Figure 3.

State diagram of one RYR tetramer to illustrate calculation of binding energies ΔE. The states are identified by their energy E_o (for o = 0–3), expressed in parentheses in terms of the model parameters. As shown, the tetramer energy depends on its occupancy o and on simultaneous occupancy of adjacent protomers in adjacent tetramers. E_o may therefore adopt multiple values for the same occupancy. Among the many states available to a tetramer, the diagram illustrates those with four of five possible o values and up to two simultaneously occupied adjacent protomers. Arrows represent transitions.² The diagonal gray arrows represent binding to (or unbinding from) a protomer that has the adjacent protomer occupied. In this example, the lower value of ΔE₂ illustrates a case of Eq. 3 and the higher value illustrates Eq. 4 with starting occupancy o = 2 in both cases. Parameters a_o and S have arbitrary values, which result in positive ΔE₂; other possibilities are explored later.