Figure 5.

Two Mn²⁺ ions are required to rescue the reaction of S_3′S with G_NH2. (A) Effect of Mn²⁺ on the reaction E⋅S + G_X → products [(k_c/K_m)^G_X; G_X = G or G_NH2] with S_3′O and G (○), S_3′S and G (●), and S_3′S and G_NH2 (■). Determined as in Fig. 4 with subsaturating G_X. The data for reactions of S_3′O with G and of S_3′S with G are from Fig. 4A and are shown for comparison. (B) The effect of Mn²⁺ on the rate of reaction of S_3′S with G_NH2 relative to that of S_3′O with G [k^rel = (k_c/K_m)_S3′S^G_NH2/(k_c/K_m)_S3′O^G]. The solid line is the Mn²⁺ concentration dependence of the relative reactivity predicted from a model in which two Mn²⁺ ions, Mn_A²⁺ and Mn_C²⁺, independently rescue the reaction of S_3′S and G_NH2, respectively (see Eq. 2). The dashed line is the best fit of data to a model in which a single Mn²⁺ ion rescues the reaction of S_3′S with G_NH2.