Thylakoid lumen

Plant cells contain a unique family of organelles, the plastids, of which the chloroplast is the prominent example. Chloroplasts have a double membrane envelope, an inner volume called the stroma, and an internal membrane system rich in thylakoid membranes, which enclose a third compartment, the thylakoid lumen. Chloroplasts are significantly larger than mitochondria. Other plastids are found in specialized structures such as fruits, flower petals, and roots and have specialized roles. 

Photosynthetic electron transport, which pumps into the thylakoid lumen, can occur in two modes, both of which lead to the establishment of a transmembrane proton-motive force. Thus, both modes are coupled to ATP synthesis and are considered alternative mechanisms of photophosphorylation even though they are distinguished by differences in their electron transfer pathways. The two modes are cyclic and noncyclic photophosphorylation. 

As discussed in Section 22.7, illumination of chloroplasts leads to light-driven pumping of protons into the thylakoid lumen, which causes pH changes in both the stroma and the thylakoid lumen (Figure 22.27). The stromal pH rises, typically to pH 8. Because rubisco and rubisco activase are more active at pH 8, COg fixation is activated as stromal pH rises. Fructose-1,6-bisphosphatase, ribulose-5-phosphate kinase, and glyceraldehyde-3-phosphate dehydrogenase all have alkaline pH optima. Thus, their activities increase as a result of the light-induced pH increase in the stroma. 

Horton P., Ruban, A.V., Rees D., A. Pascal, Noctor, G.D., and Young, A.J. 1991. Control of the light-harvesting function of chloroplast membranes by the proton concentration in the thylakoid lumen aggregation states of the LHCII complex and the role of zeaxanthin. FEBS Lett. 292 1-4. 

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

Kieselbach T, Hagman A, Andersson B, Schroder WP. The thylakoid lumen chlo-roplasts. Isolation and characterization. J Biol Chem 1998 273 6710-6716. 

Bogsch, E., Brink, S., and Robinson, C. (1997). Pathway specificity for a ApH-dependent precursor thylakoid lumen protein is governed by a sec-avoidance motif in the transfer peptide and a sec-incompatible mature protein. EMBO J. 16, 3851-3859. 

Howe, C., and Wallace, T. (1990). Prediction of leader peptide cleavage sites for polypeptides of the thylakoid lumen. Nucl. Acids Res. 18, 3417-3417. 

Like Complex III of mitochondria, cytochrome b6f conveys electrons from a reduced quinone—a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, PQb in chloroplasts)—to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts). As in mitochondria, the function of this complex involves a Q cycle (Fig. 19-12) in which electrons pass, one at a time, from PQBH2 to cytochrome bs. This cycle results in the pumping of protons across the membrane in chloroplasts, the direction of proton movement is from the stromal compartment to the thylakoid lumen, up to four protons moving for each pair of electrons. The result is production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration—a powerful driving force for ATP synthesis. 

Because the four protons produced in this reaction are released into the thylakoid lumen, the oxygen-evolving complex acts as a proton pump, driven by electron transfer. The sum of Equations 19-12 through 19-15 is... 

Electron-transferring molecules in the chain of carriers connecting PSII and PSI are oriented asymmetrically in the thylakoid membrane, so photoinduced electron flow results in the net movement of protons across the membrane, from the stromal side to the thylakoid lumen (Fig. 19-57). In 1966 Andre Jagendorf showed that a pH gradient across the thylakoid membrane (alkaline outside) could furnish the driving force to generate ATP. 

Figure 23-35 Proposed sequence of S-states of the manganese cluster of photosystem II. The successive states as two molecules of H20 (green oxygen atoms) are converted to 02 is shown with the successive states S0-S4 labeled. To save space and possible confusion tyrosine 161 (Yz) and the nearby His 190 are shown only by S4. The Yz radical is thought to remove a hydrogen atom or H+ from one bound H20 and an electron from one Mn ion at each of the four S-states S0-S3 functioning in each case to eject a proton into the thylakoid lumen and to transfer an electron to P+ of the reaction center. However, the exact sequence of e transfer and H+ release may not be shown correctly. After Hoganson and Babcock.392 3923...

Flow of Electrons from H20 to NADP+ Drives Proton Transport into the Thylakoid Lumen Protons Return to the Stroma through an ATP-Synthase Carbon Fixation Utilizes the Reductive Pentose Cycle Ribulose-Bisphosphate Carboxylase-Oxygenase Photorespiration and the C-4 Cycle... 

Transport of two electrons from photosystem II through the cytochrome bhf complex to photosystem I results in the movement of four protons from the chloroplast stroma to the thylakoid lumen. The proton translocation probably occurs in a Q cycle resembling that illustrated in figure 14.11. Two more protons are released in the lumen for each molecule of H20 that is oxidized to 02, and one additional proton is removed from the stroma for each molecule of... 

NADP+ reduced to NADPH. (Two protons are taken up when the flavoprotein is reduced by photosystem I one ends up on NADPH, and the other is returned to the solution.) The flow of protons from the thylakoid lumen back to the stroma through an ATP-synthase (CF0-CF ) drives the formation of ATP. The abbreviations used in this figure are the same as in figure 15.17. 

Electron flow through the cytochrome b6f complex results in proton translocation from the stroma to the thylakoid lumen. In addition, protons are released in the lumen when H20 is oxidized and are taken up from the stromal space when NADP+ is reduced. Protons move from the thylakoid lumen back to the stroma through an ATP-synthase, driving the formation of ATP. 

Phycobiliproteins are found also in cryptophytes but, differently from cyanobacteria and red algae, they are not organized into a phycobilisome, but instead they are located in the thylakoid lumen. Unique for cryptophytes, their phycobiliproteins do not exhibit a trimeric aggregation state characteristic for cyanobacteria, but instead they are present as ai(3a2(3 heterodimers, with each a subunit having a distinct amino acid sequence. [40]... 

From the quinones, the electron is transferred to plastocyanin and then to cytochrome bf. The two H+ ions (protons) left behind remain in the thylakoid lumen. As the electrons move down this electron transport chain, protons are pumped into the thylakoid lumen. Eventually the transported electron is given up to the oxidized P700 chlorophyll of Photosystem I. 

This oxidation transfers four electrons to the Manganese Center, a complex metalloprotein, which then donates the electrons through an intermediate to oxidized P680. The protons derived from water are transported into the thylakoid lumen. The protons pumped into the thylakoid lumen by PSII are used to make ATP through the action of coupling factor, in a mechanism similar to that of mitochondrial ATP synthesis. 

Plastocyanin 11 2 1 0.37 Blue protein (reduced form is colorless) that accepts electrons from the Cyt bff complex and donates them to Photosystem I contains one Cu soluble in aqueous solutions, but occurs in the thylakoid lumen... 

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