Photosynthetic bacteria oxidation-reduction potential

If an enzyme binds a flavin radical much more tightly than the fully oxidized or reduced forms, reduction of the flavoprotein will take place in two one-electron steps. In such proteins the values of E° for the two steps may be widely separated. The best known examples are the small, low-potential electron-carrying proteins known as flavodoxins.266 269a These proteins, which carry electrons between pairs of other redox proteins, have a variety of functions in anaerobic and photosynthetic bacteria, cyanobacteria, and green algae. Their functions are similar to those of the ferredoxins, iron-sulfur proteins that are considered in Chapter 16. 

These are involved in a wide range of electron-transfer processes and in certain oxidation-reduction enzymes, whose function is central to such important processes as the nitrogen cycle, photosynthesis, electron transfer in mitochondria and carbon dioxide fixation. The iron-sulfur proteins display a wide range of redox potentials, from +350 mV in photosynthetic bacteria to —600 mV in chloroplasts. 

HiPIP Formerly used abbreviation for high-potential iron-sulfur protein, now classed as a ferredoxin. An ELECTRON-TRANSFER PROTEIN from photosynthetic and other bacteria, containing a [4FE-4S] CLUSTER which undergoes oxidation-reduction between the [4Fe-4S]2+ and [4Fe-4S]3+ states. 

Fig. 6. (A) Redox titration of the primary electron acceptor in Rb. sphaeroides chromatophores at pH 11. The amplitude of absorption changes Induced by short flashes is plotted as a function of the redox potential solid dots and empty circles represent reductive and oxidative titrations, respectively. The solid line is the theoretical Nernst curve. (B) Equilibrium midpoint potentials of the primary acceptor as determined in (A) plotted as a function of pH. Figure source Prince and Dutton (1978) Protonation and the reducing potential of the primary electron acceptor. In RK Clayton and WR Sistrom (eds) The Photosynthetic Bacteria, p 443, 444. Plenum.

In some early studies of cytochrome reactions in photosynthetic bacteria. Chance and Nishimura made a remarkable observation with regard to the temperature dependence of photooxidation of the c-type cytochromes in photosynthetic bacteria. They found that in Chromatium the low-potential Cyt c553 (also called Cyt c423.5 according to the wavelength of the Soret band) can still undergo photooxidation at low temperatures. Fig. 5 shows that the oxidation rate of Chromatium Cyt c is about the same at 300 as at 250 K, while the rate of re-reduction is deaeased about 6-fold. What is remarkable is that at 77 K the rate of cytochrome oxidation appears to be even faster than at ambient temperature. The oxidized... 

The simple constitution of the active center, iron and sulfur, contrasts with the diversified role played by these proteins in key biological oxidation-reduction processes, such as carbon, hydrogen, sulfur and nitrogen metabolism, using a very wide range of redox potentials (+ 350 mV in photosynthetic bacteria to — 600 mV in chloro-plasts). 

The photochemically oxidized reaction-center chlorophyll of PSII, Peso, is the strongest biological oxidant known. The reduction potential of Peso is more positive than that of water, and thus it can oxidize water to generate Q2 and H ions. Photosynthetic bacteria cannot oxidize water because the excited chlorophyll a in the bacterial reaction center is not a sufficiently strong oxidant. (As noted earlier, purple bacteria use H2S and H2 as electron donors to reduce chlorophyll in linear electron flow.)... 

The property of chemotropicity testifies to the balance of the redox layer system with respect to the vertical fluxes of the oxidants and reductants supplied. This should be the well-defined sequence of changes with depth of the favorability of the potential redox reactions [ 17,75] that can be realized by the bacterial community. The development of bacteria in this case should affect the distributions of nutrients. By modern estimation [79] the chemosynthetic production is comparable with photosynthetic production, and that should in the same manner affect the consumption of inorganic nutrients and production of their organic forms. Besides this the possible abiotic chemical reactions and the sedimentation of particulate matter of different densities should also play their roles in this mechanism. 

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