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In: "Concepts in Photobiology: Photosynthesis and Photomorphogenesis", Edited by GS Singhal, G Renger, SK Sopory, K-D Irrgang and Govindjee, Narosa Publishers/New Delhi; and Kluwer Academic/Dordrecht, pp. 11-51.

John Whitmarsh
Photosynthesis Research Unit, Agricultural Research Service/USDA
Department of Plant Biology and Center of Biophysics and Computational Biology,
University of Illinois at Urbana-Champaign

Govindjee
Department of Plant Biology and Center of Biophysics and Computational Biology
University of Illinois at Urbana-Champaign

Summary

The primary source of energy for nearly all life is the Sun. The energy in sunlight is introduced into the biosphere by a process known as photosynthesis, which occurs in plants, algae and some types of bacteria. Photosynthesis can be defined as the physico-chemical process by which photosynthetic organisms use light energy to drive the synthesis of organic compounds. The photosynthetic process depends on a set of complex protein molecules that are located in and around a highly organized membrane. Through a series of energy transducing reactions, the photosynthetic machinery transforms light energy into a stable form that can last for hundreds of millions of years. This introductory chapter focuses on the structure of the photosynthetic machinery and the reactions essential for transforming light energy into chemical energy.

Table of Contents

1.

2.

3.

3.1 Oxygenic Photosynthetic Organisms

3.2

4.

5.

5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

5.9

5.10

6.

6.1

6.2

6.3

6.4

7.

8.

). The first step is the conversion of a photon to an excited electronic state of an antenna pigment molecule located in the antenna system. The antenna system consists of hundreds of pigment molecules (mainly chlorophyll or bacteriochlorophyll and carotenoids) that are anchored to proteins within the photosynthetic membrane and serve a specialized protein complex known as a reaction center. The electronic excited state is transferred over the antenna molecules as an exciton. Some excitons are converted back into photons and emitted as fluorescence, some are converted to heat, and some are trapped by a reaction center protein. (For a discussion of the use of fluorescence as a probe of photosynthesis, see e.g., Govindjee et al., 1986 and Krause and Weis, 1991.) Excitons trapped by a reaction center provide the energy for the primary photochemical reaction of photosynthesis - the transfer of an electron from a donor molecule to an acceptor molecule. Both the donor and acceptor molecules are attached to the reaction center protein complex. Once primary charge separation occurs, the subsequent electron transfer reactions are energetically downhill.

In oxygenic photosynthetic organisms (see section 5), two different reaction centers, known as photosystem II and photosystem I, work concurrently but in series. In the light photosystem II feeds electrons to photosystem I. The electrons are transferred from photosystem II to the photosystem I by intermediate carriers. The net reaction is the transfer of electrons from a water molecule to NADP+, producing the reduced form, NADPH. In the photosynthetic process, much of the energy initially provided by light energy is stored as redox free energy (a form of chemical free energy) in NADPH, to be used later in the reduction of carbon. In addition, the electron transfer reactions concentrate protons inside the membrane vesicle and create an electric field across the photosynthetic membrane. In this process the electron transfer reactions convert redox free energy into an electrochemical potential of protons. The energy stored in the proton electrochemical potential is used by a membrane bound protein complex (ATP-Synthase) to covalently attach a phosphate group to adenosine diphosphate (ADP), forming adenosine triphosphate (ATP). Protons pass through the ATP-Synthase protein complex that transforms electrochemical free energy into a type of chemical free energy known as phosphate group-transfer potential (or a high-energy phosphate bond) (Klotz, 1967). The energy stored in ATP can be transferred to another molecule by transferring the phosphate group. The net effect of the light reactions is to convert radiant energy into redox free energy in the form of NADPH and phosphate group-transfer energy in the form of ATP. In the light reactions, the transfer of a single electron from water to NADP+ involves about 30 metal ions and 7 aromatic groups. The metal ions include 19 Fe, 5 Mg, 4 Mn, and 1 Cu. The aromatics include quinones, pheophytin, NADPH, tyrosine and a flavoprotein. The NADPH and ATP formed by the light reactions provide the energy for the dark reactions of photosynthesis, known as the Calvin cycle or the photosynthetic carbon reduction cycle. The reduction of atmospheric CO2 to carbohydrate occurs in the aqueous phase of the chloroplast and involves a series of enzymatic reactions. The first step is catalyzed by the protein Rubisco (D-ribulose 1,5-bisphosphate carboxylase/oxygenase), which attaches CO2 to a five-carbon compound. The reaction produces two molecules of a three-carbon compound. Subsequent biochemical reactions involve several enzymes that reduce carbon by hydrogen transfer and rearrange the carbon compounds to synthesize carbohydrates. The carbon reduction cycle involves the transfer and rearrangement of chemical bond energy.

