Fig 8. Myosin flux through the 4-pathway network contrasting 3 phases of muscle contraction in the beating heart.

Myosin begins and ends detached from actin and with ATP bound in the contraction cycle. The green box with dashed line boundaries group the strong actomyosin bound states. Blue vector v at the end of the myosin lever-arm is positive net force on, and positive velocity of, the thick filament in units where amplitudes are equal. Fluxes through the network, f_i, differ depending on contraction phases (values in Table 1). Measured values for f_i are in green while computed values are indicated in black. Four pathways cross from beginning to end of the contraction cycle. The top pathway populated by flux f₁ executes an 8 nm step-size. The middle pathway populated by flux f₃ executes a 3 nm step-size. It releases Pi while weakly actin bound without doing work. The bottom pathway populated by flux f₄ is branched and executes 5 or 5+3 nm step-sizes. The branch from the bottom pathway is populated by flux f₇ and executes the 5+3 nm step-size. Strain sensitivity is modeled with mechanisms in two subpathways within the shaded regions. The upper mechanism is populated by fluxes f₂ and f₈ from the 8 and 3 nm steps when the ELC N-terminus binds actin for actomyosin in rigor. The taut (blue line), intermediate (green curve), and slack (red wave) ELC N-terminus for muscle in near-isometric, auxotonic, or unloaded phases have high, modest, or zero strain when net force v is zero in isometric, intermediate in auxotonic, or high in unloaded phase. The linear (blue) actin bound ELC N-terminus is proposed to inhibit ATP binding by lowering active site accessibility for ATP at the small arrow near the myosin head. Inhibited ATP binding extends actomyosin attachment time indicated by the clock icon and quantitated in our single myosin measurements as a 0 length step. The lower mechanism is populated by flux f₆ from the 5 nm step with ADP bound. Near-isometric, auxotonic, or unloaded phases have high, intermediate, or zero strain (of an unspecified myosin element) when net force is zero, intermediate, or high. Strain lowers ADP release rate. Short duration ADP rate inhibition flux, f₇, leaves to continue with the 3 nm step. Long duration ADP rate inhibition flux, f₆ − f₇, continues with the 0 length step. For either the ATP accessibility or ADP release rate mediated mechanisms (top or bottom strain sensing mechanisms), low net force inhibits myosin cycling by extending the time myosin is strongly actin bound by >2Δt.