Photosynthesis, Biochemistry 9e PDF

680 CHAPTER 22 Photosynthesis Photosystem I can be excited by light of wavelengths shorter than 700 nm, but photosystem II requires light of wavelengths shorter...

680 CHAPTER 22 Photosynthesis Photosystem I can be excited by light of wavelengths shorter than 700 nm, but photosystem II requires light of wavelengths shorter than 680 nm for excita- tion. Both photosystems must operate for the chloroplast to produce NADPH, ATP, and O2, because the two photosystems are connected by the electron trans- port chain. The two systems are, however, structurally distinct in the chloroplast; photosystem I can be released preferentially from the thylakoid membrane by treatment with detergents. The reaction centers of the two photosystems provide different environments for the unique chlorophylls involved. The unique chlorophyll of photosystem I is referred to as P700, where P is for pig- ment and the subscript 700 is for the longest wavelength of absorbed light (700 nm) that initiates the reaction. Similarly, the reaction-center chlorophyll of photosystem II is designated P680 because the longest wavelength of absorbed light that initiates the reaction is 680 nm. Note particularly that the path of elec- trons starts with the reactions in photosystem II rather than in photosystem I. The reason for the nomenclature is that photosystem I was studied extensively at an earlier date than photosystem II because it is easier to extract photosystem I from the thylakoid membrane than it is to extract photosystem II. There are two places in the reaction scheme of the two photosystems where the absorption of light supplies energy to make endergonic reactions take place (Figure 22.7). Neither reaction-center chlorophyll is a strong enough reducing agent to pass electrons to the next substance in the reaction sequence, but the absorp- tion of light by the chlorophylls of both photosystems provides enough energy for such reactions to take place. The absorption of light by Chl (P680) allows electrons to be passed to the electron transport chain that links photosystem II and photosystem I and generates an oxidizing agent that is strong enough to split water, producing oxygen. When Chl (P700) absorbs light, enough energy is provided to allow the ultimate reduction of NADP1 to take place. (Note that the energy difference is shown on the vertical axis of Figure 22.7. This type of diagram is also called a Z scheme. The Z is rather lopsided and lies on its side, but the name is common.) In both photosystems, the result of supplying en- ergy (light) is analogous to pumping water uphill. Phytosystem II: Oxidation of Water to Produce Oxygen The oxidation of water by photosystem II to produce oxygen is the ultimate source of electrons in photosynthesis. These electrons are subsequently passed from photosystem II to photosystem I by the electron transport chain. The electrons from water are needed to “fill the hole” that is left when the absorp- tion of one photon of light leads to donation of an electron from photosystem II to the electron transport chain. The electrons released by the oxidation of water are first passed to P680, which is reduced. There are intermediate steps in this reaction because four electrons are required for the oxidation of water, and P680* can accept only oxygen-evolving complex the part of photosystem one electron at a time. A manganese-containing protein complex and sev- II that splits water to produce oxygen eral other protein components are required. The oxygen-evolving complex Figure 22.7 The Z scheme of photosynthesis. (A) The Z scheme is a diagrammatic representation of photosynthetic electron flow from H2O to NADP1. The energy relationships can be derived from the E°' scale beside the Z diagram, with lower standard potentials and therefore greater energy as you go from bottom to top. Energy input as light is indicated by two broad arrows, one photon appearing in P680 and the other in P700. P680* and P700* represent excited states. Electron loss from P680* and P700* creates P680 and P700. The representative components of the three supramolecular complexes (PSI, PSII, and the cytochrome b6–f complex) are in shaded boxes enclosed by solid black lines. A number of components are represented by letters of the alphabet— chlorophylls and quinones by A and Q, respectively, and ferredoxins by Fiand are further distinguished by subscripts. Proton translocations that establish the proton-motive force driving ATP synthesis are illustrated as well. (B) Figure showing the functional relationships among PSII, the cytochrome b6–f complex, PSI, and the photosynthetic CF1CF0iATP synthase within the thylakoid membrane. Note that e 2 acceptors QA (for PSII) and A1 (for PSI) are at the stromal side of the thylakoid membrane, whereas the e 2 donors to P680 and P700 are situated at the lumenal side of the membrane. The consequence is charge separation (stroma, lumen) across the membrane. Also note that protons are translocated into the thylakoid lumen, giving rise to a chemiosmotic gradient that is the driving force for ATP synthesis by CF1CF0iATP synthase. Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  22-2 Photosystems I and II and the Light Reactions of Photosynthesis 681 A Photosystem I –1.20 P * 700 A0 A1 FA –0.80 FB FX Fd Photosystem II Fp (FAD) –0.40 P * Chl a 680 Pheo QA H+ + NADP+ NADPH QB 0 o' (Cyt b6)N (Cyt b6)P PQ +0.40 Fe-S Cyt f PC hn P 700 +0.80 H2O Protons Protons Mn taken up released complex from stroma into lumen 1 D hn 2 O2 +1.20 P Protons 680 released in lumen +1.60 B 2 H+ H+ ATP ADP + P Stroma H+ + NADP+ NADPH hn hn CF1CF0– Photosystem II Photosystem I ATP Fd synthase Fp Fd (FAD) FeSA FeSB QA QB Cyt b6 Fe FeSX Cyt b6 A1 Pheo Pheo PQ Fe-S A0 P680 P700 Cyt f Mn PC PC complex H2O H+ H+ 2 H+ + 1 2 O2 Lumen Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 682 CHAPTER 22 Photosynthesis Light Light Light Light of photosystem II passes through a series of five oxidation states (designated e– e– e– e– as S0 through S4) in the transfer of four electrons in the process of evolving oxygen (Figure 22.8). One electron is passed from water to PSII for each S1 S2 S3 S4 quantum of light. In the process, the components of the reaction center go S0 successively through oxidation states S1 through S4. The S4 decays spontane- “dark” ously to the S0 state and, in the process, oxidizes two water molecules to one oxygen molecule. Note that four protons are released simultaneously. The im- O2+4 H+ 2H2O mediate electron donor to the P680 chlorophyll, shown as D in Figure 22.7, is a tyrosine residue of one of the protein components that does not contain Figure 22.8 The PSII reaction center passes manganese. Several quinones serve as intermediate electron transfer agents to through five different oxidation states, S0 through accommodate four electrons donated by one water molecule. Redox reactions S4, in the course of oxygen evolution. of manganese also play a role here. Even this mechanism is an oversimplifica- tion. Attempts to observe the direct production of oxygen by the S4 state imply that some intermediate (S4') directly produces oxygen after deprotonation of S3 and loss of an electron by S4. The main point is that the oxygen-evolving complex is very complex indeed. In photosystem II, as in photosystem I, the absorption of light by chloro- phyll in the reaction center produces an excited state of chlorophyll. The wave- length of light is 680 nm; the reaction-center chlorophyll of photosystem II is also referred to as P680. The excited chlorophyll passes an electron to a primary acceptor. In photosystem II, the primary electron acceptor is a molecule of pheophytin a photosynthetic pigment that differs pheophytin (Pheo), one of the accessory pigments of the photosynthetic appa- from chlorophyll only in having two hydrogens in ratus. The structure of pheophytin differs from that of chlorophyll only in the place of magnesium substitution of two hydrogens for the magnesium. The transfer of electrons is mediated by events that take place at the reaction center. The next electron ac- plastoquinone a substance similar to coenzyme Q, ceptor is plastoquinone (PQ). The structure of plastoquinone (Figure 22.9) is part of the electron transport chain that links the similar to that of coenzyme Q (ubiquinone), a part of the respiratory electron two photosystems in photosynthesis transport chain (Section 20-3), and plastoquinone serves a very similar purpose in the transfer of electrons and hydrogen ions. The electron transport chain that links the two photosystems consists of pheo- phytin, plastoquinone, a complex of plant cytochromes (the b6–f complex), a plastocyanin a copper-containing protein; it is copper-containing protein called plastocyanin (PC), and the oxidized form of part of the electron transport chain that links the P700 (see Figure 22.7). The b6–f complex of plant cytochromes consists of two two photosystems in photosynthesis b-type cytochromes (cytochrome b6) and a c-type cytochrome (cytochrome f ). This complex is similar in structure to the bc1 complex in mitochondria and oc- cupies a similar central position in an electron transport chain. This part of the photosynthetic apparatus is the subject of active research. There is a possibility that a Q cycle (recall this from Section 20-3) may operate here as well, and the object of some of this research is to establish definitely whether this is so. In plastocyanin, the copper ion is the actual electron carrier; the copper ion ex- ists as Cu(II) and Cu(I) in the oxidized and reduced forms, respectively. This electron transport chain has another similarity to that in mitochondria, that of coupling to ATP generation. When the oxidized chlorophyll of P700 accepts electrons from the