Statistical mechanics of myosin molecular motors in skeletal muscles

Statistical mechanics provides the link between microscopic properties of matter and its bulk properties. The grand canonical ensemble formalism was applied to contracting rat skeletal muscles, the soleus (SOL, n = 30 ) and the extensor digitalis longus (EDL, n = 30 ). Huxleyʹs equations were used to calculate force ( π ) per single crossbridge (CB), probabilities of six steps of the CB cycle, and peak muscle efficiency (Effmax). SOL and EDL were shown to be in near-equilibrium (CB cycle affinity ⪡ 2.5 kJ / mol ) and stationary state (linearity between CB cycle affinity and myosin ATPase rate). The molecular partition function (z) was higher in EDL (1.126±0.005) than in SOL (1.050±0.003). Both π and Effmax were lower in EDL (8.3±0.1 pN and 38.1±0.2%, respectively) than in SOL (9.2±0.1 pN and 42.3±0.2%, respectively). The most populated step of the CB cycle was the last detached state (D3) (probability PD3: 0.890±0.004 in EDL and 0.953±0.002 in SOL). In each muscle group, both π and Effmax linearly decreased with z and statistical entropy and increased with PD3. We concluded that statistical mechanics and Huxleyʹs formalism provided a powerful combination for establishing an analytical link between chemomechanical properties of CBs, molecular partition function and statistical entropy.