Figure 5.

Allosteric mechanism of HCN4 nonadditivity for the Rp/G double-ligand cycle. The observed nonadditivity (${\gamma }_{\text{Exp}}$ > e) for the HCN4 CNBD Rp/G double-ligand cycle is dissected in terms of three main contributions (${\gamma }_1$, ${\gamma }_2$, and ${\gamma }_{12}$), which represent state- and transition-specific terms, respectively. The ${\gamma }_1$ factor is close to unity because the lid is disengaged from the base in state 1 (wavy red dotted line), resulting in negligible nonadditivity. The ${\gamma }_{12}$ factor is >e because both substituents drive the P₁$\iff$ P₂ two-state equilibrium in the same direction (black dashed lines) relative to the scaffold S, i.e., the unmodified cAMP ligand, giving rise to significant nonadditivity. The ${\gamma }_2$ factor is >e primarily because of the allosteric frustration in the P₂:Rp-cAMPS complex. To see this figure in color, go online.