Figure 6.

Proposed model showing the interaction between ryanodine and either the wtRyR1 or the E4032A RyR mutant. With wtRyR, agents that increase channel P_o are hypothesized to reduce the free energy (ΔG*_C→O) associated with the transition from closed to open conformations. A high concentration of ryanodine promotes sequential binding to allosterically coupled sites which bring the channel into a persistently inhibited state with a large energy barrier for transition to the open state (ΔG*_I→O). By contrast, E4032A exhibits a large energy barrier that is not affected by Ca²⁺, caffeine, and AMP-PCP, singly or in combination. A high concentration of ryanodine promotes sequential binding to allosterically coupled sites on E4032A, but the outcome is a dramatic decrease in free energy associated with channel gating between closed (C′) and open (O′) states in the ryanodine-modified E4032A RyR1.