Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogurtsov 2011

Molecular mechanisms of photosynthesis
Photosystem of purple bacteria

Molecular oxygen is produced during Photosynthesis in cyanobacteria, Algae, and plants, but is absent in green and purple Bacteria. The latter possess only type I Photosystems (Photosystem I, PSI), which drive the reduction of NADP⁺ to NADPH, whereas cyanobacteria, algae, and plants additionally feature type II photosystems (Photosystem II, PSII), which generate O₂ from H₂O.

The Mechanism of Charge Separation in the photosystem of purple bacteria is identical to the mechanism described above for plant photosystems. Light is absorbed by the so-called "special pair"—a pigment dimer of bacteriochlorophyll a molecules. Within an extremely short time (less than 4 picoseconds), an excited electron is transferred to a pheophytin molecule (pheophytin, Ph), leaving the chlorophyll dimer positively charged.

Next, within 200 ps, the electron is transferred to the menaquinone molecule Q_A, and finally, in 200 µs (the slowest step), the electron is transferred to the ubiquinone molecule Q_B (Figure 183) to form the semiquinone Q_B^- (see Figure 166).

Absorption of a second photon by the same reaction center adds a second electron to this semiquinone, resulting in a fully reduced quinone (dihydroquinone, QH₂, (Figure 166)), which is released from the reaction center and diffuses through the bacterial Cell/33.html">Plasma Membrane to the Q₀ center on the exoplasmic side of the cytochrome $bc_1$ complex.

The binding of the QH₂ quinone to the Q₀ center leads to the release of two protons into the periplasmic space (the region between The Plasma Membrane and the Introduction/37.html">Bacterial Cell wall). Thus, Light absorption results in the pumping of protons out of the Cytosol and the generation of a proton-motive force across the bacterial plasma membrane.

Simultaneously, the QH₂ quinone releases two electrons, which are transferred within the cytochrome $bc_1$ complex in a manner entirely analogous to the mitochondrial CoQH₂-cytochrome c reductase complex (Figure 167). The Q-cycle in the bacterial cytochrome $bc_1$ complex, much like the mitochondrial Q-cycle, pumps an additional proton from the cytosol into the intermembrane (periplasmic) space, thereby further increasing the proton-motive force.

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Figure 183 - Electron Transport Chain in the photosystem of purple bacteria

The electron acceptor for the electrons passing through the cytochrome $bc_1$ complex is a small, Water-soluble, iron-containing carrier protein, cytochrome, located in the periplasmic space of the bacterium. Upon receiving an electron, the iron ion in the cytochrome changes its oxidation state (is reduced) from Fe³⁺ to Fe²⁺.

The reduced cytochrome (similarly to cytochrome c in Mitochondria (Figure 167)) dissociates from the cytochrome $bc_1$ complex and diffuses to the reaction center, completing the Q-cycle—the electron is transferred to the positively charged chlorophyll a⁺, thereby returning both the chlorophyll and the cytochrome iron ion to their initial states.

As a result of this cyclic (periodic) Circulation of the electron through a closed circuit, no oxygen is produced and no Coenzymes are reduced.

An alternative utilization of the bacterial photosystem electron chain is also possible, in which electrons move unidirectionally (linearly) rather than cyclically along the chain. In this case, electrons excited in the reaction center chlorophylls are transferred to the coenzyme NAD^⊕ (rather than NADP⁺, as in plants), reducing it to NADH. Oxidized chlorophyll a⁺ is subsequently reduced by an electron supplied by cytochrome c. To reduce cytochrome c, the bacteria utilize electrons either from hydrogen sulfide H₂S (forming elemental sulfur S) or directly from molecular hydrogen H₂.

In neither case is water H₂O used as an electron donor; therefore, even in this alternative "linear" mode of operation of the bacterial photosystem electron transport chain, molecular oxygen O₂ is not formed.

A proton-motive force is generated in both scenarios—whether electron transfer is cyclic or linear in bacterial photosystems. Just as in other organisms, this proton-motive force is utilized by Proteins embedded in the plasma membrane: ATP synthases for ATP synthesis, and carrier proteins for transporting molecules against their concentration gradient.