Thylakoid lumen proton transport into

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... 

Figure 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

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. 

Because photosystem 11 and the cytochrome b/f complex release protons from reduced plastoquinone into the lumen (via a Q. cycle), photosynthetic electron transport establishes an electrochemical gradient across the thylakoid membrane (see p. 126), which is used for ATP synthesis by an ATP synthase. ATP and NADPH+H", which are both needed for the dark reactions, are formed in the stroma. 

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. 

Mitchell s theory holds that an electrochemical proton gradient across the membrane (which is only slightly permeable to many ionized species and particularly to H ") is formed by the vectorial transport of into the thylakoid lumen coupled to electron transport, as a consequence of the alternate disposition across the membrane of electron carriers which can bind protons and others which cannot be protonated. 

In this way, the loss of redox free energy occurring during electron transport is partially conserved as electrochemical potential energy of the proton gradient. The synthesis of ATP occurs when the protons accumulated inside the thylakoid lumen are transported out into the external water phase by an anisotropic, proton-translocating ATP synthase-ATPase (the complex CFq-CFi), which catalyses the reaction... 

Figure 19.25. Comparison of Photosynthesis and Oxidative Phosphorylation. The light-induced electron transfer in photosynthesis drives protons into the thylakoid lumen. The excess protons flow out of the lumen through ATP synthase to generate ATP in the stroma. In oxidative phosphorylation, electron flow down the electron-transport chain pumps protons out of the mitochondrial matrix. Excess protons from the intermembrane space flow into the matrix through ATP synthase to generate ATP in the matrix.

The flow of electrons from photosystem II to photosystem I drives the transport of protons into the thylakoid lumen. Electron transfer through the iron-sulfur proteins FeA and FeB is not understood. ° values are approximate. MSP contains a manganese cluster. (MSP = manganese stabilizing protein)... 

At two stages in the process of transporting electrons, protons are released into the thylakoid lumen. The transfer of protons into the lumen produces a pH gradient across the thylakoid membrane. The resulting proton gradient is used to drive the synthesis of ATP in a manner very similar to that used in oxidative phosphorylation. In oxidative phosphorylation, mitochondria establish a proton gradient that allows the synthesis of ATP to be coupled to electron transport. 

Cytochrome b6f is a is part of the electron transport chain that transfers electrons from photosystem II to photosystem I. Cytochrome b6f is a complex of proteins that includes cytochromes f, b6, and an iron sulfur protein. Cytochrome b6f accepts electrons from plastoquinone QH2 and passes them to plastocyanin (Figure 17.12). In addition to transferring electrons, the cytochrome b6f complex pumps protons into the thylakoid lumen, helping to build the proton gradient, which is used by the CFO-CFl complex to make ATP. 

The active site of the oxygen-evolving enzyme is arranged so that the protons formed during water oxidation are released into the thylakoid lumen. These protons contribute to the electrochemical proton potential. The thylakoid membrane contains aprotein that functions to transport Cl across the membrane. Proton accumulation in the thylakoid lumen is electrically balanced in large part by Cl uptake. As a result, thylakoids accumulate HCl and the membrane potential across the membrane is low. The pH inside the lumen during steady-state photosynthesis is about 5.0. 

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