Ferredoxin-NADP+-Oxidoreductase

Onda Y, Hase T (2004) FAD assembly and thylakoid membrane binding of ferredoxin NADP oxidoreductase in chloroplasts. FEBS Lett 564 116-120... 

Another flavoprotein that makes use of both one-and two-electron transfer reactions is ferredoxin-NADP+ oxidoreductase (Eq. 15-28). Its bound FAD accepts electrons one at a time from each of the two... 

The soluble electron carriers released from the reaction centers into the cytoplasm of bacteria or into the stroma of chloroplasts are reduced single-electron carriers. Bacterial ferredoxin with two Fe4S4 clusters is formed by bacteria if enough iron is present. In its absence flavodoxin (Chapter 15), which may carry either one or two electrons, is used. In chloroplasts the carrier is the soluble chloroplast ferredoxin (Fig. 16-16,C), which contains one Fe2S2 center. Reduced ferredoxin transfers electrons to NADP+ (Eq. 15-28) via ferredoxin NADP oxidoreductase, a flavoprotein of known three-dimensional structure.367 369... 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

Photosystem I contains three iron-sulfur clusters firmly associated with the reaction center. These are designated Fe-Sx, Fe-SA, and Fe-SB in figure 15.17. The cysteines of Fe-Sx are provided by the two main polypeptides of the reaction center, which also bind P700 and its initial electron acceptors Fe-SA and Fe-SB are on a separate polypeptide. The quinone that is reduced in photosystem I probably transfers an electron to Fe-Sx, which in turn reduces Fe-SA and Fe-SB. From here, electrons move to ferredoxin, a soluble iron-sulfur protein found in the chloroplast stroma, then to a flavoprotein (ferredoxin-NADP oxidoreductase), and finally to NADP+. 

Figure 2. Schematic of photoinduced electron transport and phosphorylation reactions considered to occur in chloroplast lamellae [from Moreland and Hilton (2)]. Open arrows indicate light reactions solid arrows indicate dark reactions and the narrow dashed line represents the cyclic pathway. Abbreviations used PS I, photosystem I PS II, photosystem II Y, postulated electron donor for photosystem II Q, unknown primary electron acceptor for photosystem II PQ, plastoquinones cyt b, b-type cytochromes cyt f, cytochrome f PC, plastocyanin P700, reaction center chlorophyll of photosystem I FRS, ferredoxin-reducing substance Fd, ferredoxin Fp, ferredoxin-NADP oxidoreductase FeCy, ferricyanide asc, ascorbate and DPIP, 2,6-dichloropheno-lindophenol. The numbers la, lb, 2, 3, and 4 indicate postulated sites of action by...

Ferredoxin-NADP+ oxidoreductase 37 1 — — An enzyme containing one flavin adenine dinucleotide per molecule bound to outside of lamellae... 

As is indicated in Table 5-3, P680, P70o> the cytochromes, plastocyanin, and ferredoxin accept or donate only one electron per molecule. These electrons interact with NADP+ and the plastoquinones, both of which transfer two electrons at a time. The two electrons that reduce plastoquinone come sequentially from the same Photosystem II these two electrons can reduce the two >-hemes in the Cyt b(f complex, or a >-heme and the Rieske Fe-S protein, before sequentially going to the /-heme. The enzyme ferre-doxin-NADP+ oxidoreductase matches the one-electron chemistry of ferredoxin to the two-electron chemistry of NADP. Both the pyridine nucleotides and the plastoquinones are considerably more numerous than are other molecules involved with photosynthetic electron flow (Table 5-3), which has important implications for the electron transfer reactions. Moreover, NADP+ is soluble in aqueous solutions and so can diffuse to the ferredoxin-NADP+ oxidoreductase, where two electrons are transferred to it to yield NADPH (besides NADP+ and NADPH, ferredoxin and plastocyanin are also soluble in aqueous solutions). 

Figure 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

Ferredoxin-NADP" oxidoreductase is a flavoprotein bound to the membrane that can nevertheless be isolated as a water soluble homogeneous preparation [247]. The isolated enzyme has a molecular weight of 40000 and contains one mol of FAD per mol of apoprotein [248] its midpoint potential is -0.38 V at pH 7 [249]. The enzyme can also accept NAD" " as electron acceptor, but is very specific for ferredoxin as donor the formation of a 1 1 complex with this latter protein could be demonstrated by differential spectrophotometry [250]. Immunological studies have revealed the location of this flavoprotein on the stromal surface of the membrane, in the vicinity of the ATPase complexes and therefore predominantly on... 

