Figure 7. Myosin head properties and implications for the acto-myosin cycle.

A, dogfish: the measured isometric force per myosin head is shown as •. The full line shows how the force varies with a rapid length change. The intercept (S_x) is the strain in the myosin head before length change. The dashed lines show forces with all myosin heads: in the A2 state (▿, long dashes), or in the A1 state (▵, short dashes). All three lines are parallel because both states have equal stiffness. The observed proportion of heads in the A2 state at 5°C is 0.60 (Fig. 5C). Thus with all the heads in the A2 state, the length change required to reduce force to zero, z, is 3.78 nm (=S_x/0.6). B, dogfish: effect of temperature on two different estimates of the maximum work in the transition from state A1 to state A2. Each point is for a different temperature and shows the extra work above that at 2.3°C: □ 5°C, ▵ 9°C, ▿ 12°C, ◊ 16°C and ○ 20°C. The abscissa values (units: zJ per attached myosin head) are calculated from temperature and the corresponding equilibrium constant (Q, see Fig. 5C), and k_b (Boltzmann's constant). The ordinate values (units: zJ per myosin head) are calculated from the values of z and the observed force (T₀) at each temperature. The slope of the regression line (n_ATT= attached myosin heads/total myosin heads) is 0.29 ± 0.04. For a more detailed discussion of this method of finding n_ATT see Woledge et al. (2009). C, dogfish: full lines: x dependence of free energy of the A1 and A2 states with ◆ and ◊ marking the free energy minima. x: displacement from the length at which the force in state A1 is zero. The free energy curves are positioned vertically so that the proportion of A2 at equilibrium is 0.60 (at x= 0). The energy charge to the myosin head is the difference between the minima. Broken lines show the free energy profile of two further hypothetical states A3 and A4 and the broken vertical line marks the shortest length at which force could be exerted by these states (see text). D, frog: as C except that there are three further hypothetical states.