evolving or heat-absorbing states. We find that a5 binds Ca2+ solution according simple independent-site model (solution-state affinity). In phosphatidylserine-containing liposomes, differentiates into two classes sites, both which have higher affinity compared with solution-state affinity. As scenario, sites within each class were described model. Transitioning from state lower membrane-associated, state, results cooperative binding. discuss how weak association prior influx is basis for response toward Ca2+, role organization this response.