Electrons in higher energy levels can return to ground state in a manner analogous to a ball falling down a staircase. In doing so they release energy. This is the process which is exploited by a photosynthetic reaction centre.
When an electron rises to a higher energy level it increases the reduction potential of the molecule. This means it has a greater tendency to donate electrons, the key to the conversion of light energy to chemical energy.
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Toggle navigation PDB Educational portal of. Molecule of the Month. Photosystem II Photosystem II captures the energy from sunlight and uses it to extract electrons from water molecules Photosystem II from cyanobacteria.
The membrane is shown schematically in gray. Three billion years ago, our world changed completely. Before then, life on Earth relied on the limited natural resources found in the local environment, such as the organic molecules made by lightning, hot springs, and other geochemical sources.
However, these resources were rapidly being used up. Everything changed when these tiny cells discovered a way to capture light and use it to power their internal processes.
The discovery of photosynthesis opened up vast new possibilities for growth and expansion, and life on the earth boomed. With this new discovery, cells could take carbon dioxide out of the air and combine it with water to create the raw materials and energy needed for growth.
Today, photosynthesis is the foundation of life on Earth, providing with a few exotic exceptions the food and energy that keeps every organism alive. Modern cells capture light using photosystem proteins, such as the one pictured here from PDB entry 1s5l. These photosystems use a collection of highly-colored molecules to capture light.
These light-absorbing molecules include green chlorophylls, which are composed of a flat organic molecule surrounding a magnesium ion, and orange carotenoids, which have a long string of carbon-carbon double bonds. There are four proteins, the "L" and "M" subunits dark and light blue whose alpha-helices span the membrane bilayer, the "H" subunit yellow on the side of the reaction center that accepts the electrons released by absorption of photons, and the cytochrome subunit that ligates the electron donor hemes gold.
Figure 2 shows the cofactors of the reaction center without the proteins, and illustrates their mirror-image symmetry. Figure 2. The cofactors of the bacterial reaction center, revealed by removal of the protein scaffolding. In the upper figure, the cofactors are shown as space-filling spheres left , as they were depicted in Figure 1. On the right, one sees these cofactors represented as stick models. The hemes are red, bacteriochlorophylls are green, the bacteriopheophytins are blue, and the quinones are gold.
In the lower figure, the cytochrome hemes have been removed, and a line has been drawn to show the approximate mirror-image symmetry of cofactors in the reaction center, which is called a C 2 symmetry axis in the figure. Figure 3 identifies the cofactors, and with this information, we are confronted with the big question.
Figure 3. The organic cofactors of the reaction center. Four bacteriochlorophylls BChl are shown in green. One pair of these forms a dimer that is called the "special pair". Two bacteriopheophytins are shown in blue. Finally, the quinone electron acceptors, menaquinone and ubiquinone, and shown here in gold. To illustrate how charge separation occurs along one "leg" of the reaction center, Figure 4 shows the events that are triggered when a photon is absorbed by the special pair of reaction center bacteriochlorophylls.
The reacting species are identified sequentially by a red color in the structural model. The final, charge-separated state contains the oxidized special pair and a reduced ubiquinone. A second charge separation event and protonation yields a fully reduced ubiquinone that leaves its binding site in the reaction center. An oxidized quinone enters the vacant site so that charge separation can proceed.
Reduced ubiquinone that is released from the reaction center is oxidized by a cytochrome bc1 complex in the bacterial membrane and the electrons return to the reaction center through the heme groups shown in Figures 1 and 2. Detailed diagrams of the cycle of reactions for bacterial photosynthesis can be found in recent biochemistry texts Berg et al. Figure 4. Light absorption triggers rapid charge separation in the bacterial reaction center.
The pathway of electron transfer is indicated by the change in color to red as electron transfer proceeds through the cofactors. The Photosystem II Reaction Center The details about reaction center structure that were obtained from crystals of bacterial reaction centers provided the basis for theoretical models of the reaction center of photosystem II PSII. Efforts to purify this reaction center showed that the cofactors were associated with a pair of proteins of similar amino acid sequences, called "D1" and "D2" on account of their diffuse appearance on SDS-polyacrylamide gels.
A pair of small, membrane spanning polypeptides were also present that ligate the heme of a cytochrome, b, whose function is still unclear. The number of chlorophyll a molecules in such a preparation was higher six, rather than four as in bacteria , but two pheophytin a molecules were present, and a pair of plastoquinones were known to be present in this reaction center Nelson and Yocum, When the first crystals of PSII were obtained from thermophilic cyanobacteria Thermosynechococcus elongatus and Thermosynechococcus vulcanus , it came as no surprise that the organization of the cofactors was shown to be very similar to those in the reaction centers of photosynthetic bacteria Ferreira et al.
There are outstanding differences as well. In the case of PSII, the electron donor is a tyrosine residue on the D1 protein, instead of the cytochromes that fulfill this function in bacteria. Close mobile search navigation Article Navigation.
Volume Photosynthetic Reaction Centers. Warwick Hillier , Warwick Hillier. Oxford Academic. Google Scholar. Gerald T. Select Format Select format. Permissions Icon Permissions. Following the initial capture of a photon by antenna pigments, the photon is transferred to the RC pigments, where it gives rise to a separation and stabilization of charge across the photosynthetic membrane.
Figure 1 depicts this process and illustrates the time scales typically involved. One feature of the photochemistry is that all photosynthetic RCs undergo charge separation with a quantum yield approaching unity, which makes them marvelous molecular machines. Open in new tab Download slide. Insight into the molecular organization of the RC has been derived, initially, from spectroscopic studies and, subsequently, from the development and analysis of high-resolution crystal structures of several photosynthetic organisms.
This was soon followed by the elucidation of several other purple bacterial structures. Good progress is also being made toward achieving two- and three-dimensional structures of photosystem II PSII crystals.
It is surprising that the structures of all of the different RCs show a dimeric core with a pseudo-C 2 axis of symmetry. This feature is illustrated in Figure 2 in the example of a purple bacterial RC. The holoprotein is shown on the left. The charge-separating RC pigments contained within the structure Fig. Exciton coupling between these two pigments provides a red shift in the optical spectrum that contributes substantially to forming the low-energy trap discussed above. The conversion of photons to chemical potential involves photoexcitation and initial charge separation to produce an oxidized bacterio chlorophyll and reduction of one of the other chlorin pigments in the RC.
From this chlorin, the electron migrates to reduce a quinone in less than a nanosecond Fig. It is interesting that the strength of the dimer exciton coupling has changed substantially during the course of oxygenic RC evolution from photosynthetic bacteria.
The weaker coupling in the oxygenic RCs increases the thermodynamic efficiency of photon capture so that a significant improvement in useful free energy capture from the photon is realized.
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