20.1: Light Absorption in Photosynthesis - An Overview


Introduction

We have seen how we can transduce the chemical potential energy stored in carbohydrates, into chemical potential energy of ATP. This occurs namely through coupling the energy released during the thermodynamically favored oxidation of carbon molecules through intermediaries (high energy mixed anhydride in glycolysis or a proton gradient in aerobic metabolism) to the thermodynamically uphill synthesis of ATP. There is a situation that occurs when we wish to actually reverse the entire process:

$\ce{CO2 + H2O → carbohydrate + O2} \nonumber$

This process is, of course, photosynthesis, which occurs in plants and certain photosynthetic bacteria and algae. Given that this process must by nature be an uphill thermodynamic battle, let us consider the major requirements that must be in place for it to occur:

• A strong oxidizing agent must be formed which can oxidize water to dioxygen. We know that redox reactions occur in the direction of stronger to weaker oxidizing agent (just as acid base reactions are thermodynamically favored in the direction of strong to weak acid). Somehow we must generate a stronger oxidizing agent than product dioxygen, which often has the most positive standard reduction potential in common tables.
• Plants must have high concentrations of a reducing agent for the reductive biosynthesis of glucose from CO2. The reducing agent used for most biosynthetic reactions in nature is NADPH, which differs from NADH only by the addition of a phosphate to the ribose ring. This phosphate differentiates the pool of nucleotides in the cells used for reductive biosynthesis (NADPH/NADP+) from those used for oxidative catabolism (NADH/NAD+)
• Finally, plants need an abundant source of ATP which will be required for reductive biosynthesis.

The light reaction of photosynthesis produces these three molecules, O2, NADPH and ATP. We will explore the light reaction in the next few sections of this chapter. The dark reaction, which as the name implies can occur in the dark, involves the actual fixation of carbon dioxide into carbohydrate using the ATP and NADPH produced in the light reaction.

The energy to power the light reactions comes directly from sunlight. Clue two is that plants have an organelle that animal cells don't - the chloroplast. Its structure is in many ways similar to a mitochondria in that it has an outer membrane, an intermembrane space, and an inner membrane. In addition, it has a series of stacked, interconnected compartments called thylakoids bounded by a thylakoid membrane surrounding a lumen. A schematic is shown in Figure $$\PageIndex{1}$$ below..

Stack of thylakids are called grana. Each thylakoid represents one granum. The thylakoids are where the light reactions occur. In contrast, the stroma contains the enzymes for dark reaction of photosynthesis.

The enzyme complexes involved in the light reaction are aligned in the thylakoid membrane just as the membrane complex involved in mitochondrial electron transport/oxidative phosphorylation were aligned in the mitochondrial inner membrane. That process used Complex I, Complex III, Complex II (succinate dehydrogenase) and Complex IV to transfer electrons from NADH and FADH2 to increasingly potent oxidizing agents (ubiquinone, cytochrome C) ending with dioxygen. The energy released during that thermodynamically favored process was capture in the formation of a proton gradient, which collapsed through the F0F1ATPase to drive ATP synthesis.

For the light reaction, three complexes, the Light Harvesting ComplexII -Photosystem II (also called the LHC-PSII supercomplex), cytochrome b6f, and the light harvesting complex -Photosystem I (called the LHC-PSII supercomplex) are used to carry out the light reactions of photosynthesis. These include the photoxidation of water to produce O2 and transfer of the lost electrons through a series of mobile electron carriers (plastoquinone and plastacyanin) to a terminal acceptor, NADP+ to form the reducing agent NADPH needed for carbohydrate synthesis in the dark reaction. The energy released during the electron transfer process is likewise captured in the form of a proton gradient as protons are move from the chlorplast stroma to the lumen of the thylakoid. The collapse of the resulting proton gradient powers ATP synthesis. These reactions are shown in Figure $$\PageIndex{2}$$ below.

In this section we will explore the absorption of light by the light harvesting complex (LHCII) of the LHC-PSII supercomplex. Before we get into too much detail, let's start with a simplified review of light absorption in the LHCII.

