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The Central Photochemical Event: Light-Driven Electron Flow: -Cyanobacteria Use the Cytochrome b6f Complex and Cytochrome c6 in Both Oxidative Phosphorylation and Photophosphorylation

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  738

2026-06-24

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The Central Photochemical Event: Light-Driven Electron Flow:- Cyanobacteria Use the Cytochrome b6f Complex and Cytochrome c6 in Both Oxidative Phosphorylation and Photophosphorylation

Cyanobacteria can synthesize ATP by oxidative phos phorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly con voluted plasma membrane. Two protein components function in both processes (Fig. 19–55). The proton-pumping cytochrome b6 f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bc1 in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria; it can also carry electrons from the cytochrome b6 f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b6 f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin.

FIGURE 19–55 Dual roles of cytochrome b6f and cytochrome c6 in cyanobacteria. Cyanobacteria use cytochrome b6f, cytochrome c6, and plastoquinone for both oxidative phosphorylation and pho tophosphorylation. (a) In photophosphorylation, electrons flow (top to bottom) from water to NADP⁺. (b) In oxidative phosphorylation, electrons flow from NADH to O₂. Both processes are accompanied by proton movement across the membrane, accomplished by a Q cycle.