Aspartate transcarbamylase from Escherichia coli I. Inhibition by inorganic anions

Abstract

• 1.

  1. The effect of different inorganic anions on the catalytic activity of native and subunit aspartate transcarbamylase (carbamoylphosphate: l-aspartate carbamoltransferase, EC 2.1.3.2) has been investigated at pH 7.0 and 25°.

• 2.

  2. Several inorganic anions were found to inhibit both native and subunit aspartate transcarbamylase. The order of effectiveness for the best inhibitors was: $\text{PP}{}_{\mathrm{i}}\text{> F}{}^{\mathrm{-}}\text{> P}{}_{\mathrm{i}}\text{> SO}{}_4{}^{\mathrm{2-}}$.

• 3.

  3. For each ion except F⁻, a strict competitive relationship was observed between the anion inhibitors and the substrate carbamyl phosphate.

• 4.

  4. The concentration of l-aspartate also greatly influenced the magnitude of the inhibition. The inhibition increased with increasing concentration of l-aspartate.

• 5.

  5. The effect of F⁻ was shown to be due to a displacement of the pH curve along the pH axis. F⁻ inhibited native asparatate transcarbamylase below pH 8 and activated it above this pH.

• 6.

  6. Possible mechanisms of inhibition are discussed, and it is suggested that an inhibitor-l-aspartate complex is formed at the active site.