As described, the absorption of a photon by P700 leads to the release of an energized electron. This electron is then passed through a series of electron carriers, the first of which is a chlorophyll a molecule (A0). As the electron is donated sequentially to phylloquinone (Q) and to several iron-sulfur proteins (the last of which is ferre-doxin), it is moved from the lumenal surface of the thylakoid membrane to its stromal surface. Ferredoxin, a mobile, water-soluble protein, then donates each electron to a flavoprotein called ferredoxin-NADP oxidoreductase (FNR). The flavoprotein uses a total of 2 electrons and a stromal proton to reduce NADP+ to NADPH. The transfer of electrons from ferredoxin to NADI is referred to as the noncyclic electron transport pathway. In some species (e.g., algae), electrons can return to PSI by way of a cyclic electron transport pathway (Figure 13.13). In this process, which typically occurs when a chloroplast has a high NADPH/NADP1 ratio, no NADPH is produced. Instead, electrons are used to pump additional protons across the thylakoid membrane. Consequently, additional molecules of ATP are synthesized. 

As 2 electrons move from each water molecule to NADP1 (blue arrows), about two H+ are pumped from the stroma into the thylakoid lumen. Two additional H+ are generated within the lumen by the oxygen-evolving complex. The flow of protons through the proton pore in CF0 drives the synthesis of ATP in CF (MSP = manganese stabilizing protein ph = pheophytin Fd = ferredoxin FNR = ferredoxin-NADP oxidoreductase)... 

Shah MM and Spain JC, Elimination of nitrite from the explosive 2,4,6-trinitrophenyl-methylnitramine (tetryl) catalyzed by ferredoxin NADP oxidoreductase from spinach, Biochem. Biophys. Res. Commun., 220, 563, 1996. 

The biological activity of wild-type ferredoxin and mutants thereof can be assessed in several different assays, as the protein is the physiological electron carrier for a variety of different oxidoreductases in P.Juriosus and both direct and coupled assays are available. The direct assay uses pyruvate ferredoxin oxidoreductase (FOR), " which reduces ferredoxin according to Eq. (1). The coupled assay involved FOR and ferredoxin NADP oxidoreductase (FNOR). - FNOR accepts electrons from reduced ferredoxin and reduces NADP [Eq. (2)]. Hence with a combination of FOR, FNOR, and Fd, pyruvate oxidation can be coupled to NADP reduction,... 

The concentrated sample (150 ml) is applied to a column (5 x 12 cm) of Blue Sepharose (Pharmacia LKB) equilibrated with buffer B. The column is eluted with a linear gradient (1.4 liters) from 0 to 2.0 M NaCl in buffer B and 50 ml fractions are collected. Three peaks of BVNOR activity are separated by this step. The first peak contains 20% of the total activity, and the enzyme responsible has not been characterized. The second peak represents 40% of the activity and, in this case, the reduction of benzyl viologen is catalyzed by ferredoxin NADP oxidoreductase (FNOR). The third peak of activity contains about 40% of the total and corresponds to NROR. This starts to elute as 1.7 M NaCl is applied to the column. Those fractions containing NROR activity are combined (600 ml) and concentrated to 30 ml by ultrafiltration (PM30 membrane). 

Pulich (4) had observed the autooxidation of low potential cytochrome c in extracts of Nostoc when supplied with NADPH under aerobic conditions and he suggested that cytochrome reduction was catalysed by ferredoxin-NADP oxidoreductase in his preparation. We could easily repeat Pulich s observation using purified proteins but found the rate of cytochrome reduction was very low. The addition of crude ferredoxin to the reaction mixture greatly stimulated the rate of electron transfer. When extracts of the soluble proteins of cyanobacteria are chromatographed on DEAE cellulose ion exchange columns, two forms of ferredoxin are occasionally observed. When cells are grown with sufficient iron, one finds a ferredoxin which elutes from the column al 0.4 to 0.5 M NaCl and which is usually identified with electron transfer from Photosystem I to FNR. This is called ferredoxin I. 

BIOSYNTHESIS AND DISTRIBUTION OF THE FERREDOXIN-NADP OXIDOREDUCTASE BINDING PROTEIN... 

Ferredoxin-NADP oxidoreductase is the final enzyme of the photosynthetic electron transport chain (for a review see Ref. 1). This fla-voprotein was isolated from spinach and lettuce chloroplasts closely associated with a trimer of a 17.5 kDa polypeptide (2,3). The purified complex showed some of the allotopic properties of the membrane-bound enzyme like pH and temperature profiles of diaphorase activity (2,4,5). Immunological evidence indicates that the binding protein protrudes from the thylakoids into the stroma (6) and that it is affected by the extent of thylakoids energization, suggesting that it may be the regulatory subunit of the reductase. 

---