Scheme 15.
Estimated carbon acid pKas for C-6 deprotonation of uridine and fluorouridine in water [R = H, (pKa)w] and of UMP and FUMP at the active site of OMPDC [(R = -PO32-), (pKa)E]. The values of [(pKa)w] for ionization in water were estimated from the second-order rate constant kLO for the lyoxide-catalyzed deuterium reaction,74 as described by Scheme 4B. The values of [(pKa)E] were determined in an analysis of the kinetics for OMPDC-catalyzed deuterium exchange reactions of UMP and FUMP, which showed that substrate binding to OMPDC results in a ≥ 5 × 109-fold increase in the equilibrium constant for deprotonation at C-6. The values of (pKa)E are upper limits, because of the uncertainty in the rate constant for protonation of the UMP and FUMP carbanions at the enzyme active site.