TABLE III.
Rate Constants Assumed in the Model
| 1 | 2 | 3 | 4 |
|---|---|---|---|
| Reaction | Forward rate | Reverse rate |
Ratio |
| s−1 | |||
| A | |||
| Ca2+ + ATP ⇔ CaATP | 0.1566 × 108 M−1 s−1 * | 34,461 | 2,200 μM |
| B | |||
| Ca2+ + Fluo ⇔ CaFluo | 2.676 × 108 M−1 s−1 | 137 | 0.51 μM |
| Pr + Fluo ⇔ PrFluo | 0.1149 × 108 M−1 s−1 | 4,216 | 367 μM |
| Ca2+ + PrFluo ⇔ CaPrFluo | 0.172 × 108 M−1 s−1 | 32.9 | 1.91 μM |
| Pr + CaFluo ⇔ CaPrFluo | 0.1149 × 108 M−1 s−1 | 15,777 | 1,373 μM |
| C | |||
| Ca2+ + Parv ⇔ CaParv | 0.479 × 108 M−1 s−1 | 0.574 | 0.012 μM |
| Mg2+ + Parv ⇔ MgParv | 0.000379 × 108 M−1 s−1 | 3.45 | 91.0 μM |
| D | |||
| Ca2+ + E ⇔ CaE | 108 M−1 s−1 | 103 | 10 μM |
| CaE ⇔ CaE′ | 500 s−1 | 1,200 | 2.4 |
| Ca2+ + CaE′ ⇔ Ca2E′ | 108 M−1 s−1 | 10 | 0.1 μM |
| Ca2E′ ⇔ E + (2 Ca2+) | 20 s−1 | 0 | 0 |
| E | |||
| Ca2+ + Trop ⇔ CaTrop | 1.017 × 108 M−1 s−1 | 132 | 1.3 μM |
| F | |||
| Ca2+ + Trop ⇔ CaTrop | 2.033 × 108 M−1 s−1 | 2,642 | 13 μM |
| Ca2+ + CaTrop ⇔ Ca2Trop | 1.017 × 108 M−1 s−1 | 13.21 | 0.13 μM |
Letters A–F correspond to the reactions in panels A–F of Fig. 1, where forward and reverse rate constants are denoted by k+i and k−i (i = 1, 2, 3, or 4), respectively. Rates for reactions A, B, C, and E were adjusted to 18°C from those given in Baylor and Hollingworth (1998) and Hollingworth et al. (2000) (Q10 = 2), except for the reaction rates of fluo-3 with protein in B (see materials and methods). Rates for reactions D and F are described in materials and methods. (A) The forward rate for ATP, indicated by an asterisk, is an apparent rate constant in the presence of 1 mM free [Mg2+] (pH = 7, viscosity = 2 centipoise, [K+] = 0.15 M; see Baylor and Hollingworth, 1998). (C) At 50 nM [Ca2+] and 1 mM free [Mg2+], the steady-state occupancy of parvalbumin is 25.8% with Ca2+ and 68.0% with Mg2+; 6.2% is metal-free. (D) The four states of the SR Ca2+ pump are denoted E, CaE, CaE′, and Ca2E′; the parentheses in the fourth line indicate that two Ca2+ ions are released into the lumen of the SR. (E and F) Reaction F is mathematically equivalent to reaction E if (a) the total troponin concentration in reaction F is half that in reaction E, and (b) the values of k−1 and k−2 in Table III F are reduced 20-fold and increased 20-fold, respectively.