In anoxygenic photosynthetic organisms (see section 6) water is not used as the electron donor. Electron flow is cyclic and is driven by a single photosystem, producing a proton electrochemical gradient that is used to provide energy for the reduction of NAD+ by an external H-atom or e-donor (e.g., H2S or an organic acid) in a process known as "reverse electron flow". Fixation of CO2 occurs via different pathways in different organisms.

), although many different shapes and sizes can be found in plants. For details of chloroplast structure, see Staehlin (1986). The inner envelope membrane acts as a barrier, controlling the flux of organic and charged molecules in and out of the chloroplast. Water passes freely through the envelope membranes, as do other small neutral molecules like CO2 and O2. There is evidence that chloroplasts were once free living bacteria that invaded a non-photosynthetic cell long ago. They have retained some of the DNA necessary for their assembly, but much of the DNA necessary for their biosynthesis is located in the cell nucleus. This enables a cell to control the biosynthesis of chloroplasts within its domain.

Inside the chloroplast is a complicated membrane system, known as the photosynthetic membrane (or thylakoid membrane), that contains most of the proteins required for the light reactions. The proteins required for the fixation and reduction of CO2 are located outside the photosynthetic membrane in the surrounding aqueous phase. The photosynthetic membrane is composed mainly of glycerol lipids and protein. The glycerol lipids are a family of molecules characterized by a polar head group that is hydrophilic and two fatty acid side chains that are hydrophobic. In membranes, the lipid molecules arrange themselves in a bilayer, with the polar head toward the water phase and the fatty acid chains aligned inside the membrane forming a hydrophobic core (). The photosynthetic membrane is vesicular, defining a closed space with an outer water space (stromal phase) and an inner water space (lumen). The organization of the photosynthetic membrane can be described as groups of stacked membranes (like stacks of pita or chapati bread with the inner pocket representing the inner aqueous space), interconnected by non-stacked membranes that protrude from the edges of the stacks (). Experiments indicate that the inner aqueous space of the photosynthetic membrane is likely continuous inside of the chloroplast. It is not known why the photosynthetic membrane forms such a convoluted structure. To understand the energetics of photosynthesis the complicated structure can be ignored and the photosynthetic membrane can be viewed as a simple vesicle.

. In chlorophyll b, CH3 in ring II is replaced by CHO group. Plants appear green because of chlorophyll, which is so plentiful that regions of the earth appear green from space. The absorption spectrum of chloroplast chlorophyll a and b and carotenoids along with the action spectrum of photosynthesis of a chloroplast is shown in . Light is collected by 200-300 pigment molecules that are bound to light- harvesting protein complexes located in the photosynthetic membrane. The light-harvesting complexes surround the reaction centers that serve as an antenna. The three-dimensional structure of the light-harvesting complex (Kühlbrandt et al., 1994) shows that the protein determines the position and orientation of the antenna pigments. Photosynthesis is initiated by the absorption of a photon by an antenna molecule, which occurs in about a femtosecond (10-15 s) and causes a transition from the electronic ground state to an excited state. Within 10-13 s the excited state decays by vibrational relaxation to the first excited singlet state. The fate of the excited state energy is guided by the structure of the protein. Because of the proximity of other antenna molecules with the same or similar energy states, the excited state energy has a high probability of being transferred by resonance energy transfer to a near neighbor. Exciton energy transfer between antenna molecules is due to the interaction of the transition dipole moment of the molecules. The probability of transfer is dependent on the distance between the transition dipoles of the donor and acceptor molecules (1/R6), the relative orientation of the transition dipoles, and the overlap of the emission spectrum of the donor molecule with the absorption spectrum of the acceptor molecule (see van Grondelle and Amesz, 1986). Photosynthetic antenna systems are very efficient at this transfer process. Under optimum conditions over 90% of the absorbed quanta are transferred within a few hundred picoseconds from the antenna system to the reaction center which acts as a trap for the exciton. A simple model of the antenna and its reaction center is shown in .

