Plant Physiology - Musiyenko M.M. 2001

Photosynthesis: Physiological, Biochemical, and Ecological Aspects
Photosystem I

Photosystem I (PSI) complex drives the light-induced oxidation of plastocyanin, the reduction of ferredoxin, and the generation of an asymmetric transmembrane electrical charge distribution.

PSI Polypeptides primarily facilitate The formation of pigment-Structure/178.html">Protein Complexes, properly orient the pigments of the PSI light-harvesting complex (LHC I), and arrange the Components of the PSI reaction center (RC I)—namely the Donors and acceptors involved in electron migration and charge generation—in the required order.

The PSI complex comprises at least 13 distinct polypeptides, approximately 200 chlorophyll molecules, an undetermined number of carotenoids, three iron-sulfur (Fe-S) centers, and one or more quinone molecules.

The core of the PSI complex consists of 6 different polypeptide subunits (I-VI). It is believed that the core contains a dimer of subunit I, which forms the PSI reaction center, alongside single copies of all other polypeptides. Furthermore, compelling evidence indicates that the subunits of the first dimer are not identical.

Subunits IV, V, and VI are likely associated with the Fe-S centers, which serve as electron acceptors in PSI. Subunit III is linked to plastocyanin, whereas the functional role of subunit II remains unclear and it may not even participate in electron transport.

The remaining polypeptides contribute to the assembly of LHC I and ensure the proper orientation of chlorophylls. Recently, polypeptides binding chlorophyll b have been discovered, although it was previously thought that a hallmark of PSI was the absence of chlorophyll b-binding Proteins. Approximately 3-4 proteins with molecular masses of 19-24 kDa assemble into complexes and cooperatively bind both chlorophyll a and chlorophyll b (Fig. 53).

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Fig. 53. Structural scheme of the photosystem I complex (A, B, C, D—polypeptides)

The primary electron donor in the photosystem I reaction center is P₇₀₀. Whether it is a monomer or a dimer remains a subject of debate. It has a low redox potential (480 mV), meaning P₇₀₀ is a weak oxidizing agent that can be identified as a chlorophyll a cation—P₇₀₀⁺. Within 10 picoseconds (ps) following the absorption of quanta from LHC I, the excited P₇₀₀ molecule transfers an electron to the PSI RC acceptor molecule. The proper orientation of the RC within the membrane ensures Charge Separation in a direction perpendicular to the membrane surface, thereby establishing a transmembrane electrical potential difference.

It is highly probable that the primary acceptor A₀, which is reduced by the P₇₀₀ donor, is a chlorophyll molecule based on NMR spectra. The charge separation resulting from the photochemical reaction between P₇₀₀ and A₀ is subsequently stabilized by The transfer of the reduced equivalent from A₀ to Q_A (or Q), occurring in approximately 200 ps. Chemically, Q is no longer of a chlorophyll type, but is likely a specific semiquinone.

Direct electron donors for PSI can be hydrophilic proteins capable of diffusing within the thylakoid lumen. Electron transfer on the acceptor side of photosystem I is primarily associated with three 4Fe-4S type Fe-S centers characterized by exceptionally low redox potentials: —550 mV (IV), —590 mV (V), and —730 mV (VI), respectively. The exact spatial arrangement of these centers within photosystem I has not yet been elucidated. It is believed that Fe-S centers IV and V function in parallel, jointly forming the terminal electron acceptor in the PSI reaction center. Consequently, both are responsible for the reduction of soluble ferredoxin on the outer surface of the thylakoid membrane. Fe-S center VI may act as an acceptor positioned upstream of the preceding two centers (IV and V), though very little evidence suggests it serves as an electron donor for them; it may operate in parallel instead.

Ferredoxin, dissolved in the stromal phase of the chloroplast, transports electrons from the PSI reaction center to NADP+, resulting in the formation of NADPH. The reduction of NADP is catalyzed by ferredoxin-NADP⁺ oxidoreductase, which is bound to the outer surface of the thylakoid membrane outside the regions of membrane apposition.