Photosynthesis proton transport

The Fe-S Reaction Center (Type I Reaction Center) Photosynthesis in green sulfur bacteria involves the same three modules as in purple bacteria, but the process differs in several respects and involves additional enzymatic reactions (Fig. 19-47b). Excitation causes an electron to move from the reaction center to the cytochrome bei complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis, just as in purple bacteria and in mitochondria. 

Fig. 1. Scheme of electron transport in oxygenic photosynthesis. The solid arrows (- ) indicate the direction of electron transport —>, proton transport across the thylakoid membrane ------------>, light re-... 

The fact that the pKa of the various oxidation states of quinones differ dramatically has been exploited in a recent design of artificial systems that mimic the light-driven transmembrane proton transport characteristic of natural photosynthesis [218]. Triad artificial reaction centers structurally related to 41 were vectorially inserted into the phospholipid bilayer of a liposome (vesicle) such that the majority of the quinone moieties are near the external surface of the membrane, and the majority of the carotenoids extend inward, toward the interior surface. The membrane... 

Of the other blue copper proteins, only amicyanin shows a similar effect of pH (79), and a TpK of 7.18 has been obtained for the Cu(I) state. As with plastocyanin, no corresponding effect is observed for Cu(II) amicyanin, at least down to pH 4.5. The physiological relevance in the case of both proteins is at present unclear. Because in photosynthesis the pH of the inner thylakoid is less than 5.0, one possibility is that this is related to proton transport. Alternatively, it quite simply may be a control mechanism for electron transport. 

Animals and bacteria are heterotrophs they obtain carbon in various forms as food and metabolize many forms of it to provide energy and body structure. Plants are autotrophs all their carbon comes from C02 powered by photosynthesis. Photosynthesis occurs within the thylakoid membranes of chloroplasts in plant leaves, and it is mediated by chlorophyll. The light reaction splits water into 02, electrons, and protons (H+). NADPH is produced by electron transport and ATP synthesis by associated proton transport. 

Of particular interest to us are the membrane proteins, numbered I-V. Four of them are part of the electron transport chain in accordance with the chemiosmotic model. FT in the chains is driven by the energy of food or by photosynthesis. Protons are pumped across the membrane to a more acid location. This is done in Complex I, Complex III, and Complex IV. Complex II is used in reduction of ubiquinone to ubiquinol. Another molecule of this type is Complex V, an ATP synthase where ATP is synthesized from ADP and P, as just mentioned. This complex does not have any electron transport chain. 

These proteins, which were the first Zn-containing enzymes to be characterized, are ubiquitous in nature where they are involved in processes such as respiration, photosynthesis, ion transport, and pH homeostasis [209, 210]. The reaction catalyzed by CA is the following where proton transfer to the bulk solvent is the rate-hmiting step in the reaction. 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

Photosynthesis occurs in the plant cell organelle called the chloroplast. During the process of photosynthesis, electrons are transferred from H20 to NADP+ via an electron carrier system. The energy released by electron transport is converted into the form of a proton gradient and coupled to ADP phosphorylation. In this experiment a method is introduced to demonstrate the formation of the proton gradient across the chloroplast membranes. 

When electrons flow from photosystem I to photosystem II, protons are transported across the chloroplast membranes as indicated in Figure E9.1. This aspect of photosynthesis will be discussed in a later section. 

Proton transfer is a particularly important transport process. Beyond acid-base reactions, proton transfer may be coupled to electron transfer in redox reactions and to excited-state chemistry. It is of enormous significance in biochemical processes where it is an essential step in hydrolytic enzyme processes and redox reactions spanning respiration, and photosynthesis where proton motion is responsible for sustaining redox gradients. In relatively recent times, proton transfer in the excited state has undergone significant study, primarily fueled by advances in ultrafast spectroscopy. 

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