Interaction of Ferredoxin–NADP+Reductase with its Substrates: Optimal Interaction for Efficient Electron Transfer

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Abstract
Electron transfer (ET) reactions in systems involving proteins require an oriented interaction between electron donor and acceptor in order to accommodate their respective redox centres in optimal orientation for efficient ET. Such type of reactions are critical for the maintenance of the physiological functions of living organisms, since they are implicated in vital actions, as is, for example, in the photosynthetic ET chain that leads to NADPH reduction. In this particular case, a small redox protein ET chain is responsible for ET from Photosystem I (PS I) to NADP+. In this system the enzyme responsible for NADP+ reduction is ferredoxin–NADP+ reductase (FNR), a FAD-containing NADP+ dependent reductase. In order to produce such reduction, this enzyme receives electrons from a [2Fe–2S] plant-type ferredoxin (Fd), which is previously reduced by PS I. Moreover, in the case of some algae and cyanobacteria, an FMN-dependent protein, flavodoxin (Fld), has been shown to replace Fd in this function. The processes of interaction and ET between FNR and all of its substrates involved in the photosynthetic ET chain, namely Fd, Fld and NADP+/H have been extensively investigated in recent years using a large number of techniques, including the introduction of site-specific mutations in combination with kinetic and structural studies of the produced mutants. The present manuscript summarises the information so far reported for an efficient interaction between FNR and its substrates, compares such information with that revealed by other systems for which the FNR structure is a prototype and, finally, discusses the implications of the processes of association in ET between FNR and its substrates.