Absorption of Light

Plants have many pigments (chlorophyll, phycoerthryins, carotenoids, etc.) whose absorption spectra overlap that of the solar spectra. The main pigment, chlorophyll, has a protophorphryin IX ring (same as in heme groups) with Mg2+ at its center instead of Fe2+. When the chlorophyll absorbs light, the excited electrons must eventually relax to their ground state. It can do this by either radiative or nonradiative processes. In radiative decay, a photon of lower energy is emitted (after some energy has already been lost by vibrational transitions) in a process of either fluorescence or phosphorescence. In nonradiative decay, the energy of an excited electron can be transferred to another similar molecule (in this case other chlorophyll molecules) in a process which excites the electron in the second molecule to the same excited state. It is as if a photon is released by the first excited molecule, which then is absorbed by an electron in a second molecule to excite it to the same exited state. However no photon is involved in the energy transfer. In this fashion, energy is transferred from one chlorophyll to another. This type of energy transfer is called resonance energy transfer or exciton transfer, as shown in the Figure $$\PageIndex{3}$$ below.

Because of its unique environment, one type of chlorophyll has slightly different characteristics. The energy level of first excited state in the chlorophyll reaction center is lower than in the rest of the chlorophyll molecules, in much the same way that pKa values of amino acid side chains differ with local and solvent environment, and the standard reduction potential of FAD molecules that are tightly bound to enzymes differ due to the different environment of bound FAD/FADH2.

Instead of radiationless transfer of energy to this special chlorophyll, an actual electron from the excited state chlorophyll is transferred, which by definition is a redox reaction. The electron donor (the excited chlorophyll) looses an electron (an oxidation reaction) as the recipient molecule gains one (a reduction). This charge (electron) transfer reaction produces charge separation on the formation of a positively charged chlorophyll (now an oxidizing agent) and a negatively charged chlorphyll (now a reducing agent). The chlorophylls directly involved in this final process are collectively called the reaction center. This process is shown in Figure $$\PageIndex{4}$$ below.

The reaction center chlorophylls absorbs light at 680 nm so sometimes these chlorophylls are labeled P680. There are 4 unique chlorophylls (PD1, PD2, ChlD1, and ChlD) that are are the main players in the reaction center. Both sets of labels are shown in the figure above.

Figure $$\PageIndex{5}$$ below shows a cartoon of the absorption of photons and subsequent handoff of energy to "antennnae" chlorophylls leading to photoexcitation of the reaction center and subsequent transfer of an electron to the special chlorophyll which has a lower first excited state energy. This molecule is named pheophytin A. It is identical to chlorophyll A but lacks the central Mg2+ ion.

Photosystem I and II contain many chlorophyll molecules that act as antennas which transfer energy to the reaction centers. The "antenna" proteins involved in photon adsorption and energy transfer in Photosystem I and II are shown in the Figure $$\PageIndex{6}$$ below. Note the D1, D2, cp47 and cp43 protein subunits in the figure below are also shown in the figure above above. The D1 and D2 subunits contain the reaction center chlorphylls.

The different chlorophylls have absorption spectra that overlap reasonably well the solar spectrum, as illustrated in Figure $$\PageIndex{7}$$ below.

The black lines shows the solar flux spectrum (photons per meter square per wavelength) from 300 to 1200 nm. The yellow line is the photon energy (E=hc/λ) at each wavelength. The absorbance spectra of the chlorophyll species are normalized to the same maximal value.

If you use the energy required to make glucose from CO2 and H2O as a standard, about 95% of the incoming solar energy is wasted since not all of the incident light photons have the right wavelength. Waste also occurs due to reflectance and heat generation.

