Electron Transfer Chemistry in Photosynthesis
Art van der Est, Department of Chemistry, Brock University, Ontario, Canada
In oxygenic photosynthesis, light-induced electron transfer reactions occur in two photosystems embedded in the thylakoid membrane. These complexes bind many cofactors and are among the handful of membrane proteins for which the X-ray crystal structures have been determined. The availability of this structural information, in combination with advanced spectroscopic techniques, has allowed the electron transfer reactions to be studied in detail. This work has revealed the pathways, rates, and yields of the electron transfer reactions but has also raised several intriguing questions about how these properties are governed by the protein-cofactor interactions. In this article, a brief overview of the main features of these very complex reactions is given, along with a summary of some techniques used to study them. A short survey of some recent advances in our understanding of the chemistry of electron transfer in photosynthesis is given.
The energy needed by all living organisms is derived ultimately from the sun. However, most energy contained in sunlight is dissipated as heat, which is insufficient to break and form all but the weakest chemical bonds. Hence, to use the energy of the sun, living organisms require a mechanism for capturing individual photons and storing the energy they contain. Moreover, a system that can perform multiple turnovers is required, because even the energy contained in a single photon of visible light is not sufficient for many biochemical reactions. Photosynthesis is the process by which this energy storage is achieved, and because all living organisms are dependent on it, it is arguably the most important biologic process.
At the heart of this solar energy conversion process are two light-induced electron transfer reactions that generate oxidizing and reducing agents and a transmembrane proton concentration gradient. The nature of this reaction and the type of oxidant it produces distinguishes the various types of photosynthetic organisms.
Occurrence of photosynthesis in the three domains of life
Photosynthesis is most commonly associated with higher plants, which are eukaryotes. However, in terms of photosynthetic electron transfer chemistry, a wider variety of species exists among the bacteria. Indeed, eukaryotic photosynthesis is believed to have evolved through a symbiotic relationship between a non-photosynthetic eukaryote and an oxygenic photosynthetic bacterium. Hence, a complete classification of the various types of photosynthetic electron transfer apparatus can be performed within the domain of the bacteria.
Types of photosynthetic organisms
Photosynthetic bacteria can be divided into two classes: anoxygenic and oxygenic. The anoxygenic bacteria are thought generally to have evolved before the advent of oxygenic photosynthesis, and in these species, the electron transfer oxidizes electron-rich compounds such as thiols. The limited availability of such compounds strongly restricts the environments in which anoxygenic bacteria can survive. In contrast, the oxygenic organisms can live in a much wider variety of habitats because they have evolved the ability to oxidize water, which is much more abundant. Although considerable diversity exists, both at a cellular and molecular level between the various photosynthetic organisms, all of them possess photosynthetic reaction centers, which are membrane-bound oxido-reductase enzymes in which the light-induced electron transfer takes place. The reaction centers are divided generally into two classes on the basis of the terminal electron acceptors, and all known reaction centers can be placed into one of these two types. A unique feature of oxygenic organisms is that they have two different reaction centers, one of each type. Hence, the oxygenic
Components of the photosynthetic apparatus in oxygenic photosynthesis
The two different reaction centers in oxygenic species seem to have evolved so that they can generate the very positive oxidation potential needed to split water without sacrificing the reducing potential generated on the other side of the membrane. In both reaction centers, an extensive network of chlorophyll molecules, used to capture light, is housed in the same complex as the electron transfer cofactors, and hence, these complexes are called photosystems. The two photosystems are called more correctly light-driven oxidoreductases, and they are referred to as Photosystem I (PS I) and Photosystem II (PS II) or plastocyanin:ferredoxin oxidoreductase and water:plastoquinone oxidoreductase, respectively. In addition to the photosystems, the membrane protein complex cytochrome b6f and the soluble proteins, plastocyanin, ferredoxin, and ferredoxin-NADP+ reductase (FNR) participate in the electron transfer in oxygenic photosynthesis. The proton gradient generated by these enzymes is used by ATP synthase to phosphorylate ADP. These components are shown in Fig. 1, along with a schematic representation of the overall electron transfer pathway. The representations of the protein complexes in Fig. 1 are derived from their X-ray structures. The fact that these structures are known for all components makes oxygenic photosynthesis one of the best characterized biologic pathways.
