Photosystem I

Both Photosystem II (PSII) and Photosystem I (PSI) contain an electron transport chain, which extracts energy when an excited electron loses its energy of excitation in a stepwise fashion. The photosystem carries out a series of oxidation/reduction reactions similar to those in electron transport. The terminal electron acceptor of PSI is NADP+, forming NADPH.

PSI is a multi-protein complex, containing at least 11 polypeptide chains, antenna cholorophylls, and a reaction center chlorophyll (P700) which can absorb light of up to 700 nm. Photon absorption by antenna chlorophylls raises P700 electrons to an excited state of about -1.3 V. Each excited electron then passes through a transport chain as follows (see also Figure 17.12):

> 1. A 0 (a chlorophyll-like acceptor);

> 2. A 1 (protein-bound phylloquinone, also known as vitamin K 1);

> 3. Three FeS proteins - F X, F B, and F A , each of which has an FeS cluster like those shown in Figure 15.4;

> 4. Souble ferredoxin (Fd), another FeS protein present in the stroma;

> 5. NADP+ -> NADPH

The transfer of electrons from reduced Fd to NADP+ is catalyzed by the enzyme ferredoxin:NADP+ oxidoreductase (FNR):

> 2Fd (reduced) + H+ -> 2Fd (oxidized) + NADPH

In a sense Fd, rather than NADP+, is the direct recipient of electrons from PSI. For example, much of the reduced Fd is used to reduce NADP+, but some is used for other reductive reactions, too, such as the reduction of thioredoxin (Figure 17.23). Ferredoxin can also use electrons to reduce substances and may also be considered the final electron accepting molecule of PSI.

The NADPH produced by Fd oxidation is released into the stroma, where it will be used in the Calvin cycle (the photosyntheic dark reactions).

At this point, P700 has been left in the oxidized state, P700+. In cells with two photosystems, the electrons are replaced by plastocyanin from PSII.

The overall reaction of PSI is summarized here.

See also: , Photosystem Summary, Cyclic Electron Flow, Chlorophyll, Reaction Center, FNR, Figure 17.16