The Light Harvesting Complex (LHCII) - Photosystem II (PS II) Supercomplex

Now let's look in more detail at the chloroplast thylakoid membrane complex that interacts with light and results in the oxidation of water to form O2. This first structure is called the Light Harvesting Complex II (LHCII) - Photosystem II (PS II) Supercomplex. It is definitely super complex (a pun) to understand. The supercomplex has a PSII core complex interacting with a variable number of light harvesting complex IIs (LHCIIs) complexes. Sometimes the entire super complex is more simply called Photosystem II. The supercomplex consists of

• a PSII core C with a Mn4CaO5 cluster called the oxygen evolving complex (OEC or OEX) that oxidizes water to O2;
• peripheral antennae complexes M and S, also called light harvesting complexes II (LCHIIs)

The core part of the Photosystem II supercomplex contains the key to the existence of aerobic organisms as it produces the dioxygen in our atmosphere. Its chemistry is phenomenal. Many are seeking to modify it to produce, in addition to O2, a green energy source, H2. We will explore the mechanisms for oxidation of water to O2 by a PSII bound inorganic Mn4CaO5 cluster, the oxygen evolving complex (OEC) in the next section. Lets first look at PSII in light of the discussion above. Where are all the chlorophylls? The mystical "reaction center"?

The PSII Core Complex (within the supercomplex) has over 20 subunits. These include:

• reaction center subunits D1 and D2
• inner antennae subunits CP43, CP47
• many other small protein subunits
• peripheral subunits that project into the lumen that interact with the OEC

The peripheral antennae complexes (light harvesting complexes - LHCIIs M and S - consist of proteins encoded by 6 different Lhcb genes (1-6).

• Lhcb1-3 monomers form trimeric (homo- or hetero) LHCIIs (Lhcb1-3). An example is (Lhcb3)3. That trimer is often called the M-LHCII. There is also an S-form trimer as well as L- and N-forms. S represents strong association with the reaction center, M for moderate and L for loose.
• Lhcb4-6 are called the minor component and consists of the monomeric proteins named CP29, CP26, and CP24, respectively.

Light harvesting complex proteins

Each Lhc apoprotein consists of 3 alpha-helices and binds 8 chlorophylls a, 6 chlorphyll b and 4 carotenoid molecules. Figure $$\PageIndex{8}$$ below shows an interactive iCn3D model of the monomeric pea chlorophyll a-b binding protein AB80 (LHCII type I CAB-AB80) (2BHW)

Figure $$\PageIndex{8}$$: Monomeric pea chlorophyll a-b binding protein AB80 (LHCII type I CAB-AB80) (2BHW). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...QtEEWeQ5Y9n2i8

The color scheme is as follows:

• protein - secondary structure colors
• Chlorophyll a - Green
• Chlorophyll b - yellow
• carotinoids - cyan

Now lets look at a trimeric form of the same protein. of LHCII, the chlorophyll binding protein, from PSII. It's also called LHCII type I CAB-AB80. We showed the role of the LHCII trimer in the figures above. The subunits clearly function to absorb light for the light reaction and through resonance transfers energy to the reaction centers in PSII and in addition PSI. Figure $$\PageIndex{9}$$ below shows an interactive iCn3D model of the trimeric Light Harvesting Complex (LHC_II) from pea photosystem II (2bhw).

Figure $$\PageIndex{9}$$: Light Harvesting Complex (LHC_II) from pea photosystem II (2bhw). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...BK6WvzrxABPDh6

The protein is shown in grey. To simplify the model, chlorophyll A molecules are shown in magenta and chlorophyll B isomeric variants in green. Note the large number of chlorophylls (42) in the LHCII complex, which differentiates it from most protein complexes which might have a just a few ligands bound.

Photosynthetic bacteria and plants have an abundance of molecules that interact with visible light. The main one found in PS II is chlorophyll a whose structure is shown in Figure $$\PageIndex{10}\ below, along withe variant, chlorophyll b. Remember the pheophytin A is identical to chlorophyll A but lacks the central Mg2+ ion. Photosystem II (PS II) Supercomplex Structures The most common vesion of the supercomplex is a dimer. A simplified cartoon of the monomeric version of the PSII-LHCII supercomplex is shown in Figure \(\PageIndex{11}$$ below. The PSII core is shown in the blue rectangle.The CP29, CP26, and CP24 in the peripheral antennae complexes (light harvesting complexes - LCHIIs) are shown outside of the blue-outlined rectangle. They surrounds the PSII core.

