Evolution of Photosynthetic Reaction Centers

Meyer TE Evolution of photosynthetic reaction centers and light-harvesting chlorophyll proteins. Biosystems 1994, 33(3) 167-175. 

Guergova-Kuras, M., Boudreaux, B., Joliot, A., JoHot, R, and Redding, K., Evidence for two active branches for electron transfer in photosystem 1, Proc. Natl. Acad. Sci. U.S.A., 98, 4437, 2001. Nitschke, W, Mattioli, T., and Rutherford, A.W., The FeS-type photosystems and the evolution of photosynthetic reaction centers, in Origin and Evolution of Biological Energy Conversion, Baltscheff-sky, H., Ed., VCH Publishers, Weinheim, Germany, 1994, chap. 7. 

C-P-Q-Q Tetrads. Electron transfer in the P-Q and C-P-Q species bears some resemblance to the tetrapyrrole to quinone Q electron transfer observed in photosynthetic reaction centers. Thus, a logical next step in the evolution of the artificial photosynthetic systems would be the construction of molecular devices containing two quinone moieties, as per the Qa-Qb system in reaction centers. Indeed, P-Qa-Qb triad 14 was reported some years ago by Sakata, Mataga and co-workers [31]. 

Aromatic residues have been found in proteins at positions that probably enhance the electronic coupling in systems that have been selected by evolution for efficient ET. Examples are the tryptophan mediated reduction of quinone in the photosynthetic reaction center (31), the methylamine dehydrogenase (MADH) amicyanin system, where a Trp residue is placed at the interface between the two proteins (32), as well as the [cytochrome c peroxidase-cytochrome c] complex, where a Trp seems to have a similar function (33). 

Manganese plays a critical role in oxygen evolution catalyzed by the proteins of the photosynthetic reaction center. The superoxide dismutase of bacteria and mitochondria, as well as pyruvate carboxylase in mammals, are also manganese proteins. How the multiple manganese atoms of the photosynthetic reaction center participate in the removal of four electrons and protons from water is the subject of intense investigation by spectroscopists, synthetic inorganic chemists, and molecular biologists. ... 

These differences in the chlorophyll pair structures are interesting in view of the evolution of the photosynthetic reaction centers. 

Vermaas WF Evolution of heliobacteria implications for photosynthetic reaction center complexes. Photosynth Res 1994, 41 285-294. 

The probability for drastic changes is small. RNA or photosynthetic reaction centers cannot have been created in seconds, just because all constituent atoms and molecules happened to meet at a certain time. Life may have started from a single cell, when such a cell finally existed after millions of years of prebiotic evolution. Still, there have been many possibilities for variation during the biological evolution. On another planet, plants may be red or black and ducks may be able to talk and use boats and sails. There is no reason to believe that present life is perfect. 

The situation is somewhat different in the solid state [50], where stable radical pairs are usually formed by electron transfer over a distance of more than 1 nm and are then localized rather than diffusing. In this case it becomes significant that the To state is a coherent superposition of spin eigenstates of the pair the same is true for the S state [51]. Evolution of this zero-quantum coherence can be observed indirectly, and information on the coupling between the two spins can be obtained from out-of-phase ESEEM experiments [52]. In photosynthetic reaction centers, this technique was used to measure distances between the radicals up to approximately 3 nm [53]. In a cascade of electron transfers, out-of-phase ESEEM spectra depend on the couplings in both the primary and secondary pair and on the time constant for the secondary electron transfer [54]. 

Fyfe PK, Jones MR, and Heathcote P. Insights into the evolution of the antenna domains of Type-I and Type-II photosynthetic reaction centers through homology modeling. FEES Lett. 2002 530 117-123. 

The quantum yield of photosynthesis, the amount of product formed per equivalent of light input, has traditionally been expressed as the ratio of COg fixed or Og evolved per quantum absorbed. At each reaction center, one photon or quantum yields one electron. Interestingly, an overall stoichiometry of one translocated into the thylakoid vesicle for each photon has also been observed. Two photons per center would allow a pair of electrons to flow from HgO to NADP (Figure 22.12), resulting in the formation of 1 NADPH and Og. If one ATP were formed for every 3 H translocated during photosynthetic electron transport, 1 ATP would be synthesized. More appropriately, 4 hv per center (8 quanta total) would drive the evolution of 1 Og, the reduction of 2 NADP, and the phosphorylation of 2 ATP. 

The authors begin with the basic equation of photosynthesis and an overview of the process. Next comes a description of the chloroplast, of chlorophyll, and of the relatively simple reaction center from a photosynthetic bacterium. They then describe the overall structures, components, and reactions of photosystems II and I, and the cytochrome bj complex, including the absorption of fight, charge separation, electron-transport events, and the evolution of O2. They explain how these light reactions lead to the formation of proton gradients and the synthesis of ATP and NADPH. 

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