FEATURE ARTICLE
How Plants Produce Dioxygen
At its core, oxygen production comes down to the chemistry of a poorly understood manganese-containing complex in the membranes of plant chloroplasts
Veronika Szalai, Gary Brudvig
Photosystem II
Understanding O2 production in plant photosynthesis involves understanding both the structure and the function of the PSII protein complex. Determination of the structure of PSII would make it possible to locate essential parts of the protein relative to each other. In effect, we would have a map of where everything is located in PSII. Unfortunately, until recently, the large amounts of highly purified PSII required for structure-determination measurements have been difficult to obtain. Luckily, this has not been as severe a limitation as one might imagine because the structure of the reaction center from bacteria has been solved, and parts of that structure are similar to PSII. Of course, the drawback of comparing bacteria and higher plants is that bacteria do not generate O2, and, therefore, the O2-evolving components that we are attempting to study are not present in the structure of the bacterial reaction center. Even without a complete structural model of PSII, we can still study the way in which the protein functions by using spectroscopic techniques. These techniques have revealed that PSII contains at least five distinct nonprotein compounds, called cofactors, and a tyrosine amino acid residue from the surrounding protein, which directly participate in O2 evolution. These cofactors are small molecules or metal ions that are bound within the protein. They relay electrons to one another in a series of oxidation and reduction reactions (also called redox reactions) and catalyze reactions of PSII. In addition to these six "involved" redox centers, PSII contains multiple other cofactors whose functions under physiological conditions are still unknown.
A model of the arrangement of these various cofactors in PSII, based on the information gathered so far, has been assembled into Figures 8 and 10, which show the six redox centers that directly participate in O2 evolution and the direction in which electrons are transferred. What emerges is a sort of relay in which electrons are transferred from one to another molecule in the PSII complex.
The relay starts with the chlorophyll dimer in PSII that absorbs light energy and initiates the charge separation. This chlorophyll dimer is called P680, so named because it is a pigment for which one of the maxima in its visible absorption spectrum occurs at 680 nanometers. A few picoseconds after photoexcitation, P680 transfers an electron to Pheo—a member of the chemical family of pheophytins and a relation to chlorophyll—to create the charge-separated state P680+Pheo–. Structurally, a pheophytin is a chlorophyll molecule that lacks a central magnesium ion.
After approximately 200 picoseconds, the unpaired electron on Pheo– is transferred to the species called QA, a quinone molecule with a long tail that is tightly bound to the protein. It only accepts one electron and, once it has done so, forms the semiquinone radical anion QA–. In 100–200 microseconds, the electron on QA– is transferred to another quinone species, designated QB. The QB binding site acts as a sort of depot for protons and electrons. In PSII, free quinone binds in the QB site, accepts two electrons and two protons (H+), leaves the QB site and is replaced by a new quinone molecule. Because the quinone that binds in the QB site eventually leaves, it is the terminal electron acceptor in the chain of electron-transfer processes in PSII. The free quinone then shuttles the protons and electrons to the cytochrome b6/f complex in the thylakoid membrane.
The two remaining redox centers that have not been discussed are the species TyrZ and OEC. The abbreviation "Tyr" denotes the amino acid tyrosine, one of the amino acids in the polypeptide in which the OEC is embedded; the subscript Z is used to differentiate this tyrosine from other tyrosines in PSII. During O2 production, TyrZ is reversibly oxidized and reduced. Almost as soon as the strong oxidant P680+ is formed, TyrZ rapidly donates an electron to it, thus regenerating neutral P680. Although P680+ removes an electron from TyrZ, TyrZ also loses a proton in the same step. The result is that TyrZ becomes the neutral tyrosine radical TyrZ? rather than a charged tyrosine cation TyrZ+. This also means that to regenerate TyrZ, TyrZ? must regain both an electron and a proton, which in fact happens in the very next step.
TyrZ? takes an electron and a proton from the manganese-containing complex called the oxygen-evolving complex, or OEC. During this step, TyrZ? returns to its neutral state, and the OEC loses an electron and a proton. To replace its lost electrons and protons, the OEC strips electrons and protons away from water—the electron donor in the light reaction of plant photosynthesis. Just as quinone is the terminal electron acceptor in PSII, water is the terminal electron donor.
The OEC is the site at which water binds and is oxidized to O2. It is known that positively charged manganese ions are bound to PSII. It has also been found that when manganese is removed from PSII, the system no longer produces O2. These observations suggest that water molecules bind directly to manganese atoms and then are converted to O2. Through the use of many techniques, including EPR and x-ray spectroscopies, the OEC has been modeled as a cluster of four manganese atoms.
In 1969, before much information about the chemical nature of the OEC was known, French researcher Pierre Joliot devised an experiment to determine whether the conversion of two water molecules to one O2 molecule happened in one step or in several. His plan was to deliver light flashes to a sample of photosynthetic algae or chloroplasts and then monitor the amount of O2 generated after each light flash. He observed that when a sample was prepared and incubated in the dark before being exposed to the light flashes, three flashes were required before O2 was produced. He also noted that after the first cycle of three light flashes, four light flashes were required before O2 was detected (Figure 11).
Based on Joliot's observation, Dutch scientist Bessel Kok proposed that PSII stored the oxidizing equivalent produced by each flash of light until the four oxidizing equivalents needed to form O2 from two molecules of H2O had been stored. Converting this idea into a model, he devised what is called the Kok or "S-state" cycle (Figure 11). Each S state is a storage state that PSII uses to accumulate oxidizing power. Although some controversy still exists, it is commonly agreed that the S states are associated with the manganese cluster in the OEC. This has been interpreted to mean that the advance from one S state to the next requires the removal of an electron from the manganese atoms in the OEC. During one full S-state cycle, two water molecules bind to the OEC and are converted to reactive species so that by the end of the cycle O2 is produced.
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