Figure 7.

Third-order rate constants k_act and substrate pK_as for activation of HAdK1-catalyzed phosphoryl transfer by EA. Substitution of −F for −H at HP results in a 6-fold falloff in k_act and reduction in substrate basicity that is consistent with β_nuc = 0.3 on a two-point Brønsted plot for phosphoryl transfer. Substitution of −OH for −H at HP causes an increase in dianion basicity that should enhance nucleophilic reactivity; the large 27-fold falloff in k_act is consistent with a large steric effect of the −OH on enzyme activation.