Figure 2.

Schematic of all potential species involved in the model for binding of Ca²⁺ and membrane to annexin. The model is analogous to the MWC model of O₂ binding hemoglobin (48), where the MWC states tensed (T) and relaxed (R) are replaced by solution and membrane-bound states. In the absence of membrane, all five Ca²⁺ sites are low affinity (K₀) and exothermic (represented as squares). In the presence of membrane, the five Ca²⁺-binding sites differentiate into two classes: two high-affinity (K_1a) endothermic Ca²⁺ sites (represented as circles) and three low-affinity (K_1b) exothermic Ca²⁺ sites (represented as squares). Transitioning to the membrane-bound state in the absence of Ca²⁺ is dictated by the affinity K_L, which is analogous to the equilibrium constant describing the transition between the T and R states in the MWC model. As in the classic MWC model, in each state (solution or membrane-bound), binding sites are considered to be independent and equivalent within each class.