The Photosystem II reaction Center

Photosynthetic reaction centre

A photosynthetic reaction centre (or photosynthetic reaction center) is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named chromophores or pigments) such as chlorophyll and phaeophytin, as well as quinones. The energy of the photon is used to promote an electron to a higher molecular energy level of a pigment. The free energy created is then used to reduce a chain of nearby electron acceptors, which have subsequently higher redox-potentials. These electron transfer steps are the initial phase of a series of energy conversion reactions, ultimately resulting in the production of chemical energy during photosynthesis.

The electron on the reduced Pheo is transferred very quickly (after about 0.4 ns) onto plastoquinone QA (P680+ Pheo– QA → P680+ Pheo QA– ). Plastoquinone QA , located on protein D2, is the first stable acceptor of electrons in PSII . At the same time, the oxidized molecule of chlorophyll P680+ is neutralized by an electron taken from tyrosine (Tyrz) in the water-splitting complex on the electron donor side of PSII (Tyrz P680+ → Tyrz+ P680). The localization of separated charges on the stable acceptor and the stable donor prevents their recombination, which ensures effective use of energy during the following stages of photosynthesis

Structures of Two Bacterial Reaction Centers

In 1984, Hartmut Michel, Johann Deisenhofer, Robert Huber, and coworkers in Munich solved the three-dimensional structure of the reaction center from the purple photosynthetic bacterium Rhodopseudomonas viridis (Deisenhofer and Michel 1989). This landmark achievement, for which a Nobel prize was awarded in 1988, was the first high-resolution X-ray structure determination for an integral membrane protein and the first structure determination for a reaction center complex.

Structure of the reaction center of the purple bacterium Rhodopseudo-monas viridis, resolved by X-ray crystallography. The 11 tubes within the membrane represent transmembrane protein helices. The chlorophyll-type pigments are shown in light green; the heme groups of the tightly bound iron-containing cytochrome are shown in dark gray at the top of the diagram. The C (cytochrome subunit) points into the periplasm or outside the cell; the H subunit is in contact with the cytoplasm of the cell. The quinones and the cytochrome participate in electron transfer reactions with the bacteriochlorophylls. (Courtesy of J. Richardson.)

The protein part of the complex consists of four separate polypeptides. Two of them, called L and M (for light and medium mass) bind all of the bacteriochlorophyll, quinone, and carotenoid cofactors of the complex. The structure has a twofold symmetry about an axis perpendicular to the plane of the membrane, hinting at a dimeric nature of the reaction center. The ten transmembrane portions of the L and M peptides (five from each) are arranged in α helices, and there are almost no charged amino acid residues in the interior of the membrane. The H (heavy) protein has a single transmembrane helix and is localized mostly on the cytoplasmic side of the membrane. The C (cytochrome) subunit is located in the periplasmic region (the region between the bacterial plasma membrane and the outer membrane). The geometric arrangement of the pigments and the quinones (electron acceptors) is shown in Web Figure 7.5.B, with the protein removed. A similar arrangement is found in the reaction center of another purple photosynthetic bacterium, Rhodobacter sphaeroides, except that the C subunit is not present.

Web Figure 7.5.B Geometric arrangement of pigments and other prosthetic groups in the bacterial reaction center: bacteriochlorophyll (BChl); bacteriopheophytin (BPh), a chlorophyll molecule in which the magnesium atom has been replaced by two hydrogen atoms; and QA and QB, the first and second quinone acceptors. The cytochrome heme groups have been deleted from this diagram; if present, they would be at the top. The two bacteriochlorophylls at the top (P870) are the “special pair” that react with light within the reaction center. The electron transfer sequence begins at the special pair and proceeds down the right side of the diagram to bacteriopheophytin. Next, the electrons are transferred to QA and then to QB. The distances between the depicted molecules and their angles within the planes of symmetry of the reaction center have been determined with precision, making it possible to analyze the path of photons and electrons in the reaction center in remarkable detail.

Two of the bacteriochlorophyll molecules are in intimate contact with each other and are known as the special pair. This dimer, whose existence was predicted from magnetic-resonance studies, is the photoactive portion of the complex. An electron is transferred from this dimer along the sequence of electron carriers on the right side of the complex. Detailed analysis of these structures, along with analysis of numerous mutants, has revealed many of the principles involved in the energy storage processes that are carried out by all reaction centers.

The bacterial reaction center structure is thought to be similar in many ways to that found in photosystem II from oxygen-evolving organisms, especially in the electron acceptor portion of the chain (see textbook Figure 7.25). The proteins that make up the core of the bacterial reaction center are relatively similar in sequence to their photosystem II counterparts, implying an evolutionary relatedness.

