Iron-sulfur clusters electron-transfer series

NADH-coenzyme Q (CoQ) oxidoreductase, transfers electrons stepwise from NADH, through a flavoprotein (containing FMN as cofactor) to a series of iron-sulfur clusters (which will be discussed in Chapter 13) and ultimately to CoQ, a lipid-soluble quinone, which transfers its electrons to Complex III. A If, for the couple NADH/CoQ is 0.36 V, corresponding to a AG° of —69.5 kJ/mol and in the process of electron transfer, protons are exported into the intermembrane space (between the mitochondrial inner and outer membranes). 

At the time PS 11 absorbs a photon, PS-1 complex also absorbs a photon, resulting in a charge separation to form the oxidized primary electron donor P700 and the reduced primary electron acceptor Aq. The oxidized primary donor P700 is then reduced by an electron from reduced plastocyanin. The electron on the primary acceptor Aq is rapidly transferred step-wise through a series of acceptors consisting of a phylloquinone (0 Q), three iron-sulfur clusters in FeS-X, FeS-A and FeS-B, and finally ferredoxin (Fd) which reduces NADP" to NADPH in a reaction catalyzed by NADP -ferredoxin reductase (FNR). The PS-1 complex is also called the plastocyanin-ferredoxin oxido-reductase. ... 

Figure 18.10 Coupled electron-proton transfer reactions through NADH-Q oxidoreductase. Electrons flow in Complex I from NADH through FMN and a series of iron-sulfur cluster to ubiquinone (Q). The electron flow results in the pumping of four protons and the uptake of two protons from the mitochondria matrix. [Based on U, Brandt et al, FEB5 Letters 54S(2003) 9-17, Figure 2.]...

From a proteic point of view, the location of the light-induced reactions in PS1 appears to be very different from that in PS2 or in bacterial RCs. Indeed, in the bacterial RCs the membrane-embedded part of the photosystems that carry the first electron donors and acceptors consists of two proteic subunits of 250-350 amino acids, whereas psa A and psa B are each constituted of 750 amino acids. If one compares the function of the different photosystems, it clearly appears that many of the electron tranfer steps are similar between PS2 and bacterial RCs. In these RC, after excitation of the primary electron donor, the electron rapidly jumps from a chlorophyllic structure (a dimer of BChl in bacterial RCs) to a (bacterio)pheophytin. From the (bacterio)pheophytin the electron is transferred to a quinone then to a second quinone. In PS1, after the excitation of the primary electron donor, the electron jumps rapidly from the primary electron donor P700 (most likely a dimer of Chls) to a chlorophyll (Aq) from Aq it is transferred to the A acceptor, and thence to a series of iron-sulfur clusters (Fx. Fg and Fb) (4). If some structural analogy may be found between all the photosystems. It obviously will concern the proteic features related to the first electronic steps, e.g. those which are located in the local environment of the primary donor and/or primary acceptors of electrons. 

Oxidation of the nickel to Ni(III), by a series of electron transfer steps through the iron-sulfur cluster chain, to an external electron acceptor. A proton is released, possibly binding to a base near the [NiFe]-center. 

The second electron is transferred, by a series of electron transfer steps along the chain of iron-sulfur clusters to the external electron acceptor. Meanwhile a proton is transferred to the solution. 

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