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

Jacob W. Gauer, Kristofer J. Knutson, Samantha R. Jaworski, Anne M. Rice, Anika M. Rannikko, Barry R. Lentz, Anne Hinderliter

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

Abstract

Isothermal titration calorimetry was used to characterize the binding of calcium ion (Ca2+) and phospholipid to the peripheral membrane-binding protein annexin a5. The phospholipid was a binary mixture of a neutral and an acidic phospholipid, specifically phosphatidylcholine and phosphatidylserine in the form of large unilamellar vesicles. To stringently define the mode of binding, a global fit of data collected in the presence and absence of membrane concentrations exceeding protein saturation was performed. A partition function defined the contribution of all heat-evolving or heat-absorbing binding states. We find that annexin a5 binds Ca2+ in solution according to a simple independent-site model (solution-state affinity). In the presence of phosphatidylserine-containing liposomes, binding of Ca 2+ differentiates into two classes of sites, both of which have higher affinity compared with the solution-state affinity. As in the solution-state scenario, the sites within each class were described with an independent-site model. Transitioning from a solution state with lower Ca 2+ affinity to a membrane-associated, higher Ca2+ affinity state, results in cooperative binding. We discuss how weak membrane association of annexin a5 prior to Ca2+ influx is the basis for the cooperative response of annexin a5 toward Ca2+, and the role of membrane organization in this response.

Original languageEnglish (US)
Pages (from-to)2437-2447
Number of pages11
JournalBiophysical journal
Volume104
Issue number11
DOIs
StatePublished - Jun 4 2013

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