During photosynthesis, ferredoxin-NADP+ reductase (FNR) catalyzes the electron transfer from ferredoxin to NADP+ via its FAD cofactor. The final hydride transfer event between FNR and the nucleotide is a reversible process. Two different transient charge-transfer complexes form prior to and upon hydride transfer, FNRrd-NADP+ and FNR ox-NADPH, regardless of the hydride transfer direction. Experimental structures of the FNRox:NADP+ interaction have suggested a series of conformational rearrangements that might contribute to attaining the catalytically competent complex, but to date, no direct experimental information about the structure of this complex is available. Recently, a molecular dynamics (MD) theoretical approach was used to provide a putative organization of the active site that might represent a structure close to the transient catalytically competent interaction of Anabaena FNR with its coenzyme, NADP +. Using this structure, we performed fully microscopic simulations of the hydride transfer processes between Anabaena FNRrd/FNR ox and NADP+/H, accounting also for the solvation. A dual-level QM/MM hybrid approach was used to describe the potential energy surface of the whole system. MD calculations using the finite-temperature string method combined with the WHAM method provided the potential of mean force for the hydride transfer processes. The results confirmed that the structural model of the reactants evolves to a catalytically competent transition state through very similar free energy barriers for both the forward and reverse reactions, in good agreement with the experimental hydride transfer rate constants reported for this system. This theoretical approach additionally provides subtle structural details of the mechanism in wild-type FNR and provides an explanation why Tyr303 makes possible the photosynthetic reaction, a process that cannot occur when this Tyr is replaced by a Ser. © 2012 American Chemical Society.