shows a schematic view of photosystem II that is consistent with current data.

Photochemistry in photosystem II is initiated by charge separation between P680 and pheophytin, creating P680+/Pheo-. Primary charge separation takes about a few picoseconds (). Subsequent electron transfer steps have been designed through evolution to prevent the primary charge separation from recombining. This is accomplished by transferring the electron within 200 picoseconds from pheophytin to a plastoquinone molecule (QA) that is permanently bound to photosystem II. Although plastoquinone normally acts as a two-electron acceptor, it works as a one-electron acceptor at the QA-site. The electron on QA- is then transferred to another plastoquinone molecule that is loosely bound at the QB-site. Plastoquinone at the QB-site differs from QA in that it works as a two-electron acceptor, becoming fully reduced and protonated after two photochemical turnovers of the reaction center. The full reduction of plastoquinone requires the addition of two electrons and two protons, i.e., the addition of two hydrogen atoms. The reduced plastoquinone () then debinds from the reaction center and diffuses into the hydrophobic core of the membrane. After which, an oxidized plastoquinone molecule finds its way to the QB-binding site and the process is repeated. Because the QB-site is near the outer aqueous phase, the protons added to plastoquinone during its reduction are taken from the outside of the membrane.

Photosystem II is the only known protein complex that can oxidize water, resulting in the release of O2 into the atmosphere. Despite years of research, little is known about the molecular events that lead to water oxidation. Energetically, water is a poor electron donor. The oxidation- reduction midpoint potential (Em,7) of water is +0.82 V (pH 7). In photosystem II this reaction is driven by the oxidized reaction center, P680+ (the midpoint potential of P680/P680+ is estimated to be +1.2 V at pH 7). How electrons are transferred from water to P680+ remains a mystery (Govindjee and Coleman, 1990). It is known that P680+ oxidizes a tyrosine on the D1 protein and that Mn plays a key role in water oxidation. Four Mn ions are present in the water oxidizing complex. X-ray absorption spectroscopy shows that Mn undergoes light-induced oxidation. Water oxidation requires two molecules of water and involves four sequential turnovers of the reaction center. This was shown by an experiment demonstrating that oxygen release by photosystem II occurs with a four flash dependence (; Joliot et al., 1969; Joliot and Kok, 1975). Each photochemical reaction creates an oxidant that removes one electron. The net reaction results in the release of one O2 molecule, the deposition of four protons into the inner water phase, and the transfer of four electrons to the QB-site (producing two reduced plastoquinone molecules) (reviewed by Renger, 1993; Klein et al., 1993; and Lavergne and Junge , 1993).

Photosystem II reaction centers contain a number of redox components with no known function. An example is cytochrome b559, a heme protein, that is an essential component of all photosystem II reaction centers (discussed by Whitmarsh and Pakrasi, 1996). If the cytochrome is not present in the membrane, a stable PS II reaction center cannot be formed. Although the structure and function of Cyt b559 remain to be discovered, it is known that the cytochrome is not involved in the primary enzymatic activity of PS II, which is the transfer of electrons from water to plastoquinone. Why PS II reaction centers contain redox components that are not involved in the primary enzymatic reactions is a puzzling question. The answer may be found in the unusual chemical reactions occurring in PS II and the fact that the reaction center operates at a very high power level. Photosystem II is an energy transforming enzyme that must switch between various high energy states that involve the creation of the powerful oxidants required for removing electrons from water and the complex chemistry of plastoquinone reduction which is strongly influenced by protons. In saturating light a single reaction center can have an energy throughput of 600 eV/s (equivalent to 60,000 kW per mole of PS II). Operating at such a high power level results in damage to the reaction center. It may be that some of the "extra" redox components in photosystem II may serve to protect the reaction center.

