Fig. 9.

Normalized fluorescence–pCa curves for thin filaments in solution containing control Tm or Δ23Tm. The IAANS-Tn fluorescence intensity increased when Ca²⁺ was added to thin filaments containing Δ23Tm (diamonds and thin line), cTm (squares and solid line), or full length recombinant ala-ser Tm (multiplication symbol and bold line). For each Tm, pooled data for several titrations were globally fit to a model of cooperative binding to a linear lattice [44]. Cooperativity parameter Y is the fold-increase in Ca²⁺ affinity for a troponin with a nearest neighbor Tn in the Ca²⁺-bound state, and Y1/2 ≈ n_H. Binding was less cooperative in the presence of Δ23Tm: Y = 2.77 ± 0.12 vs values for cTm and ala-ser Tm of 4.49 ± 0.32 and 5.96 ± 0.45. Corresponding Ca²⁺ affinities differed minimally by no more than a few percent: 1.52 ± 0.03 × 106, 1.71 ± 0.03 × 106, and 1.61 ± 0.03 × 106 M⁻¹