1.1.1.87 (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate + NAD+ = 2-oxoadipate + CO2 + NADH + H+ - - 1.1.1.87 (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate + NAD+ = 2-oxoadipate + CO2 + NADH + H+ there are 2 groups acting as acid-base catalysts in the reaction. One residue with a pKa of 6.5-7.0 serves as the general base to accept a proton as the beta-hydroxy acid is oxidized to the beta-keto acid, and this residue participates in all three of the chemical steps, acting to shuttle a proton between the C2 hydroxyl and itself. The metal ion then acts as a Lewis acid to catalyze the decarboxylation of the beta-ketoacid, with the general base donating a proton to the keto oxygen as the enol of alpha-ketoadipate is formed. A second residue with a pKa of 9.5 likely catalyzes the tautomerization step by donating a proton to the enol to give the final product. Catalytic rapid equilibrium random kinetic mechanism, overview 739958 1.1.1.87 (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate + NAD+ = 2-oxoadipate + CO2 + NADH + H+ two enzyme groups act as acid-base catalysts in the reaction. A group with a pKa of 6.5-7 acts as a general base accepting a proton as the beta-hydroxy acid is oxidized to the beta-keto acid, and this residue participates in all three of the chemical steps, acting to shuttle a proton between the C2 hydroxyl and itself. The second group acts as a general acid with a pKa of 9.5 and likely catalyzes the tautomerization step by donating a proton to the enol to give the final product 685232