• The PSII core C (outlined in the blue box) contains two 2 reaction center subunits D1 and D2, as well as two inner antennae subunits CP43 and CP47. The core also contains extrinsic subunits (pink) surround the OEC (red spacefill)
• A Strongly bound LHCIIs protein, S -LHCII, left (maroon rectangle) interacting with CP26. The S-LHCii complex is actually a trimer invidual Lhcb monomers.
• A Moderately bound LHCIIs protein complex, M-LHCII, right (lighter blue rectangle) interacting with the CP24/CP29 dimer. The M-LHCII complex is actually a trimer of 3 lcbh-3 subunits (Lcbh3)3.

There can be a variable number of LHCIIs in the compex. The predominant form of the supercomplex is the C2S2M2 complex. There are also many other proteins in the supercomplex that are not shown in Figure 11. A cartoon showing the top down view of the actual supercomplex dimer, C2S2M2, is show in Figure $$\PageIndex{12}$$ below.

You could image forming the dimer by spinning the monomer CSM 1800 and reproducing the monomer there.

Figure $$\PageIndex{13}$$ below shows an interactive iCn3D model of the full C2S2M2-type PSII-LHCII supercomplex from Pisum sativum (5XNL). (long load time)

Figure $$\PageIndex{13}$$: C2S2M2-type PSII-LHCII supercomplex from Pisum sativum (5XNL). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...F4WFfHoevxaVV8 (long load time)

Only a single CSM monomer of the C2S2M2 dimeric supercomplex is colored. The colors match those in Figure 8 and 9. The molecules shown in gray sticks that outline a bilayer include chlorophyll Bs (98 of them), chlorophyll As (216) and carotene or its derivatives (88) as well. The different proteins in the supper complex are colored as shown below.

• CP47 Rx Center (inner antenna protein) Dark Blue
• CP43 Rx Center (inner antenna protein) - Purple
• D1: green
• D2: yellow
• OEC: spacefill Red
• O, P, Q - OEC peripheral: Light pink
• CP24 (peripheral antenna protein): yellow
• CP29 (peripheral antenna protein): orange
• CP26: Light blue
• LHCIIs (S and M) - not shown
• membrane bilayers - cyan spheres

PSII Supercomplex Regulation

Plants have evolved a great ability to absorb light over the entire visible range of the spectra. Can they absorb to much energy? The answer is yes, so plants have developed many ways to protect themselves. IF too much light is absorbed, the pH gradient developed across the thylacoid membranes becomes greater. This is sensed by a protein, PsbS, and through subsequent conformational changes transmitted through the light-harvesting antennae, the excess light energy is dissipated as thermal energy. Mutants lacking PsbS showed decreased seed yield, a sign that it became less adaptable under conditions of stress (such as exposure to rapidly fluctuating light levels). Molecules called xanthophylls and other carotinoids such as zeaxanthin are also important in excess energy dissipation. These molecules appear to cause excited state chlorophyll (a singlet like excited state dioxygen) to become deexcited. Without the xanthophylls, the excited state chlorophyll could deexcite by transfer of energy to ground state triplet dioxygen, promoting it to the singlet, reactive state, which through electron acquisition, could also be converted to superoxide. These reactive oxygen species (ROS) can lead to oxidative damage to proteins, lipids and nucleic acids, alteration in gene transcription, and even programmed cell death. Carotenoids can also acts as ROS scavengers. Hence both heat dissipation and inhibition of formation of ROS (by such molecules as vitamin E) are both mechanism of defense of excessive solar energy

Given that both plants and animals must be protected from ROS, antioxidant molecules made by plants may prove to protect humans from diseases such as cancer, cardiovascular disease, and general inflammatory diseases, all of which have been shown to involve oxidative damage to biological molecules. Humans, who can't synthesize the variety and amounts of antioxidants that are found in plants, are healthier when they consume large amounts of plant products. These phytomolecules also have other properties, including regulation of gene transcription which can also have a significant effect on disease propensity.

Plants have to respond to different qualities and quantities of lights. When light is low, they seek to maximize light capture. Too much light could damage a plant so molecular adaptions are made to prevent it. Part of the regulation occurs in the stoichiometry of the supercomplex by altering the antenna protein (the LHCs) composition. The most abundant composition of the complex is C2S2M2 and C2S2M. C2S and C2S2 increase as an adaption to increasing levels of light.