Figure 1. Schematic representation and structure of the electron transfer components of the thylakoid membrane. The overall pathway of electron and proton flow through the system is also shown. The structural representations have been generated from the following protein databank files using the program molmol (1): Photosytem I 1JB0 (2), Photosystem II 2AXT (3), Cytochrome 1VF5 (4); ATP synthase F1 subunit 1E79 (5), F0 subunit 1 C17 (6); Plastocyanin 1JXD (7); Ferredoxin 1FXI (8); and Ferredoxin-NADP+ reductase (FNR) 1FNB (9). Note that the a and b subunits of the ATP synthase are not shown.
Electron Transfer in Oxygenic Photosynthesis
The electron transfer pathway shown in Fig. 1 is driven by light-induced charge separation in the two photosystems. The first of these systems is PS II, shown on the left of Fig. 1. In PS II, the radical cation generated by the charge separation extracts electrons from a manganese cluster located on the lumen side of the membrane. After four turnovers of PS II, the manganese complex can remove four electrons from two water molecules to produce molecular oxygen and four protons. On the stromal side of the complex, four turnovers of the complex reduce two molecules of plastoquinone to plastoquinol, which diffuses into the membrane. The plastoquinonol is then reoxidized to plastoquinone in the cytochrome b6f complex. The electrons and protons released in this process are both transferred to the stromal side of the membrane. The electrons reduce the soluble protein plastocyanin, whereas the two protons are released into the stroma. In the second photosystem, PS I, the radical cation generated by light-induced charge separation reoxidizes reduced plastocyanin. The electrons transferred through PS I reduce ferredoxin, which is a soluble protein located in the stroma. Ferredoxin is then reoxidized by ferredoxin-NADP+ reductase (FNR), which converts NADP+ to NADPH. The oxidation of water in PS II and the conversion of plastoquinone to plastoquinol and back again lead to the accumulation of protons in the lumen, which creates a proton gradient across the thylakoid membrane. Cyclic electron transport, in which reduced ferredoxin passes electrons to the cytochrome b6f, also can contribute to establishing a difference in H+ concentration across the membrane. This proton gradient is used to drive the ATP synthase enzyme shown on the right, which converts ADP to ATP on the stromal side of the membrane. The final step of photosynthesis, the production of carbohydrates from CO2 via carbon fixation, derives its energy from ATP and NADPH. This step is not shown in Fig. 1.
The elucidation of the overall mechanism of photosynthesis described above is the product of many years of intense research. Initially, the focus of most of this work was to determine the nature of the light-induced reactions and their energetics. In 1960, Hill and Bendall (10) presented a hypothesis that suggested that two light-induced steps were involved and showed a diagram in which the energy was plotted along the horizontal axis. Because the two reactions in this diagram resembled the letter “Z,” it became known as the Z-scheme. Now it is drawn according to the more usual convention of energy as a vertical axis, and many details of the electron transfer reactions have been added (11). This scheme is shown in Fig. 2. The vertical axis in the figure indicates the midpoint potential for oxidation or reduction for the various species. The two vertical steps indicate the absorbance of light by PS I and PS II, and it is apparent that the midpoint potentials of the two photosystems are shifted by ~ 0.8 V with respect to one another. Because of this shift, PS II can use 680 nm photons to generate an oxidizing potential of ~ +1.0 V, while PS I uses photons of similar energy (700 nm) to produce a reducing potential of ~ —0.7 V). The various steps after light absorption by PS I and PS II refer to electron transfer along the chain of acceptors in the two systems. Here, we will focus on the details of these electron transfer reactions. Electron transfer in the cytochrome bf complex is a multistep process also, but these steps are not shown in Fig. 2 and readers are referred to a recent review (12) for details.
Fig. 3 shows the arrangement of electron transfer cofactors in PS I and PS II as given by their X-ray crystal structures (2, 3). The two structures are similar, and each has a so-called “special pair” of chlorophylls located on the stromal side of the complex and shown in green in Fig. 2. Extending across the membrane from the respective special pairs are two branches of cofactors that act as the electron acceptors.
