In this article we will discuss about the electron transport system in bacterial photosynthesis.
In the phototrophic bacteria, cytochrome a and other type of cytochrome oxidase are not present because photosynthesis takes place under anaerobic conditions and there is no need of interaction with molecular O2.
The electron transport system i.e. the mechanism of reduction of NADP to NADPH + H+ called electron transport system in photosynthesis, while the mechanism of production of ATP from ADP and pi with the help of light energy is called photophosphorylation.
The ETS consists of an intermediate electron acceptor (I), a primary electron acceptor (X), secondary electron acceptor (Y) which is generally ubiquinone and b and c types of cytochromes. The nature of I is unknown. All the electron transport carriers are asymmetrically located in the cell membrane, just to set up the hydrogen ion gradient (Fig. 13.2).
(i) Purple Bacteria:
Both purple and sulphur bacteria have anoxygenic photosynthesis (i.e. no O2 evolution occurs during photosynthesis). Recently, the similar phenomena has also been discovered in heliobacteria.
The electron transport from NAD+ or NADP+ to NADH2 or NADPH2 comprises of electron transport system (ETS). On the other hand production of ATP is called phosphorylation. It occurs in the presence of light, hence it is called photophosphorylation. Both ATP and reducing power are required to reduce CO2 for carbohydrate synthesis.
In addition, in purple bacteria light energy is trapped in the reaction centre by their surrounding antennas which provide a large surface for capturing the light. The transfer of light energy from antennas to reaction centre (P870) take place in excitons. A special BChl a pigment accepts the electrons which later on moves via different electron carrier molecules, bacteriopheophytin (BPh), quinone A, quinone B and quinone pool.
After reduction of these quinone molecules, electron transport occurs slowly to Cyt bc1, Cyt C2 and finally to reaction centre. The whole electron transport is cyclic during which proton motive force develops to yield ATP formation. In this process no consumption of electron takes place as found in ATP formation during respiration.
The electron comes from reduced sulphur compounds such as H2S, S° or thiosulphate, H2 in case of photohthotrophs and of succinate, malate or butyrate in photo-organotrophs as given below:
6CO2 +12 H2S → C6H12O6 + 6H2O + 12S°
The electrons are transferred from reduced carrier to NADP+ so as to give rise to NADPH, involved direct transfer or from more-electro positive quinone to NADP+. In later case, electrons from the quinone pool are forced backward against the electro-potential gradient to reduce NADP+ to NADPH.
This process is called reversed electron transport. In this process, membrane potential is required to utilize the electron donor of high reduction potential such as quinones. Most of the chemolithotrophic bacteria have this phenomenon (Fig. 13.3A)
(ii) Green Bacteria:
Among green bacteria electron flow occurs after accepting the electron by P840 of high electro-potential (0.5 V). Since it is too strong electro-negative, sometimes primary electron acceptor directly reduces ferredoxin and pyridine nucleotide (NADP). No reverse electron flow is required similar to photosystem I of cyanobacteria.
Electron transfer also may occur via iron sulphur protein complex to quinone, Cyt bc1, Cyt C553. Electrons are finally accepted by reaction centre. The reaction centre is then capable to absorb energy leading to ATP production in cyclic reaction because electrons repeatedly move in a closed circle. Electron flow in green bacteria is given in Fig. 13.3 B.
The reaction centre P798 absorbs the light energy and photosynthetic electron flow occurs via modified form of chlorophyll a called hydroxy-chlorophyll a -Fe-S-Q-bc1 Cyt – Cyt C553 to reaction centre which is slightly different from green sulphur bacteria.
In both the bacteria NADH production is light-mediated. The primary electron acceptor in such bacteria has reduction potential of -0.5 V. If it is reduced, it is able to reduce NAD+ directly, hence reverse electron flow does not require for reducing NAD+ as shown in Fig. 13.3C.