Oxygen Evolution

The chemical mechanism of photosynthetic water oxidation is not yet known, although there is a great deal of indirect evidence about the process. If a sample of dark-adapted photosynthetic membrane is exposed to a sequence of very brief, intense flashes, a characteristic pattern of oxygen production is observed. Little or no oxygen is produced on the first two flashes, and maximal oxygen is released on the third flash and every fourth flash thereafter, until eventually the yield per flash damps to a constant value. This remarkable result was first observed by Pierre Joliot in the 1960s.

A schematic model explaining these observations, proposed by Kok and coworkers, has been widely accepted (Kok et al. 1970). This model for the photooxidation of water, called the S state mechanism, consists of a series of five states, known as S0 to S4, which represent successively more oxidized forms of the water-oxidizing enzyme system, or oxygen-evolving complex (Web Figure 7.7.A). The light flashes advance the system from one S state to the next, until state S4 is reached. State S4 produces O2 without further light input and returns the system to S0. Occasionally, a center does not advance to the next S state upon flash excitation, and less frequently, a center is activated twice by a single flash. These misses and double hits cause the synchrony achieved by dark adaptation to be lost and the oxygen yield eventually to damp to a constant value. After this steady state has been reached, a complex has the same probability of being in any of the states S0 to S3 (S4 is unstable and occurs only transiently), and the yield of O2 becomes constant. States S2 and S3 decay in the dark, but only as far back as S1, which is stable in the dark. Therefore, after adaptation to the dark, approximately three-fourths of the oxygen-evolving complexes appear to be in state S1 and one-fourth in state S0. This distribution of states explains why the maximum yield of O2 is observed after the third of a series of flashes given to dark-adapted chloroplasts.

Web Figure 7.7.A  

Rensselaer researchers to map step-by-step mechanism of photosynthesis

"Photosystem II powers the planet with solar energy. If there is a design that is perfect for harnessing the energy of the sun, this is it.

This S state mechanism explains the observed pattern of O2 release, but not the chemical nature of the S states or whether any partly oxidized intermediates, such as H2O2, are formed. Additional information has been obtained by measurement of the pattern of proton release  as the S states are advanced with flashes. In addition to oxygen, hydrogen ions are a product of water oxidation. The release of protons is not strictly coupled to O2 release.[/b][/b]

The PS-I reaction center

The PS-I reaction center is composed of a multiprotein complex. The reaction center chlorophyll P700 and about 100 core antenna chlorophylls are bound to two proteins, PsaA and PsaB, with molecular masses in the range of 66 to 70 kDa (Golbeck 1992; Krauss et al. 1993, 1996; Chitnis 1996; Jordan et al. 2001). PS-I reaction center complexes have been isolated from several organisms and found to contain the 66 to 70 kDa proteins, along with a variable number of smaller proteins in the range of 4 to 25 kDa (see textbook Figure 7.29). Some of these proteins serve as binding sites for the soluble electron carriers plastocyanin and ferredoxin. The functions of some of the other proteins are not well understood. An 8-kDa protein contains some of the bound iron-sulfur centers that serve as early electron acceptors in photosystem I. The structure of the PS-I complex from pea has been determined to a resolution of 4.4 Å, and the positions of many of the chlorophylls and electron transfer components have been located (Krauss et al. 1996; Schubert et al. 1997; Jordan et al. 2001; Nelson and Ben-Shem 2004; see textbook Figure 7.29).

In their reduced form, the electron carriers that function in the acceptor region of photosystem I are all extremely strong reducing agents. These reduced species are very unstable and thus difficult to identify. Evidence indicates that one of these early acceptors is a chlorophyll molecule, and another is a quinone species, phylloquinone, also known as vitamin K1 (Nugent 1996).

Additional electron acceptors include a series of three membrane-associated iron–sulfur proteins, or bound ferredoxins, also known as Fe–S centers Fe–SX, Fe–SA, and Fe–SB. Fe–S center Fe–SX is part of the P700-binding protein; centers Fe–SA and Fe–SB reside on an 8 kDa protein that is part of the PS-I reaction center complex. Electrons are transferred through centers Fe–SA and Fe–SB to ferredoxin, a small, water-soluble iron–sulfur protein. The membrane-associated flavoprotein ferredoxin–NADP reductase (FNR) reduces NADP+ to NADPH, thus completing the sequence of noncyclic electron transport that begins with the oxidation of water (Karplus et al. 1991).

Electron transport between PSII and PSI is mediated by plastohydroquinone, the cytochrome b6f complex, and plastocyanin (see textbook Figure 7.28). The cytochrome b6f complex contains two b-type hemes and one c-type heme (Web Figure 7.8.A)



Web Figure 7.8.A   Structure of prosthetic groups of b- and c-type cytochromes. The protoheme group (also called protoporphyrin IX) is found in b-type cytochromes, the heme c group in c-type cytochromes. The heme c group is covalently attached to the protein by thioether linkages with two cysteine residues in the protein; the protoheme group is not covalently attached to the protein. The Fe ion is in the 2+ oxidation state in reduced cytochromes and in the 3+ oxidation state in oxidized cytochromes.

