Figure 3.

Partial agonists are IP₃-like in their interactions with the IBC. (a) Equilibrium-competition binding to IBC (top) and NT (bottom) with ³H-IP₃ and either IP₃ or 2, n ≥ 17. (b) ΔΔG (ΔG^IBC-ΔG^NT), reflecting ΔG used to rearrange the IBC-SD relationship, is shown for each ligand, and compared with the efficacy of each (EC₅₀/K_d, with K_d determined for full-length IP₃R1). Results (a,b) are means ± SEM. (c) Estimated ΔG for conformational changes associated with IP₃R activation. The affinity (K_d) of IP₃ for IP₃R1 truncated as shown was measured herein (Table 1) or by others: by⁴ for ΔSD (IP₃R1 lacking residues 1-223), and by³² for Δloop (IP₃R1 lacking residues 2428-2437); ΔG was then calculated from ΔG = -RTlnK_d. The K_d for IP₃ was not directly measured in³², but under the conditions used the 4-fold increase in IP₃ binding after deletion of residues 2428-2437 (ie Δloop) is likely to reflect a 4-fold decrease in K_d. We assume that deletion of IP₃R fragments through which conformational changes must pass increases IP₃ affinity because less binding energy is diverted into re-arranging the protein (Fig. 1b). Deletions of many other regions (shown in blue) do not increase IP₃ affinity⁴, suggesting that the IP₃-evoked conformational changes do not pass through them. This analysis is consistent with each IP₃ binding event diverting ~9kJ/mol into conformational changes of the IP₃R, of which ~6kJ/mol rearranges the SD-IBC relationship, and ~3kJ/mol is used by the SD to gate the pore.