Proteolysis of barley (Hordeum vulgare L.) T. ferredoxin-glutamate synthase affects ferredoxin- and methyl viologen-dependent enzyme activities differently

Ferredoxin-glutamate synthase (Fd-GOGAT; E.C. 1.4.7.1) T. from barley undergoes a spontaneous proteolytic cleavage in crude extracts, yielding two fragments of 69 and 96 kDa. The breakdown of Fd-GOGAT polypeptide affects ferredoxin- and methyl viologen dependent activities of the enzyme differently: whereas ferredoxin-dependent activity is severely diminished, methyl viologen-dependent activity is fully retained. Free ferredoxin protects the Fd-GOGAT against proteolytic cleavage and the specific decay of ferredoxin-dependent enzyme activity. Therefore, it is shown for the first time that both enzyme activities can be functionally distinguished in this enzyme. The results are also, in our opinion, of crucial importance in measurements of Fd-GOGAT, given that both reduced ferredoxin and methyl viologen have been used indistinctly as far as electron donors for the Fd-GOGAT reaction are concerned.