The uncatalysed and metal-ion catalysed decarboxylation of 3-oxoglutaric acid: a model for an enzyme system

Abstract

The uncatalysed decarboxylation of 3-oxoglutaric acid, HO₂C·CH₂·CO·CH₂·CO₂H → CH₃·CO·CH₂·CO₂H + CO₂ has been studied at 42°. The rate constants for the un-ionised acid, the monoanion, and the dianion are 12 × 10^–3 min^–1, 45 × 10^–3 min^–1, and 2·75 × 10^–3 min^–1 respectively. Because of the higher reactivity of the monoanion, the pH-rate profile for the uncatalysed decarboxylation is bell-shaped with a rate maximum at pH 3·5 (l= 0·1M). The enhanced reactivity of the monoanion appears to be due to intramolecular hydrogen bonding between the carbonyl group and the un-ionised carboxy-group. The practical ionisation constants of 3-oxoglutaric acid are pK₁= 3·23 and pK₂= 4·27 at 0·01M. The decarboxylation of 3-oxoglutaric acid, unlike that of acetoacetic acid, is catalysed by transition-metal ions, so that in the presence of metal ions there is a rapid loss of one molecular equivalent of carbon dioxide followed by a slower loss of a second molecular equivalent in the uncatalysed reaction. The catalytic effects of copper(II), nickel(II), and manganese(II) have been studied in some detail. It has been found that 2,2′-bipyridyl which is capable of π-bonding with the metal ions enhances the catalytic activity of manganese(II) by a factor of 10 while the effect with copper(II) and nickel(II) is less marked (ca. 2 times). The possible significance of these effects in the action of the metal-activated decarboxylases is discussed, as manganese(II) is the biologically important metal ion in the enzymatic reactions.