In fact, light levels can also promote grana membrane association mediated by interactions of a PSII-LHCII supercomplex (PSII-LHCIIsc) on one thylakoid membrane interacting with a PSII-LHCIIsc on an adjacent thylakoid membrane to form a large PSII-LHCIIsc dimer. The structure and properties of paired PSII-LHCIIsc are illustrated in Figure $$\PageIndex{14}$$ below.

The figure also shows the variation in the PSII–LHCIIsc with light intensity. At low light C2M2S2 prevails while at hight light intensity C2S2 is most abundant. The system moves to regulate activity by increasing the abundance of LHCIIs in low light and decreasing them in high light intensities!

Figure 1 shows the stacking of individual granum in the chloroplasts. The stacking is maintained at various light levels and is mediated loops of the LHCII trimers that are exposed in the stroma and Lhcb4 subunits on adjacent membranes. The stromal surfaces are flat and tightly stacked in grana. Stacking is a dynamic process and depends on cross membrane interactions between and reorganization of the PSII -LHCIIsc.

The PSII–LHCIIsc is regulated through light-dependent post-translational phosphorylation, particularly on LHCII, and acetylation. which further regulates energy distribution. In plants, the core proteins CP43, D1 and D2, and PsbH are phosphorylated by the kinase Stn8. The peripheral antenna LHCII proteins are phosphorylated by Stn7. Phosphorylation of PSII is not seen in cyanobacteria and red algae. Higher light levels promote higher levels of core protein phosphorylation. Stn7 appears to be inhibited at high light levels. Figure $$\PageIndex{15}$$ shows putative Stn8 phosphorylation sites in the PSII -LHCIIsc

The structure is a model pieced together from multiple pdb files. The approximate positions of phosphorylation sites of D1, D2, CP43, PsbH, and CP29 are shown.

Most free N-terminal loops in LHCII can be phosphorylated. Both phosphorylation and lysine acetylation on Lhcb2 N-terminal loops help regulate the redistribution of LHCII from PSII (in grana stacks) to PSI (in single-layered thylakoid regions). Grana stacking by N-terminal loop association between facing PSII–LHCIIsc and their N-terminal acetylation appear to strengthen grana stacking.

How does PSII respond when not the intensity but the wavelength of light is changed. For this we have to briefly discuss photosystem I (PSI ), which we will explore in more detail in Chapter 20.3. Both PSII and PSI work together to transduce light into chemical energy so you would expect that their activities are regulated in a linked fashion. They have different absorbance spectra characteristics as well, with PSI absorbing more in the red region. If the effective absorbance (normalized for concentrations and LHCIIs) were the same, you would predict that the effective absorbance ratio over a broad wavelength range for the two photosystems, PSI/(PSI+PSII) would be 0.5. This is approximately the case over the entire spectral wavelength except for between 670-730 nm, where the ratio is close to 1, showing that PSI absorbance and hence activity is tilted toward the red end of the absorbance spectra. Changes in light characteristics (i.e. wavelength) would then effect each photosystem differently and can cause imbalances in their activities and in their states, which should lead to a restorative balance. When exposed to far red light, the systems move to state I. In this state, the major mobile antenna proteins (LHCIIs) move to PSII to restore a "photoabsorption" balance. When exposed to light depleted in the high end of visual spectra, the system move to state II, in which mobile LHCIIs move to PSI. What an interesting reciprocal way to balance activity, even if it is hard to conceptualize regulation involving moment of membrane proteins. Of course, such movement is seen often in clustering of ligand-bound membrane receptors.

As mentioned above, the location and hence movement of the LHCII is regulated by phosphorylation by LHCII kinase. We'll explore that more in Chapter 20.3 when we discuss PSI in more detail.

