The synthesis of pABA: Coupling between the glutamine amidotransferase and aminodeoxychorismate synthase domains of the bifunctional aminodeoxychorismate synthase from Arabidopsis thaliana

Aminodeoxychorismate (ADC) synthase in plants is a bifunctional enzyme containing glutamine amidotransferase (GAT) and ADC synthase (ADCS) domains. The GAT domain releases NH₃ from glutamine and the ADCS domain uses NH₃ to aminate chorismate. This enzyme is involved in folate (vitamin B9) biosynthesis. We produced a stable recombinant GAT–ADCS from Arabidopsis. Its kinetic properties were characterized, and activities and coupling of the two domains assessed. Both domains could operate independently, but not at their optimal capacities. When coupled, the activity of one domain modified the catalytic properties of the other. The GAT activity increased in the presence of chorismate, an activation process that probably involved conformational changes. The ADCS catalytic efficiency was 10⁴ fold higher with glutamine than with NH₄Cl, indicating that NH₃ released from glutamine and used for ADC synthesis did not equilibrate with the external medium. We observed that the GAT activity was always higher than that of ADCS, the excess of NH₃ being released in the external medium. In addition, we observed that ADC accumulation retro-inhibited ADCS activity. Altogether, these results indicate that channeling of NH₃ between the two domains and/or amination of chorismate are the limiting step of the whole process, and that ADC cannot accumulate.