Photosystem I Reduces NADP by Way of Iron-Sulfur Proteins

Photosystem I Reduces NADP+ by Way of Iron-Sulfur Proteins 

On the reducing site of photosystem I, the initial electron acceptor appears to be a molecule of chlorophyll a (see fig. 15.17). The second acceptor probably is a quinone, phylloquinone (vitamin K, fig. 15.10). In these respects, photosystem I resembles photosystem II and purple photosynthetic bacteria, which use pheophytin a or bac-teriopheophytin a followed by a quinone. From this point on, photosystem I is different its next electron carriers consist of iron-sulfur proteins instead of additional quinones. 

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+. 

02 Evolution Requires the Accumulation of Four Oxidizing Equivalents in the Reaction Center of Photosystem II 

Oxidation of two molecules of H20 to 02 requires removing four electrons for each 02 produced. According to the Z scheme each electron must traverse the photochemical reac- 

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