Photosystem II has another perplexing feature. Many plants and algae have been shown to have a significant number of photosystem II reaction centers that do not contribute to photosynthetic electron transport (e.g., Chylla and Whitmarsh, 1989). Why plants devote resources for the synthesis of reaction centers that apparently do not contribute to energy conversion is unknown (for reviews of photosystem II heterogeneity see Ort and Whitmarsh, 1990; Guenther and Melis, 1990; Govindjee, 1990; Melis, 1991; Whitmarsh et al., 1996; Lavergne and Briantais, 1996)

The photosystem I complex catalyzes the oxidation of plastocyanin, a small soluble Cu- protein, and the reduction of ferredoxin, a small FeS protein (). Photosystem I is composed of a heterodimer of proteins that act as ligands for most of the electron carriers (Krauss et al., 1993). The reaction center is served by an antenna system that consists of about two hundred chlorophyll molecules (mainly chlorophyll a) and primary photochemistry is initiated by a chlorophyll a dimer, P700. In contrast to photosystem II, many of the antenna chlorophyll molecules in photosystem I are bound to the reaction center proteins. Also, FeS centers serve as electron carriers in photosystem I and, so far as is known, photosystem I electron transfer is not coupled to proton translocation. Primary charge separation occurs between a primary donor, P700, a chlorophyll dimer, and a chlorophyll monomer (Ao). The subsequent electron transfer events and rates are shown in (see Golbeck, 1994).

). Electrons are transferred between these large protein complexes by small mobile molecules (plastoquinone and plastocyanin in plants). Because these small molecules carry electrons (or hydrogen atoms) over relatively long distances, they play a unique role in photosynthetic energy conversion. This is illustrated by plastoquinone (PQ), which serves two key functions. Plastoquinone transfers electrons from the photosystem II reaction center to the cytochrome bf complex and carries protons across the photosynthetic membrane (see Kallas, 1994). It does this by shuttling hydrogen atoms across the membrane from photosystem II to the cytochrome bf complex. Because plastoquinone is hydrophobic its movement is restricted to the hydrophobic core of the photosynthetic membrane. Plastoquinone operates by diffusing through the membrane until, due to random collisions, it becomes bound to a specific site on the photosystem II complex. The photosystem II reaction center reduces plastoquinone at the QB-site by adding two electrons and two protons creating PQH2. The reduced plastoquinone molecule debinds from photosystem II and diffuses randomly in the photosynthetic membrane until it encounters a specific binding site on the cytochrome bf complex. The cytochrome bf complex is a membrane bound protein complex that contains four electron carriers, three cytochromes and an FeS center. The crystal structure has been solved for cytochrome f from turnip (Martinez et al., 1994) and the FeS center from bovine heart mitochondria (Iwata et al., 1996). In a complicated reaction sequence that is not fully understood, the cytochrome bf complex removes the electrons from reduced plastoquinone and facilitates the release of the protons into the inner aqueous space. The electrons are eventually transferred to the photosystem I reaction center. The protons released into the inner aqueous space contribute to the proton chemical free energy across the membrane.

Electron transfer from the cytochrome bf complex to photosystem I is mediated by a small Cu-protein, plastocyanin (PC). Plastocyanin is water soluble and operates in the inner water space of the photosynthetic membrane. Electron transfer from photosystem I to NADP+ requires ferredoxin, a small FeS protein, and ferredoxin-NADP oxidoreductase, a peripheral flavoprotein that operates on the outer surface of the photosynthetic membrane. Ferredoxin and NADP+ are water soluble and are found in the outer aqueous phase.