A closer view of the reaction center and its local environment

Let's look at the structure of a simpler PSII complex from Thermostichus vulcanus (3WU2). It has 70 chlorophyll a molecules, 4 special chlorphylls, 4 pheophytin As (PHOs), 20 beta-carotenes, 4 plastoquinol-9s (PL9s), 4 hemes, and 2 caroten-3-ols. Figure $$\PageIndex{16}$$ below shows the arrangement of chlorophyll molecules in the CP47 and CP44 (not CP43 as shown several of the above diagrams) antennae subunits of PSII from T. vulcanus (3WU2).

.

• CP44 is the Photosystem II CP44 reaction center protein, psbC gene, Photosystem II 44 kDa reaction center protein;
• CP47 is the Photosystem II CP47 reaction center protein, psbB gene; Photosystem II CP47 chlorophyll apoprotein

Sandwiched in between them is the reaction center containing the 4 special chlorphylls, the inorganic metal cluster called the oxygen evolving complex (OEC or OEX) and a key amino acid near the OEC, Tyr 161 ( pdb 3ARC).

Two special chlorophylls, ChlD1 and ChlD2, accompanied by partner chlorophylls, PD1 and PD2, that are coplanar to each, are found near the OEC and are the key chlorophylls in the reaction center that turn OEC into a powerful enough oxidant to oxidize H2O

A special reaction center chlorophyll/pheophytin absorbs a photon of light at 680 nm so the species that absorbs the photon is given the label P680. On absorption, it forms the excite state, P680*. This transfers the excited state electron to pheophytin which forms a pheophytin radical anion, while the electron donor P680* becomes P680.+, a radical cation. The radical cation, an unstable species, can oxidize another molecule to regain stability and in a series of addition linked oxidation stems, water is oxidized (O in water has an oxidation state of 2-) to O2 (in which each oxygen has an oxidation state of 0). This happens through the oxygen evolving complex, which we will explore in the next section.

Which of the chlorophyll molecules absorbs the light (ie. which is P680)? The answer is probably all four chlorophylls in the reaction center, PD1, PD2, ChlD1, and ChlD2 through a delocalization of the excited electron, which would be describe by a wave function for the combined chlorophylls. On donation of the electron from the P680*, the positive charge, which can be also described as a "hole" (similar to transistors) delocalizes as well. Quantum mechanical calculations show a coupling between PD1 and PD2, such that 80% of the positive charge and radical character is situated on PD1 and 20% is on PD2.)

Figure $$\PageIndex{17}$$ below shows the four chlorophylls in the reaction center of T. Vulcanus. PD1 appears closest to the OC.

Figure $$\PageIndex{18}$$ below shows an interactive iCn3D model of the key chlorophylls and OEC of photosystem II from Thermostichus vulcanus (3WU2)

Figure $$\PageIndex{18}$$: Key chlorophylls and OEC of photosystem II from Thermostichus vulcanus (3WU2). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...qYxWN5sKcquGK7

Now in any enzymatically catalyzed reaction mechanism, the enzyme must return to the beginning state and a path for electron flow must be apparent. How does the radical cation P680.+ return to the ground state? As we will see in the next section, it "grabs" an electron from the nearby Y161 (also called YZ), which then forms a radical cation, Y.+. This likewise returns to the ground state by grabbing an electron from the OEC which ulitmate grabs one from water. This process repeats four times to remove the four electrons from two waters needed to form dioxgyen.

Figure $$\PageIndex{19}$$ below shows the first step in process of reforming P680 and formation of the Y161 radial cation.

A different process occurs to remove electron from the radical anion, P680.-. This is passed on to a series of other electron acceptors/carriers as part of the Z scheme for the light reaction of photosynthesis. The electron is ultimately passed on to NADP+ to form NADPH for reductive biosynthesis of carbohydrates.

Light is also absorbed in photosystem I, which does not form O2. Rather it receives electrons from a mobile electron carrier and passes them on NADP+ to form NADPH, a reducing agent needed for reductive biosynthesis of carbohydrates. It also helps produce a proton gradient which helps drive ATP synthesis.

With this background, we can now explore in greater detail the key reactions that enable the evolution of aerobic organisms.

20.1: Light Absorption in Photosynthesis - An Overview is shared under a not declared license and was authored, remixed, and/or curated by Henry Jakubowski.