Dependence of (vi)obs/(vi)o on the concentration guanidinium cation for R235A mutant ScOMPDC-catalyzed reactions, where (vi)o is the observed initial velocity in the absence of the cation activator: (●), the deuterium exchange reaction of 0.74 mM h-FUMP catalyzed by the R235A mutant of ScOMPDC at pD 8.15 (20 mM GlyGly, 20% free base), 25 °C and I = 0.14 (NaCl); (■), the decarboxylation reaction of 5 µM OMP at pH 7.1 (10 mM MOPS), 25 °C and I = 0.10 (NaCl).27