The pathway of electrons is largely determined by the energetics of the reaction and the distance between the carriers. The electron affinity of the carriers is represented in by their midpoint potentials, which show the free energy available for electron transfer reactions under equilibrium conditions. (It should be kept in mind that reaction conditions during photosynthesis are not in equilibrium.) Subsequent to primary charge separation, electron transport is energetically downhill (from a lower (more negative) to a higher ( more positive) redox potential). It is the downhill flow of electrons that provides free energy for the creation of a proton chemical gradient.

Photosynthetic membranes effectively limit electron transport to two dimensions. For mobile electron carriers, limiting diffusion to two dimensions increases the number of random encounters (Whitmarsh, 1986). Furthermore, because plastocyanin is mobile, any one cytochrome bf complex can interact with a number of photosystem I complexes. The same is true for plastoquinone, which commonly operates at a stoichiometry of about six molecules per photosystem II complex.

). The CF0 subunit spans the photosynthetic membrane and forms a proton channel through the membrane. The CF1 subunit is attached to the top of the CF0 on the outside of the membrane and is located in the aqueous space. CF1 is composed of several different protein subunits, referred to as a, b, g, d and e. The top portion of the CF1 subunit is composed of three ab-dimers that contain the catalytic sites for ATP synthesis. A recent major breakthrough has been the elucidation of the structure of ATPase of beef heart mitochondria by Abrahams et al. (1994). The molecular processes that couple proton transfer through the protein to the chemical addition of phosphate to ADP are poorly understood. It is known that phosphorylation can be driven by a pH gradient, a transmembrane electric field, or a combination of the two. Experiments indicate that three protons must pass through the ATP synthase complex for the synthesis of one molecule of ATP. However, the protons are not involved in the chemistry of adding phosphate to ADP. Paul Boyer and coworkers have proposed an alternating binding site mechanism for ATP synthesis (Boyer, 1993). One model based on their proposal is that there are three catalytic sites on each CF1 that cycle among three different states (). The states differ in their affinity for ADP, Pi and ATP. At any one time, each site is in a different state. This model is supported by the structure of ATPase elucidated by Abrahams et al. (1994). Initially, one catalytic site on CF1 binds one ADP and one inorganic phosphate molecule relatively loosely. Due to a conformational change of the protein, the site becomes a tight binding site, that stabilizes ATP. Next, proton transfer induces an alteration in protein conformation that causes the site to release the ATP molecule into the aqueous phase. In this model, the energy from the proton electrochemical gradient is used to lower the affinity of the site for ATP, allowing its release to the water phase. The three sites on CF1 act cooperatively, i.e., the conformational states of the sites are linked. It has been proposed that protons affect the conformational change by driving the rotation of the top part (the three ab-dimers) of CF1. Such a rotating model has recently been supported by recording of a rotation of the gamma subunit relative to the alpha-beta subunits by Sabbert et al. (1996). This revolving site mechanism would require rates as high as 100 revolutions per second. It is worth noting that flagella that propel some bacteria are driven by a proton pump and can rotate at 60 revolutions per second.

). The six-carbon compound is split, giving two molecules of a three-carbon compound (3-phosphoglycerate). This key reaction is catalyzed by Rubisco, a large water soluble protein complex. The 3-dimensional structure has been determined by X-ray analysis for Rubisco isolated from tobacco (Schreuder et al. 1993) from a cyanobacterium (Synechococcus) (Newman and Gutteridge, 1993) and from a purple bacterium (Rhodospirillum rubrum) (Schneider et al. 1990). The carboxylation reaction is energetically downhill. The main energy input in the Calvin cycle is the phosphorylation by ATP and subsequent reduction by NADPH of the initial three-carbon compound forming a three-carbon sugar, triosephosphate. Some of the triosephosphate is exported from the chloroplast and provides the building block for synthesizing more complex molecules. In a process known as regeneration, the Calvin cycle uses some of the triosephosphate molecules to synthesize the energy rich ribulose 1,5-bisphosphate needed for the initial carboxylation reaction. This reaction requires the input of energy in the form of one ATP. Overall, thirteen enzymes are required to catalyze the reactions in the Calvin cycle. The energy conversion efficiency of the Calvin cycle is approximately 90%. The reactions do not involve energy transduction, but rather the rearrangement of chemical energy. Each molecule of CO2 reduced to a sugar [CH2O]n requires 2 molecules of NADPH and 3 molecules of ATP.

