The kinetics of azaserine and phosphinothricin inhibition of glutamate synthase cycle enzymes from barley leaves

The interaction of chemical inhibitors with the enzymes of the glutamate synthase cycle extracted from barley (Hordeum vulgare) leaves has been examined. Azaserine, a potent inhibitor of glutamine amide transfer reactions, behaves initially as a competitive (with respect to glutamine) inhibitor of ferredoxin glutamate synthase (EC 1.4.7.1). With respect to 2-oxoglutarate, the other substrate, it is an uncompetitive inhibitor. It rapidly becomes tightly bound to the enzyme, and cannot be removed by dilution or gel filtration. From the initial inhibition kinetics an apparent Ki was determined of between 5-12.5 μM. The rate limiting step appears to be the initial formation of the enzyme-inhibitor complex