Figure 6.

Mn²⁺ rescues the reactivity of G_3′SU. (A) Effect of Mn²⁺ on the rate of the reverse reaction E⋅P + G_3′XU → E⋅S + G_3′XH [(k_c/K_m)^G_3′XU] with G_3′SU (●) and G_3′OU (○), determined as described in Materials and Methods. The effect of Mn²⁺ on the reaction of G_3′OU arises from a Mn²⁺ distinct from the Mn²⁺ ions investigated herein (unpublished results). (B) The effect of Mn²⁺ on the reactivity of G_3′SU relative to G_3′OU [k^rel = (k_c/K_m)^G_3′SU/(k_c/K_m)^G_3′OU]. The solid line is a fit of the data to Eq. 3b, derived from the model of Fig. 3A, and gives K^Mn_B = 7 ± 1 mM. The dashed line is a hypothetical Mn²⁺ concentration dependence of k^rel expected if Mn_C²⁺, which binds to E⋅P with a dissociation constant of 0.19 mM (7), were responsible for rescue of G_3′SU.