A model for the organization of electron carriers in PS-I is shown in Web Figure 7.8.B. The P700 dimer is located at the bottom of the structure and two symmetrical arms radiate from P700 (Malkin and Niyogi 2000). Each arm includes an accessory chlorophyll a and another chlorophyll molecule tentatively identified as A0. Other structures include the Fe-S center FX, and two other Fe-S centers, F1 and F2. The distance between the carriers is shown at the right.



Web Figure 7.8.B   A model for the organization of electron carriers in PS-I

The PS-I reaction center appears to have some functional similarity to the reaction center found in the anaerobic green sulfur bacteria and the heliobacteria. These bacteria contain low-potential Fe–S centers as early electron acceptors and are probably capable of ferredoxin-mediated NAD+ reduction similar to the NADP+ reduction function of photosystem I. There is almost certainly an evolutionary relationship between these complexes and photosystem I of oxygen-evolving organisms.

welcome back to our study of photosynthesis in this video we're going to talk about the basic mechanics of the photosynthetic reaction center in photosystem 2.  now i had mentioned in previous videos that photosynthesis at least the light-dependent reactions are named somewhat strangely. photosystem 2 comes before photosystem 1 in the process.  the reason that photosystem 2 is named such even though it comes first is because photo system one was the first one that was discovered and no one bothered to correct the naming system since it was already convention at that point it turns out that the main initiation of all the reactions in photosystem 2 is in what's called the p680 reaction center the p680 gets its name from the fact that it absorbs life of 680 nanometers okay and one of the key things here, as shown in the picture over to the left, is that it does absorb light okay now we're going to go into some more slides with some more detail on this but this is just a basic background of the p680 reaction center.

alright the p680 reaction center has what's in it called a special pair of chlorophyll molecules.  The special pair of chlorophyll molecules are two chlorophyll molecules that are very very close to one another within angstroms of each other and it turns out the thought that special pair of chlorophyll molecules plays a huge role in initiating the photosynthetic electron transport chain. That special pair of chlorophyll run just for now i'm going to refer to it as a special pair the special pair is able to donate electrons but it's only able to donate electrons once it receives sufficient energy ultimately from UV light from the Sun.  all right when the p680 special pair receives sufficient and enough energy from UV photons it's able to donate an electron to an electron acceptor.

That electrons except where specifically is co fighting but we're not going to concern ourselves with that now now when the special ter donates an electron it's essentially acting as reducing agents right it's reducing an electron acceptor so the p680 special pair gives up an electron therefore p680 has what we refer to as an electron hole now I don't know that I like this terminology very well but it if you put a lot of textbooks use all an electron hole is is an oxidized form of a molecule okay so if a molecule gives an electron up its charge goes up by one it becomes more positive that's what we mean by an electron hole and since it gave up an electron to something else that electron acceptor now has a charge that goes down by one all right, in other words, p680 went up by one in charge and the acceptor went down in one by charge.

This is what we're going to refer to a separation of charge and we'll look at this in more detail in another slide okay but suffice it to say the p680 now has a positive charge or at least it's one more positive than it was it has an electron hole it turns out that that electron hole which in other words it's devoid of an electron it gets that electron back by siphoning electrons from water okay remember that water is a substrate of photosynthesis remember all the synthesis produces oxygen the question is why do plants need water to live besides creating turgor pressure in the stem and allowing it to have structure water is used to feed electrons back to p680 the electrons it gave up it has to get electrons back otherwise it's going to stall the plant will die so it has to get electron from water.

So in other words the p680 reaction center in tandem with another enzyme that we're going to look at in a few slides ultimately catalyzes the conversion of water to oxygen the oxygen that we breathe in the atmosphere and when it does that it takes those electrons to replace the electron hole from the electron that was donated by the special pair of p680 now here's a question for you what is the free energy change in other words the Delta G for the oxidation of water to oxygen positive or negative ?

I don't care about a magnitude.  it's very very positive okay the standard free energy change for this reaction is extremely positive okay and that in other words means that it's very thermodynamically unfavorable so how our plants able to accomplish this well it turns out that when p680 is oxidized in other words we designate that as p680 plus this is p680 oxidized but the plus charge indicates the electron hole remember that it's going to accept electrons from water ultimately well the only reason that this conversion of water to oxygen with such a positive Delta G is able to occur is oxidized p680 is without a doubt was strongest biological oxidizing agent known it's fairly easy for this enzyme to take the electrons away from water if p680 was not a strong oxidizing agent like it is here this wouldn't occur in fact it has an estimated redox potential of approximately 1.3 volts.

