Fig. 4.

Temperature threshold of TRPM2 is optimized for physiological [ADPR] but supraphysiological [Ca²⁺]. A, Simplified model of TRPM2 gating by Ca²⁺ and ADPR; open states O₀₀… O₃₃ are omitted for clarity. Green and yellow shadings highlight the subschemes that describe gating in saturating ADPR and saturating Ca²⁺, respectively. ${\widehat{P}}_{44}$, closed–open equilibrium constant of fully liganded TRPM2; ξ₁ and ξ₂, coupling constants of the two agonists; $K_{\text{d1}}^{\mathrm{\text{C}}}$ and $K_{\text{d1}}^{\mathrm{\text{O}}}$, dissociation constants of Ca²⁺ from closed and open channels, respectively; $K_{\text{d2}}^{\mathrm{\text{C}}}$ and $K_{\text{d2}}^{\mathrm{\text{O}}}$, dissociation constants of ADPR from closed and open channels, respectively. (By the principle of detailed balance $K_{\text{d1}}^{\mathrm{\text{O}}}=K_{\text{d1}}^{\mathrm{\text{C}}}/{\xi }_{\text{1}}$ and $K_{\text{d2}}^{\mathrm{\text{O}}}=K_{\text{d2}}^{\mathrm{\text{C}}}/{\xi }_{\text{2}}$). B, C, TRPM2 open probability in (B) various [Ca²⁺] plus saturating ADPR and (C) various ADPR plus saturating [Ca²⁺] at 14°C (red symbols), 25°C (black symbols), and 40°C (blue symbols). Colored curves represent ensemble fit of the scheme in panel A to the entire data set using four free parameters (see text for details); the fitted values are ΔH⁰₄₄ = +202 kJ/mol, ΔS⁰₄₄ = +696 Jmol⁻¹K⁻¹, $K_{\text{d1}}^{\mathrm{\text{C}}}$ = 16.6 μM, and $K_{\text{d2}}^{\mathrm{\text{C}}}$ = 0.35 μM. D, P_o values calculated from the fitted model for a range of [Ca²⁺] and [ADPR] at 37°C (orange numbers) and 40°C (red numbers), and ΔP_o = P_o(40°C) − P_o(37°C) (black numbers). The red frame marks the maximum point for ΔP_o, and shaded cells identify agonist concentration ranges in which P_o is ~0 (gray) or ~1 (cyan) at both temperatures. E, Three-dimensional mesh plot of ΔP_o as a function of [Ca²⁺] and [ADPR]; the red dot identifies the maximum point.