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Role of Arg100 and Arg264 from Anabaena PCC 7119 ferredoxin-NADP+ reductase for optimal NADP+ binding and electron transfer.

Previous studies and the crystal structure of Anabaena PCC 7119 FNR suggest that the side chains of Arg100 and Arg264 may be directly involved in the proper NADP+/NADPH orientation for an efficient electron-transfer reaction. Protein engineering on Arg100 and Arg264 from Anabaena PCC 7119 FNR has been carried out to investigate their roles in complex formation and electron transfer to NADP+ and to ferredoxin/flavodoxin. Arg100 has been replaced with an alanine, which removes the positive charge, the long side chain, as well as the ability to form hydrogen bonds, while a charge reversal mutation has been made at Arg264 by replacing it with a glutamic acid. Results with various spectroscopic techniques indicate that the mutated proteins folded properly and that significant protein structural rearrangements did not occur. Both mutants have been kinetically characterized by steady-state as well as fast transient kinetic techniques, and the three-dimensional structure of Arg264Glu FNR has been solved. The results reported herein reveal important conceptual information about the interaction of FNR with its substrates. A critical role is confirmed for the long, positively charged side chain of Arg100. Studies on the Arg264Glu FNR mutant demonstrate that the Arg264 side chain is not critical for the nicotinamide orientation or for nicotinamide interaction with the isoalloxazine FAD moiety. However, this mutant showed altered behavior in its interaction and electron transfer with its protein partners, ferredoxin and flavodoxin.