Figure 2. The Z-scheme of electron transfer in oxygenic photosynthesis showing approximate lifetimes and redox midpoint potentials for the cofactors. The lifetimes are averages taken from the literature.
Figure 3. The geometric arrangement of the redox active cofactors in Photosystem I and Photosystem II as given by the X-ray crystal structures: PS I pdb entry 1JB0 (2) and PS II pdb entry 2AXT (3).
Photosystem II and type II reaction centers
PS II is representative of the Type II class of reaction centers, and in these systems, the primary acceptor is pheophytin (shown in blue and labeled Pheo in Fig. 3, left). A problem that has intrigued researchers in the field for many years is the fact that despite the apparent symmetry of the complex, the electron transfer occurs exclusively to PheoA on the left in Fig. 3 (13). The exact nature of this initial charge separation remains a subject of debate. In purple bacteria, the chlorophyll dimer acts as the donor and the electron transfer is known to proceed via the accessory chlorophyll ChlA (shown in light blue in Fig. 3) (14). However, in PS II, recent evidence suggests that the initial charge may occur between ChlA and PheoA and that the oxidized ChlA+ then is reduced rapidly by the special pair, P680 (15). After this initial charge separation, the electron is transferred to the neighboring quinone QA and then across the complex to quinone QB (both shown in red in Fig. 3). The nature of the quinone varies slightly between various organisms. In PS II, it is plastoquinone, but other quinones, such as ubiquinone, are found in anoxygenic organisms with Type II reaction centers.
In the latter organisms, the oxidized donor is re-reduced by a cytochrome whereas in PSII and P680+ is reduced by the neighboring tyrosine YZ, which in turn is reduced by the oxygen-evolving complex (OEC) (represented by the four Mn atoms shown in green in Fig. 3). A second turnover of the enzyme removes a second electron from the OEC and leads to double reduction and protonation of QB to its quinol form. The quinol does not have a high affinity for the QB site and diffuses out of the reaction center to be replaced by a quinone. The complex can then undergo two more turnovers that produce another quinol and extract an additional two electrons from the OEC. The OEC then has sufficient oxidizing potential to extract four electrons from two water molecules and release four protons and an oxygen molecule in the process.
Figure 2 shows the lifetimes for the various electron transfer steps along with the midpoint potentials for the acceptors, as indicated by the vertical axis. The charge separation to QA via PheoA takes place with an overall lifetime of ~200 ps and a quantum yield of close to unity. To achieve this extremely high quantum yield, the rates of forward electron transfer have been optimized, thus avoiding nonproductive recombination. The further the electron is from the donor, the slower the back reaction becomes, and the forward reaction rate, therefore, can be slower. The price for the high quantum efficiency is less than maximal energy efficiency. As can be observed from Fig. 2, roughly half of the photon energy is lost as the electron cascades downhill along the chain of acceptors. More detail of the initial electron transfer steps in PS II is given in a recent review (16). The energetics of electron transfer in reaction centers and the factors governing the rates are discussed in Reference 17.
One of the most interesting and intensively researched aspect of the electron transfer chemistry in PS II is the function of the OEC. This function is also the most poorly understood part of the PS II enzyme because of its complexity and the difficulty in obtaining spectroscopic signatures that can be interpreted unambiguously in terms of a mechanism. Here, only a basic outline of our current understanding of the water-splitting process is given, and readers are referred to several recent and excellent reviews for more detail (18-21). As successive electrons are removed from the OEC, it goes through a series of five states labeled S0 through S4. The S4 state is not stable and rapidly transforms into S0 accompanied by the release of an O2 molecule. This sequence of steps is known as the Kok cycle, named after Bessel Kok (22) who first observed that oxygen was evolved after every fourth flash of light given to dark-adapted PS II samples. These five states represent different oxidation states of the OEC. However, a major challenge in this area has been to determine which atoms change their oxidation states as the OEC progresses from one S state to another. Although it is clear that primarily the Mn atoms are oxidized during the Kok cycle, evidence exists that no change occurs in the Mn oxidation states during the S2 to S3 transition (see Reference 20 for a review). The OEC also contains calcium and chloride and it is known that the calcium is necessary for the complex to function, but the exact nature of the involvement of these atoms in water oxidation still is under debate. The biggest challenge facing researchers in this area has been to devise a reaction mechanism for the oxidation of water and the formation of molecular oxygen. Several of the many different models that have been proposed are discussed and compared in a recent review (18). Currently, no consensus exists regarding the mechanism. The following are among the main questions still under debate: 1) How and when are protons released from the OEC during its catalytic cycle? 2) How is the O-O bond formed? and 3) What is the role of Ca in the cycle? Although it is likely that these questions will elude definitive answers for some time to come, recent advances in determining the structure of the OEC (see below) can be expected to narrow the possibilities considerably.
