Figure 11.

Two Cd²⁺ ions at sites A and C rescue the reaction of S_3’S,P-S by the Tetrahymena ribozyme. The plot at left shows the Cd²⁺ concentration dependence of k^rel, the reaction rate constant for S_3’S,P-S relative to that of an all-oxygen substrate. The solid line fits the data according to a model in which two Cd²⁺ ions rescue the reaction, while the dashed line fits the data according to a model in which a single Cd²⁺ ion mediates rescue. [Data taken from (124).]