Rubisco is a bifunctional enzyme that, in addition to binding CO2 to ribulose bisphosphate, can also bind O2. This oxygenation reaction produces the 3-phosphoglycerate that is used in the Calvin cycle and a two-carbon compound (2-phosphoglycolate) that is not useful for the plant. In response, a complicated set of reactions (known as photorespiration) are initiated that serve to recover reduced carbon and to remove phosphoglycolate. The Rubisco oxygenation reaction appears to serve no useful purpose for the plant. Some plants have evolved specialized structures and biochemical pathways that concentrate CO2 near Rubisco. These pathways (C4 and CAM), serve to decrease the fraction of oxygenation reactions (see Chapter this volume on carbon reduction).

(Norris and van Brakel, 1986), and those of the three protein subunits L, M, and H, in . The reaction center contains four bacteriochlorophyll and two bacteriopheophytin molecules. Two of the bacteriochlorophyll molecules form the primary donor (P870). At present, there is controversy over whether a bacteriochlorophyll molecule is an intermediate in electron transfer from the P870 to bacteriopheophytin. However, there is agreement that the remaining steps involve two quinone molecules (QA and QB) and that two turnovers of the reaction center results in the release of reduced quinone (QH2) into the photosynthetic membrane. Although there is a non-heme Fe between the two quinone molecules, there is convincing evidence that this Fe is not involved directly in transferring an electron from QA to QB. Because the primary donor (P870), bacteriopheophytin and quinone acceptors of the purple bacterial reaction center are similar to the photosystem II reaction center, the bacterial reaction center is used as guide to understand the structure and function of photosystem II.

Light driven electron transfer is cyclic in Rhodobacter sphaeroides and other purple bacteria (). The reaction center produces reduced quinone, which is oxidized by the cytochrome bc complex. Electrons from the cytochrome bc complex are transferred to a soluble electron carrier, cytochrome c2, which reduces the oxidized primary donor P870+. The product of the light driven electron transfer reactions is ATP. The electrons for the reduction of carbon are extracted from an organic donor, such as succinate or malate or from hydrogen gas, but not by the reaction center. The energy needed to reduce NAD+ is provided by light driven cyclic electron transport in the form of ATP. The energy transformation pathway is complicated. Succinate is oxidized by a membrane bound enzyme (succinate dehydrogenase) that transfers the electrons to quinone, which is the source of electrons for the reduction of NAD+. However, electron transfer from reduced quinone to NAD+ is energetically uphill. By a mechanism that is poorly understood, a membrane bound enzyme is able to use energy stored in the proton electrochemical potential to drive electrons from reduced quinone to NAD+.

Photosynthesis is shown as a series of reactions that transform energy from one form to another. The different forms of energy are shown in boxes and the direction of energy transformation is shown by the arrows. The energy-transforming reaction is shown by italics in the arrows. The site at which the energy is stored is shown in capital letters outside the boxes. The primary photochemical reaction, charge separation, is shown in the oval. Details of these reactions are given in the text.

A. An electron micrograph of a plant chloroplast (Micrograph by A.D. Greenwood, courtesy of J. Barber). The chloroplast is about 6 Å long. Inside the chloroplast is the photosynthetic membrane, which is organized into stacked and unstacked regions. It is not known why the photosynthetic membrane forms such a complicated architecture. The stacked regions are linked by unstacked membranes. B. A model of the chloroplast (Ort, 1994) showing the photosynthetic membrane.