Photosystem I and type I reaction centers
As is apparent in Fig. 3, considerable similarity exists in the arrangement of the electron transfer cofactors in PS I and PS II. The main differences between the two systems are as follows: 1) PS I has three Fe4S4 iron-sulfur clusters, FX, FA, and Fb, located on the stromal side of the complex; 2) In PS I the primary acceptor is a chlorophyll, not pheophytin; and 3) the distance between the primary acceptor (A0A,B) and phylloquinone (A1A,B) in PS I is significantly shorter than the corresponding distance between PheoA,B and QA,B in PS II and Type II reaction centers. These structural differences correlate with functional differences between the two types of reaction centers. In PS II, the mobile electron carrier on the stromal side of the complex is QB, which is a lipid-soluble, two-electron acceptor. In contrast, the mobile electron carrier in PS I is ferredoxin, which is a water-soluble, one-electron acceptor. The three iron-sulfur clusters in PS I provide a channel by which electrons are funneled out of the reaction center to ferredoxin. On the donor side of the complex, plastocyanin, the reductant that replenishes electrons removed from P700, is also a water-soluble protein and is a one-electron donor. Thus, each photon absorbed by the PS I complex leads to the transfer of one electron from plastocyanin to ferredoxin. In Fig. 2, it is apparent that the midpoint potentials of the acceptors in PS I are about 500 to 700 mV more negative than those in PS II, and the primary role of PS I is to produce a strong reducing potential. From this point of view, it is surprising that PS I contains two phylloquinone molecules on its acceptor side, because quinones generally behave as oxidants and semiquinones behave as weak reductants. Thus, in PS I, the environment clearly has a strong impact on the redox properties of the acceptors. A comparison of the electron transfer rates given in Fig. 2 also shows that the electron transfer times generally are faster than in PS II. Presumably, the reason for this is because no need exists for multiple reduction of the acceptors. Therefore, the quinone can be placed closer to the primary acceptor and can pass the electron to FX faster than QA- can pass an electron to QB or QB-. As in all reaction centers, the initial charge separation is extremely fast and it has been assumed generally that it occurs between the chlorophyll dimer (P700) and A0. However, as in PS II, this assumption is being challenged by recent data, which suggest that the initial charge separation may occur between the accessory chlorophyll(s) and A0, followed by rapid donation from P700.
An unusual feature of the electron transfer in PS I is the fact that it is heterogeneous. Many steps, for example, from A1 to FX are governed by two lifetimes. At low temperature this becomes dramatic; a fraction of the complexes cannot transfer electrons past the quinone, but in the remaining fraction electron transfer through the PS I complex leads to stable reduction of FA and Fb. The origin of this behavior is not well understood. However, in recent years, much discussion has addressed the possibility that both branches of cofactors in PS I may be active and that a difference in the energetics and kinetics of electron transfer in the two branches could account for the heterogeneity (see below) (24).
Chemical Tools and Techniques
The picture of the workings of the photosynthetic apparatus presented above has come about largely because of advances in spectroscopic techniques over the past several decades. Not surprisingly, these advances have been primarily in the area of optical laser spectroscopy. However, magnetic resonance methods and X-ray scattering and spectroscopy techniques have also played an enormous role. A complete overview of this wide range of techniques goes far beyond what can be covered here, and only a brief description of the optical and electron paramagnetic resonance (EPR) methods by which the electron transfer can be observed is given. These methods are all time resolved, and they follow changes in the respective spectroscopic properties when the sample is irradiated with a flash of visible light.
