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. 2009 Aug 5;97(3):806–824. doi: 10.1016/j.bpj.2009.05.023

Table 3.

Estimates of stiffness per myosin head (S), force per myosin head (F), and y0 of individual myosin heads (y0) calculated from half-sarcomere stiffness (after subtraction of filament compliance) and isometric force generated by the soleus muscle fibers

Mut R719W Controls for Mut R719W Mut R723G(H27) Controls for Mut R723G
Stiffness per head in rigor (pN/nm) 0.45 0.17 0.84 0.29
Isometric stiffness per head at 10°C (pN/nm) 0.39 0.19 0.52 0.28
Isometric force per head at 10°C (pN) 1.77 0.99 1.99 1.49
Isometric force per head at 20°C (pN) 2.88 1.77 2.56 1.92
y0 per head at 10°C (nm) 4.54 5.14 3.83 5.32

Calculations are based on a filament compliance of 0.025 nm/kPa (Fig. 5). Force and stiffness per head during isometric contraction were calculated based on 111 × 103 myosin heads/μm2 in a half-sarcomere and a fraction of strongly bound myosin heads of 0.66 (52).

For the two controls (that were measured under slightly different conditions) the values for stiffness per head in rigor (0.17 ± 0.05 pN/nm, (n = 7) and 0.29 ± 0.09 pN/nm (n = 6)) and isometric contraction (0.19 ± 0.04 pN/nm, (n = 14) and 0.28 ± 0.07 pN/nm (n = 9)) were not significantly different (all values ± SE).

Calculations based on a fraction of 54% of myosin heads with mutation R719W, 46% wild-type heads (46), and on the values for force and stiffness shown in Table 2 (see also 10). For mutation R719W, changes of cross-bridge turnover kinetics were also taken into account that contribute an increase in Fmax and in active fiber stiffness of ∼15%.

Calculations based on a fraction of 62% heads with mutation R723G, 38% wild-type heads (14), and on values for force and stiffness shown in Table 1.

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