A Mechanism for Citrate Dissimilation

Abstract

CERTAIN bacteria dissimilate citrate anaerobically to acetate and oxaloacetate^1–4; the reaction is of particular interest since, unlike citrate biosynthesis, it is not dependent on coenzyme A^1,2. We have partially purified extracts of A. aerogenes and have demonstrated the following properties of citridesmolase: (1) As the reaction proceeds, the enzyme is progressively inactivated and ceases to function with citrate in excess. The amount of citrate decomposed is proportional to the amount of enzyme present. (2) Oxaloacetate inhibits the reaction powerfully. Acetate and pyruvate (a product of the action of crude preparations) do not. (3) The enzyme is activated by certain divalent ions, notably magnesium, zinc and ferrous iron. (4) Calcium ions do not activate, but they inhibit the action of magnesium ions competitively. Bobtelsky and Jordan⁵ proposed the structure (I) for chelate compounds between certain divalent metal ions and citrate, and they demonstrated that the hydrogen atom of the coordinated hydroxyl group was labile. On this basis a mechanism for citrate dissimilation by the enzyme is suggested (II).