Model of the photosynthetic membrane of plants showing the electron transport components and the ATP Synthase enzyme (cross sectional view). The complete membrane forms a vesicle. The pathways of electrons are shown by solid arrows. The membrane bound electron transport protein complexes involved in transferring electrons are the photosystem II and I reaction centers (PSII and PSI) and the cytochrome bf complex (Cyt bf). Abbreviations: Tyr, a specific tyrosine on the D1 protein ; P680 and P700, the reaction center chlorophyll of photosystem II and photosystem I, respectively; Pheo, pheophytin; QA, and QB bound plastoquinones; PQH2, reduced plastoquinone; Cyt bL and Cyt bH, different forms of b-type cytochromes; FeS, iron-sulfur centers; Cyt f, cytochome f; PC, plastocyanin; A0, chlorophyll; A1, phylloquinone; FX, FA and FB, iron sulfur centers; Fd, ferredoxin; FNR, ferredoxin/NADP+ oxidoreductase; NADPH, nicotinomide adenine dinucleotide phosphate (reduced form); ADP, adenosine diphosphate; ATP, adenosine triphosphate; Pi, inorganic phosphate; H+, protons; DY, the light-induced electrical potential across the membrane. In this diagram, plastoquinone (PQ,PQH2) and plastocyanin (PC) are shown with feet to indicate that they are mobile. The light- harvesting protein complexes are not shown. Details are given in the text.

Chemical structure of chlorophyll a molecule.

TOP: Estimated absorption spectra of chlorophyll a , chlorophyll b and carotenoids in chloroplasts. BOTTOM: Action spectrum of photosynthesis (oxygen evolution/incident photon) shows peaks at wavelengths where chlorophylls a and b have absorption peaks, proving that light absorbed by these pigments leads to photosynthesis (unpublished data).

A simplified scheme showing light absorption in antenna pigments followed by excitation energy transfer to a reaction center chlorophyll. The antenna and reaction center chlorophyll molecules are physically located in different proteins. Primary photochemistry (electron transfer from the primary electron donor to the primary electron acceptor) takes place in the reaction center.

Schematic drawing of photosystem II. Photosystem II is composed of numerous polypeptides, but only two of them, D1 and D2, bind the electron carriers involved in transferring electrons from YZ to plastoquinone. Abbreviations: YZ, tyrosine; P680, reaction center chlorophyll (primary electron donor); Pheo, pheophytin; QA and QB, bound plastoquinone; PQH2, reduced plastoquinone, Cyt b559, b-type cytochrome. Details are given in the text.

Photosystem II electron transport pathways and rates. The vertical axis shows the midpoint potential of the electron carriers. The heavy vertical arrow show light absorption. P680 is the electronically excited state of P680. The abbreviations are given in the legend of figs. 3.

Structure of plastoquinone (reduced form), an aromatic molecule that carries electrons and protons in photosynthetic electron transport.

Yield of oxygen from photosynthetic membranes exposed to a series of brief flashes as a function of flash number. The maximum oxygen yield exhibits a four-flash periodicity. The yield is highest after the third flash and peaks again four flashes later. The four flash dependence of the amplitude gradually decreases as the number of flashes increases due to misses and double hits. The occurrence of the peaks every 4th flash is due to the chemistry of water oxidation (4 electrons must be removed from two water molecules to yield one oxygen molecule) and the machinery of photosystem II (each reaction center works independently, binding two water molecules and releasing one molecule of oxygen every four flashes). Water oxidizing machinery works as a cyclic process that supplies electrons to the oxidized primary donor, P680+. After one flash of light, P680+ is formed, and an electron is transferred via the tyrosine Yz from a manganese complex (4 Mn atoms). After a second flash, this process is repeated and a second oxidation occurs at the Mn complex; after a third flash, a third oxidation occurs; and after a fourth flash, a fourth oxidation occurs, i.e., the Mn complex accumulates 4 positive (+) charges. This enables the Mn complex to oxidize 2 H2O, release molecular oxygen and 4 protons (H+s). This is the process known as the oxygen clock.

Schematic drawing of photosystem I. Photosystem I is composed of numerous polypeptides, but only three of them bind the electron carriers. Abbreviations: PC, plastocyanin; P700, reaction center chlorophyll (primary electron donor); A0, chlorophyll, A1, phylloquinone; FeS, FeS centers; Fd, ferredoxin. Details are given in the text.

