A Loop Unique to Ferredoxin-dependent Glutamate Synthases is Not Absolutely Essential for Ferredoxin-dependent Catalytic Activity

It had been proposed that a loop, typically containing 26 or 27 amino acids, which is only present in monomeric, ferredoxin-dependent, "plant-type" glutamate synthases and is absent from the catalytic α-subunits of both NADPH-dependent, heterodimeric glutamate synthases found in non-photosynthetic bacteria and NADH-dependent heterodimeric cyanobacterial glutamate synthases, plays a key role in productive binding of ferredoxin to the plant-type enzymes. Site-directed mutagenesis has been used to delete the entire 27 amino acid-long loop in the ferredoxin-dependent glutamate synthase from the cyanobacterium Synechocystis sp. PCC 6803. The specific activity of the resulting loopless variant of this glutamate synthase, when reduced ferredoxin serves as the electron donor, is actually higher than that of the wild-type enzyme, suggesting that this loop is not absolutely essential for efficient electron transfer from reduced ferredoxin to the enzyme. These results are consistent with the results of an in-silico study that suggests that the loop is unlikely to interact directly with ferredoxin in the energetically most favorable model of a 1:1 complex of ferredoxin with the wild-type enzyme.