Table 3.

Apparent pK_a of the Essential Residue at Free ScOMPDC and at the Enzyme Liganded by OMP, UMP and F-UMP in D₂O at 25 °C and I = 0.1 (NaCl).

Ligand	pKEa	pKESb
OMPc	8.3	9.4
UMP	8.5 ± 0.2d	8.0 ± 0.1e
F-UMP	8.5 ± 0.2d	7.1 ± 0.1f

^apK_a of the essential group at the free enzyme (Scheme 6).

^bpK_a of the essential group at the E•S complex (Scheme 6).

^cDetermined from the pH-rate profiles of k_cat/K_m (for pK_E) and k_cat (for pK_ES) for decarboxylation of OMP in H₂O [Ref. 2]. The pK_as in D₂O were estimated from the values in H₂O and the 0.6 unit higher pK_as of ammonium ions in D₂O than in H₂O [Ref. 38].

^dDetermined from the pD-rate profile for k_ex/K_d for deuterium exchange into F-UMP (Figure 4).

^eDetermined from the pD-rate profile for k_ex for deuterium exchange into UMP (Figure 5B).

^fDetermined from the pD-rate profile for k_ex for deuterium exchange into F-UMP (Figure 5A).