electron transport chain, it is reduced and subsequently passes an electron to photosys- tem I, which absorbs a second photon of light. Absorption of light by photo- O system II does not raise the electrons to a high enough energy level to reduce H3C H NADP1; the second photon absorbed by photosystem I provides the needed energy. This difference in energy makes the Z of the Z scheme thoroughly lop- sided, but the transfer of electrons is complete. H3C H O 6–10 CH3 Phytosystem I: Reduction of NADP+ Plastoquinone The absorption of light by P700 then leads to the series of electron transfer Figure 22.9 The structure of plastoquinone. The reactions of photosystem I. The substance to which the excited-state chloro- length of the aliphatic side chain varies in different phyll, P700*, gives an electron is apparently a molecule of chlorophyll a; this organisms. transfer of electrons is mediated by processes that take place in the reaction Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  22-2 Photosystems I and II and the Light Reactions of Photosynthesis 683 center. The next electron acceptor in the series is bound ferredoxin, an iron–sulfur protein occurring in the membrane in photosystem I. The bound ferredoxin passes its electron to a molecule of soluble ferredoxin. Soluble ferredoxin in turn reduces an FAD-containing enzyme called ferre- doxin-NADP1 reductase. The FAD portion of the enzyme reduces NADP1 to NADPH (Figure 22.7). We can summarize the main features of the process in two equations, in which the notation ferredoxin refers to the soluble form of the protein. Chl* 1 Ferredoxinoxidized S Chl1 1 Ferredoxinreduced Ferredoxin-NADP reductase 2 Ferredoxinreduced 1 H1 1 NADP1 S 2 Ferredoxinoxidized 1 NADPH Chl* donates one electron to ferredoxin, but the electron transfer reactions of FAD and NADP1 involve two electrons. Thus, an electron from each of two fer- redoxins is required for the production of NADPH. The net reaction for the two photosystems together is the flow of electrons from H2O to NADP1(see Figure 22.7). 2H2O 1 2NADP1 S O2 1 2NADPH 1 2H1 Cyclic Electron Transport in Photosystem I In addition to the electron transfer reactions just described, it is possible for cyclic electron transport in photosystem I to be coupled to the production of ATP (Figure 22.10). No NADPH is produced in this process. Photosys- tem II is not involved, and no O2 is generated. Cyclic phosphorylation takes place when there is a high NADPH/NADP 1 ratio in the cell: not enough NADP 1 is present in the cell to accept all the electrons generated by the excitation of P700. H+ H+ ADP + Pi ATP Photon CF1 FeSA FeSB Stroma Cyt b 6 O O FeSX A1 Cyt b 6 O O PQ A0 CF0 FeSR Cyt f P700 Lumen Figure 22.10 The pathway of cyclic PC photophosphorylation by PSI. Note that water Plastocyanin is not split and that no NADPH is produced. docking (Adapted from Arnon, D. I. (1984). The discovery of photosynthetic phosphorylation. Trends Biochem. H+ H+ Sci. 9, 258–262, with permission from Elsevier.) Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 684 CHAPTER 22 Photosynthesis Structure of Photosynthetic Reaction Centers The molecular structure of photosystems is a subject of intense interest to bio- chemists. The most extensively studied system is that from anaerobic photo- tropic bacteria of the genus Rhodopseudomonas. These bacteria do not produce molecular oxygen as a result of their photosynthetic activities, but enough similarities exist between the photosynthetic reactions of Rhodopseudomonas and photosynthesis linked to oxygen to lead scientists to draw conclusions about the nature of reaction centers in all organisms. Since the structure of this photosystem was elucidated by X-ray crystallography, the structures of PSI and PSII have also been determined and have been shown to be markedly simi- Cytochrome with lar. Consequently, the detailed process that goes on at the reaction center of 4 heme groups Rhodopseudomonas is important enough to warrant further discussion. It is well established that there is a pair of bacteriochlorophyll molecules (designated P870 from the fact that light of 870 nm is the maximum excitation wavelength) in the reaction center of Rhodopseudomonas viridis; the critical pair hn of chlorophylls is embedded in a protein complex that is in turn an integral part of the photosynthetic membrane. (We shall refer to the bacteriochloro- phylls simply as chlorophylls in the interest of simplifying the discussion.) Accessory pigments, which also play a role in the light-trapping process, have specific positions close to the special pair of chlorophylls. The absorption of light by the special pair of chlorophylls raises one of their electrons to a higher energy M L level (Figure 22.11). This electron is passed to a series of accessory pigments. The P870 first of these accessory pigments is pheophytin, which is structurally similar to chlo-