Photosystem I electron transport pathways and rates. The vertical axis shows the midpoint potential of the electron carriers. Abbreviations are given in the legend of fig. 11( FA and FB are equivalent names for FeSA and FeSB).

The electron transport pathway of plants (oxygenic photosynthesis). Abbreviations are given the legend of fig. 3. Details are given in the text.

Schematic drawing of the ATP synthase enzyme embedded in the membrane. Proton transfer through the ATP Synthase provides the energy for the creation of ATP from ADP and Pi. Abbreviations are given in the legend of fig. 3. Details are given in the text.

The ATP synthase consists of a membrane portion and an water exposed portion (see Fig. 14). The water exposed portion, which looks like a door knob, has five subunits ( 1g,1d, 1e ). The combine as ab pairs. The catalytic sites of the enzyme are on the b-subunits. The g subunit sort of connects the exposed part to the membrane part (Fo). The diagram shows a model of the top of the ATP synthase according to Boyer (1993). In this model, there are three alternate binding sites. At one site ADP and Pi bind; at another site ADP and Pi produce bound ATP; and at the third site bound ATP is released. In this model, most energy is used to release bound ATP. Each of the three sites perform all three steps, but at different times. Thus, the activity rotates on the a/b pairs. The energy of the proton gradient is converted, in this model, to conformational energy of the g protein that rotates and transfers the energy to the a/b pairs for the simultaneous binding of ADP and Pi and the release of ATP. (Evidence for such a scheme has been found by Abrahams et al. (1994) in beef-heart mitochondria and by Sabbert et al. (1996) in chloroplasts.)

An abbreviated scheme showing reduction of carbon dioxide by the Calvin Cycle. The first step is carboxylation, in which Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the addition of CO2 to the five-carbon compound, ribulose 1,5-bisphosphate, which is subsequently split into two molecules of the three-carbon compound, 3-phosphoglycerate. Next are reduction and phosphorylation reactions that form the carbohydrate, triose phosphate. Some of the triose phosphate molecules are used to form the products of photosynthesis, sucrose and starch, while the rest is used to regenerate ribulose 1,5-bisphosphate needed for the continuation of the cycle. Details are given in the text.

Relative positions of the chromophores of the reaction center of Rhodobacter sphaeroides (from Norris and van Brakel, 1986). Abbreviations: P870, reaction center bacteriochlorophyll (primary electron donor); BChl, bacteriochlorophyll; B Pheo, bacteriopheophytin, QA and QB, bound ubiquinones. Fe is non-heme iron. Diagram shows center to center distances and times for electron transfers. Details are given in the text.

Structure of the bacterial reaction center by H. Michel, J. Deisenhofer and R. Huber and co-workers. It contains three proteins: "H (shown in black) "L" (shown as dotted)" and "M" (shown as hatched bars). Both "L" and "M" have 5 helices each (labeled LA, LB, etc.) and "H" is shown on the very top of the molecule -- it has one helix (HA) that goes through the membrane. P is photoactive dimer of bacteriochlorophyll; B is monomeric bacteriochlorophyll; H is bacteriopheophytin - like bacteriochlorophyll, but without Mg2+; QA and QB are quinone molecules. Diagram courtesy of Colin Wraight.

Comparison of electron transport pathways in oxygenic and anoxygenic organisms (from Blankenship, 1992). Abbreviations: Cyt bc1, cytochrome bc complex; P840, reaction center bacteriochlorophyll; other abbreviations are given in the legend of figs. 3 and 17. 1 Some organisms derive their energy from electron donating inorganic molecules such as hydrogen gas or sulfur compounds and are not dependent on current or past photosynthesis for their survival. Examples include the bacterium Methanobacterium thermoautotrophicum, which grows in sewage sludge living on hydrogen gas and carbon dioxide and the bacterium Methanocococcus jannaschii, which grows in the ocean near hot vents.




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