Ferredoxin Fe

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

An enormous amount of electron-transfer chemistry goes on in biological systems, and nearly all of it critically depends on metal-containing electron-transfer agents. These include cytochromes (Fe), ferredoxins (Fe), and a number of copper-containing blue proteins, such as azurin, plastocyanin, and stellacyanin. 

The mechanism of electron transfer reactions in metal complexes has been elucidated by Taube who received the Nobel Prize in Chemistry for these studies in 1983 [xiv]. Charge transfer reactions play an important role in living organisms [xv-xvii]. For instance, the initial chemical step in photosynthesis, as carried out by the purple bacterium R. sphaeroides, is the transfer of electrons from the excited state of a pair of chlorophyll molecules to a pheophytin molecule located 1.7 mm away. This electron transfer occurs very rapidly (2.8 ps) and with essentially 100% efficiency. Redox systems such as ubiquinone/dihydroubiquinone, cytochrome (Fe /Fe ), ferredoxin (Fe /Fe ), nicotine-adenine-dinucleotide (NAD /NADH2) etc. have been widely studied also by electrochemical techniques, and their redox potentials have been determined [xviii-xix]. 

The photochemical apparatus of chloroplasts can be used under certain conditions to generate gaseous hydrogen, a direct proof of the photolysis of water. For this demonstration, the primarj redox system ferredoxin has to be coupled with a bacterial hydrogenase, which transfers electrons from Fe++ onto H+ leaving ferredoxin-Fe+++ and i H2. 

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