GLUTAMINE SYNTHETASE OF NEUROSPORA CRASSA - INACTIVATION BY UREA AND PROTECTION BY SOME SUBSTRATES AND ALLOSTERIC EFFECTORS

The effect of different concentrations of urea on the inactivation and dissociation of glutamine synthetase of Neurospora crassa is reported. Treatments with urea dissociate the native enzyme, a tetramer, into trimeric, dimeric, and monomeric species. The native enzyme has a molecular weight of 350,000 to 360,000 and the monomer of 90,000 to 100,000. Enzymatic activity of the trimer and the dimer obtained by centrifugation in a sucrose-density gradient containing urea, is less than that of the tetramer, the monomer showing a negligible amount of activity. Addition of glutamate to the urea gradients results in enhancement of the activity of the dimer. Magnesium protects the activity of the dimer and also leads to aggregation to a trimeric species. Anthranilic acid shows a pronounced protective effect on the dimer and the monomer but does not catalyze protomeric aggregation; histidine, on the other hand, affords both protection and subunit aggregation. Treatments with PCMB indicate that sulphydryl groups are essential for enzymatic activity. Glycine and NAD are weak protectants. © 1969.