The initial charge separation in PS I and PS II can be followed by what are known as “ultrafast” optical spectroscopy techniques. Several variations on this method exist, but they can be grouped into pump-probe absorbance difference and transient fluorescence methods (25, 26). In the first instance, the sample is irradiated with a pump pulse to initiate the electron transfer and the absorbance is measured using a probe pulse at a known delay after the pump pulse. The time resolution of this method can be as short as 10-13 seconds. Transient fluorescence methods involve detecting emitted photons and correlating the time at which the photons arrive at the detector with the time of the excitation pulse. The ultrafast techniques are usually limited to times less than a few nanoseconds. To study electron transfer steps with lifetimes longer than this, a continuous detection beam and a fast digitizer can be used to follow the absorbance changes or the emitted light. These methods have provided most lifetimes given in Fig. 2.
Electron paramagnetic resonance methods
The electron transfer in reaction centers generates a series of radical pairs that can be detected by electron paramagnetic resonance (EPR) spectroscopy (see Reference 27 for a review). The advantage of this method is that only paramagnetic species are detected. Hence there are fewer background signals and less chance of errors exists when subtracting them. However, the method is limited to a time resolution of ~10-8 seconds. Therefore, it only can be used to study the secondary electron transfer steps. A crucial advantage of magnetic resonance techniques in general is that they depend on tensorial properties and therefore give information such as the relative orientation of the radicals and their spin density distributions along with the rates of electron transfer. For photosynthetic systems, EPR has been used most widely to study trapped paramagnetic intermediates generated by the electron transfer at low temperature. In such experiments, all kinetic information is lost; however, the properties of the individual radicals can be studied in detail, and the pathway can be deduced from the species observed.
Directionality of electron transfer in photosystem I
One peculiarity of PS I is that the two branches of electron cofactors converge at FX so that no a priori reason exists for why electron transfer should use only one of them. On the other hand, no obvious need exists for both branches to be active. This problem has been addressed by a number of researchers in recent years by using the known structure of PS I to identify specific amino acid residues close to the cofactors in one branch or the other and making point mutations at these locations. Although no consistent picture of the electron transfer pathway in PS I has been developed yet, much of the data provide evidence that both branches may be active. If this model is correct, some data also suggest that it may be possible to influence the extent to which electron transfer occurs in a given branch (24).
Water oxidation and the structure of the water-splitting complex
As discussed, the holy grail of much research on PS II has been to elucidate the mechanism for water splitting. A major hurdle in achieving this goal has been that the structure of the OEC is not known. The determination of the X-ray crystal structure of PS II represents a significant advance toward solving this problem. However, currently, the electron density map generated from X-ray scattering experiments is not sufficient to unambiguously determine the structure of the OEC. The main problem is that the Mn cluster is not stable to the high intensity synchrotron radiation needed for scattering experiments. Very recently, progress in addressing this problem has been made by performing extended X-ray absorbance fine structure (EXAFS) measurements on PS II single crystals (28). Such measurements use much lower intensities and provide only the distances and arrangement of neighboring atoms to specific centers, such as the Mn atoms of the OEC. Using such data, accurate distances between the atoms of the OEC and a small number of possible arrangements for the atoms have been determined. By placing these structures into the electron density map generated by X-ray scattering, a detailed picture of the structure of the OEC has been obtained. This arrangement is shown in Fig. 4 This structure places much more stringent limits on any proposed model than was previously possible and is an important piece of the OEC puzzle.
Figure 4. Structure of the oxygen-evolving complex of PS II as derived from single-crystal EXAFS (28) and X-ray scattering (3) data. The Mn, Ca, and O atoms of the OEC are shown in green, gray, and red, respectively, and have been determined by the best fit of a model consistent with the EXAFS data into the electron density map obtained by X-ray scattering experiments (28).
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