Figure A2.

Analysis of Ca binding by TnC. (A) Computed time course of establishment of equilibrium between Ca²⁺ and EGTA. At time 0, all Ca²⁺ bound to EGTA. The kinetic model was used to compute the change in unbound Ca²⁺. The effects of including TnC (labeled +TnC) or considering EGTA alone (−TnC) are shown. In the presence of TnC, equilibration of Ca²⁺–EGTA takes longer (~2 ms), but the eventual free [Ca²⁺] level (67 nM) is the same whether TnC is included or not. (B) Calculated time courses of Ca²⁺ binding by TnC. First Ca²⁺ binds rapidly (blue) and second Ca²⁺ (red) more slowly. Unbound [Ca²⁺] attains a steady value (600 nM in this example) more rapidly than CaTn, reflecting the binding kinetics of EGTA. EGTA effectively buffers Ca²⁺ even when Ca²⁺ is being bound by TnC.