This is the only the thing that makes it possible to oxidize water into molecular oxygen now what we have in photosystem 2 which is shown on the left is unidirectional electron flow now p680 is shown right here and from on the bottom is low energy relatively and on the top is higher relative energy when light energy strikes ultimately but indirectly the p680 reaction Center an electron in the special pair of chlorophyll goes up in energy and when it goes up in energy here's p680 in the excited state or in particular that electron is in the excited state it turns out that electron is able to be transferred to an electron acceptor known as field fightin okay and I happen in this list right here at unidirectional electrons are going to flow from a carrier to carrier to carrier ultimately until we get to a complex referred to as the cytochrome b6f complex.

When p680 is in the excite stayed up here in higher energy or in other words its electron is the electron will be transferred to a nearby electron acceptor known as field fightin and it's really not so much important to know what the exact electronic scepters and donors are but what you should understand is that the electron is transferred unidirectionally in this case reveal fightin to plastic went on a two plastic women beat and then ultimately to this proton pumping complex right here referred to a cytochrome b6f okay it turns out this complex right here is going to be very important because this is one way that the chloroplast is going to generate ATP and it turns out that a PP will use we will be used in the Calvin cycle which is part of the elect the light independent reactions all right but there are several things that are really important to understand electrons are transferred unidirectionally to this proton pump right here okay now what you see over here is the water splitting complex another name for this enzyme is the oxygen-evolving complex the reason that it's called both is because number when it splits water by siphoning off waters electrons but it also evolves oxygen oxygen because sort of bubbles off you could sort of think but remember when this p680 transfers its electrons to feel fightin it now has that electron hole and it turns out that the water splitting complex or oxygen-evolving complex takes the electrons one of the time for water and feeds them to p680 in order to replenish those electrons okay and will later go into more photosystem 1 it turns out that also there's a reactions in there here called p700 that function is very similar to li 2 p680

what you'll notice here is I have the title special pair of p680 now if you look down here this is not p680 this is actually p700 part of photosystem 1 but this diagram will suffice for what we'retalking about okay what's meant to be shown down here is this is the special pair of chlorophyll molecules okay although this is a P700  p680 these except errs over here are a little bitdifferent but suffice to say this special pair of chlorophyll molecules is going to behave as one unit they're very very close in proximity to one another and it turns out that that close proximity allows them to have some very different properties that other chlorophyll molecules do not have one of those is the capacity to actually very strongly donate electrons okay it turns out that whenever this p700 special pair gets excited just like the p680 in photosystem 2 it's able to go up very high in energy and donate electrons to various electronic scepters okay now it is true these chlorophylls over here can donate electrons but they're not able to go up in energy as much the fact that these chlorophyll are paired up in other words allows them to increase in energy drastically and become a very very strong oxidizing agent okay especially in the case of p680 all right now this is where we're going to talk about charge separation all right now we talked about electron holes and all that stuff this is more of an electron diagram not molecular okay this is our starting point okay 

Our starting point is actually down here I probably should have switched the order of these pictures but it's too late this is our starting point we have two electrons here that are paired up all right this is the starting point for p680 both electrons paired up opposite spin states in the same orbital light energy H nu or hv strike these electrons and causes one of them to go up in energy once we get the electron into the excited state up here it turns out this high-energy electron is a fantastic reducing agent this one at the bottom is the elect is going to be what's called the electron hole it's not a good reducing agent it's actually a good oxidizing agent so these electrons are going to in these spaces where they are or going to behave very differently so this state right here once we have UV photons exciting this electron this is what we have here so this is our donor this is our special pair right here and this is the acceptor it turns out that this electron can be transferred to this acceptor right here notice the state over here once we get the electron transfer also notice the excited molecule to p680 which is this D is now d plus that's what we saw on a previous slide when we were looking at p680 plus ok that's the D plus it now has an electron hole and it is a very powerful oxidizing agent it will take an electron to replace that and it turns out that it replaces it with electrons from water a reaction that's catalyzed by that oxygen-evolving complex the acceptor got an electron and it turns out it has an a minus charge now ok it's an acceptor but it has one more electron so it's charge went down by one ok now hopefully this makes sense we start off with the donor these are both for the donor donor has low energy electrons paired up you get UV photons that excite or energize this electron up to the excited state as you see here and this electron then can be transferred to a nearby acceptor in the case of photosystem 2 that except there is pheophytin ok and when that electron is transferred to feel fight and co fightin would be this a minus and the p680 would be the electron hole p680 p680 plus all right in the next video we're actually going to look specifically at the oxygen-evolving complex and talk about it in a lot more detail

More readings: 
http://www.saps.org.uk/secondary/teaching-resources/283-photosynthesis-how-does-chlorophyll-absorb-light-energy