Anthranilate synthase of Neurospora crassa: reaction and labeling with glutamine analogs

The multifunctional enzyme complex, anthranilate synthase from Neurospora crassa, irreversibly loses its glutamine-dependent anthranilate synthase activity on exposure to the reactive glutamine analogs DON and azaserine. Inactivation depends on the presence of the substrate chorismate, is enhanced by the cofactor Mg⁺², and is antagonized by glutamine. Inactivation correlates well with the incorporation of ¹⁴C]DON into the protein with modification localized to the β subunit (M_r 84,000) of the complex, demonstrating directly that the β subunit provides the glutamine binding site for the glutamine-dependent anthranilate synthase reaction. The slower and less extensive loss of ammonia-dependent anthranilate synthase activity indicates that maximum expression of the ammonia-dependent anthranilate synthase activity by the α subunit also depends on the interaction with an active glutamine amidotransferase domain of the β subunit.