THE ROLE OF FLAVODOXIN IN THE REACTION CATALYZED BY THE GLUTAMATE SYNTHASE FROM MONORAPHIDIUM-BRAUNII

The ferredoxin-dependent glutamate synthase (GOGAT) from Monoraphidium braunii can use dithionite-reduced flavodoxin (Fld) as an electron do nor, showing a K-m value of 75 mu M. The incubation of both proteins i n the presence of the carboxyl group activator N-ethyl-3-(3-dimethylam inopropyl)-carbodiimide (EDC) leads to the formation of two, different stable covalent Fld:GOGAT complexes, with estimated stoichiometries o f 1:1 and 2:1 respectively. These complexes were not observed when the incubation of proteins was performed at high ionic strength, when the Fld carboxyl groups were previously blocked with glycine ethyl ester in the presence of EDC or when GOGAT was previously modified with spec ific arginine or lysine reagents. An interesting correlation between t he complex formation capacity and the GOGAT activity was observed. Whe n ferredoxin (Fd) was also present in the protein reaction mixture an Fd:GOGAT covalent complex, with 1 : 1 stoichiometry, was preferentiall y formed, indicating competition between Fd and Fld for the same GOGAT active site.