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The Central Photochemical Event: Light-Driven Electron Flow:- Antenna Chlorophylls Are Tightly Integrated with Electron Carriers

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  p734-736

2026-06-24

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The Central Photochemical Event: Light-Driven Electron Flow:- Antenna Chlorophylls Are Tightly Integrated with Electron Carriers

The electron-carrying cofactors of PSI and the light harvesting complexes are part of a supramolecular com plex (Fig. 19–51a), the structure of which has been solved crystallographically. The protein consists of three identical complexes; each composed of 11 different proteins (Fig. 19–51b). In this remarkable structure the many antenna chlorophyll and carotenoid molecules are precisely arrayed around the reaction center (Fig. 19–51c). The reaction center’s electron-carrying cofactors are therefore tightly integrated with antenna chlorophylls. This arrangement allows very rapid and efficient exciton transfer from antenna chlorophylls to the reaction center. In contrast to the single path of electrons in PSII, the electron flow initiated by absorption of a photon is believed to occur through both branches of carriers in PSI.

FIGURE 19–51 The supramolecular complex of PSI and its associated antenna chlorophylls. (a) Schematic drawing of the essential proteins and cofactors in a single unit of PSI. A large number of antenna chlorophylls surround the reaction center and convey to it (red arrows) the energy of photons they have absorbed. The result is excitation of the pair of chlorophyll molecules that constitute P700. Excitation of P700 greatly decreases its reduction potential, and it passes an electron through two nearby chlorophylls to phylloquinone (QK; also called A1). Reduced phylloquinone is reoxidized as it passes two electrons, one at a time, to an Fe-S protein (FX) near the N side of the mem brane. From FX, electrons move through two more Fe-S centers (FA and FB), then to the Fe-S protein ferredoxin in the stroma. Ferredoxin then donates electrons to NADP (not shown), reducing it to NADPH, one of the forms in which the energy of photons is trapped in chloroplasts. (b) The trimeric structure (derived from PDB ID 1JBO), viewed from the thylakoid lumen perpendicular to the membrane, showing all protein subunits (gray) and cofactors (described in (c)). (c) A monomer of PSI with all the proteins omitted, revealing the antenna and reaction center chlorophylls (green with dark green Mg2 ions in the center), carotenoids (yellow), and Fe-S centers of the reaction center (space-filling red and orange structures). The proteins in the complex hold the components rigidly in orientations that maximize efficient exciton transfers between excited antenna molecules and the reaction center.