Figure 4. The influence of third-site mutations on allostery in CAP.

(A) Predicted influence of mutation of V140 on allostery in CAP. The chart represents the ratio of the second to first dissociation constants for cAMP (K ₂/K ₁) plotted against spring constant at V140 (k _V140/k). The structures are the proposed corresponding mutations. (B) X-ray crystal structures for CAP showing the hydrophobic interactions at amino 140 in wild-type, V140L, and V140A proteins. (C–D) ITC traces (upper panel) and binding isotherms (lower panel; the different coloured symbols represent individual experiments) for the calorimetric titration of cAMP to CAP V140L (C) and V140A (D) proteins. The thermodynamic parameters obtained are shown in Tables 1 and S2.