Mechanism of arginine biosynthesis in Chlamydomonas reinhardti I. Purification and properties of ornithine acetyltransferase

• 1.

  1. Ornithine acetyltransferase (proposed name, $\text{α-N-}\text{acetyl-}\text{l}\text{-ornithine:}\text{l}\text{-glutamate}$N-acetyltransferase) which catalyzes the first step in arginine biosynthesis, the formation of N-acetylglutamate from $\text{α-N-}\text{acetyl-}\text{l}\text{-ornithine}$ and l-glutamate, has been isolated from the freshwater alga Chlamydomonas reinhardti.

• 2.

  2. The enzyme has a broad pH optimum between 7.5 and 9. The K_m value of the enzyme at pH 7.5 is 1.3 · 10⁻² M for glutamate and 5.5 · 10 ⁻³ M for $\text{α-N-}\text{acetyl-}\text{l}\text{-ornithine}$. The reaction is reversible; the equilibrium constant expressed as K = [acetylglutamate][ornithine]/[acetylornithine][glutamate] is 0.47.

• 3.

  3. Beside the transferase activity, the enzyme has also a hydrolytic activity. The rate of the hydrolytic reaction for α-N acetylornithine is 1% of that of the acetyltransferase reaction.

• 4.

  4. No specific cofactor has been found. The enzyme is inhibited by p-chloromercuribenzoate, but not by iodoacetate.