Redox potential chloroplast components

Ubiquinone or Q (coenjyme Q) (Figure 12-5) finks the flavoproteins to cytochrome h, the member of the cytochrome chain of lowest redox potential. Q exists in the oxidized quinone or reduced quinol form under aerobic or anaerobic conditions, respectively. The structure of Q is very similar to that of vitamin K and vitamin E (Chapter 45) and of plastoquinone, found in chloroplasts. Q acts as a mobile component of the respiratory chain that collects reducing equivalents from the more fixed flavoprotein complexes and passes them on to the cytochromes. 

Bend all in 1960 takes into consideration the results of many investigators and has become generally accepted as an overall description of electron flow in chloroplast lamellae. After introducing the concept of redox potential in Chapter 6 (Section 6.1 C), we will portray the energetics of the series representation (see Fig. 6-4, which includes many of the components that we will discuss next). 

We shall now turn our attention to the specific molecules that act as electron acceptors or donors in chloroplasts. A summary of the characteristics of the most common components of this complex pathway is presented in Table 5-3. Figure 6-4 should also be consulted, if the underlying concept of redox potential is already familiar. We will begin our discussion by considering the photochemistry at the reaction center of Photosystem II and then consider the various substances in the sequence in which they are involved in electron transfer along the pathway from Photosystem II to Photosystem I. We will conclude by considering the fate of the excited electron in Photosystem I. 

In Chapter 5 (Section 5.5B), we introduced the various molecules involved with electron transfer in chloroplasts, together with a consideration of the sequence of electron flow between components (Table 5-3). Now that the concept of redox potential has been presented, we will resume our discussion of electron transfer in chloroplasts. We will compare the midpoint redox potentials of the various redox couples not only to help understand the direction of spontaneous electron flow but also to see the important role of light absorption in changing the redox properties of trap chi. Also, we will consider how ATP formation is coupled to electron flow. 

As for chloroplast membranes, various compounds in mitochondrial membranes accept and donate electrons. These electrons originate from biochemical cycles in the cytosol as well as in the mitochondrial matrix (see Fig. 1-9) —most come from the tricarboxylic acid (Krebs) cycle, which leads to the oxidation of pyruvate and the reduction of NAD+ within mitochondria. Certain principal components for mitochondrial electron transfer and their midpoint redox potentials are indicated in Figure 6-8, in which the spontaneous electron flow to higher redox potentials is toward the bottom of the figure. As for photosynthetic electron flow, only a few types of compounds are involved in electron transfer in mitochondria—namely, pyridine nucleotides, flavoproteins, quinones, cytochromes, and the water-oxygen couple (plus some iron-plus-sulfur-containing centers or clusters). 

In 1965 Hill elaborated the two-photosystem scheme further as shown in Fig. 15 (B). In this Z-shaped scheme, two groups of chloroplast components with known redox potentials were placed at the bends of the Z Cyt/, plastocyanin and P700, close to -1-0.4 V, and plastoquinone and Cyt b( close to 0 V. Ferredoxin, with a potential of-0.43 V, is close to the midpoint potential ofhydrogen electrode. For oxygen production, the midpoint potential of the unknown component must exceed that of the oxygen electrode. Over the past thirty years, a variety of Z-schemes have been published in the literature to illustrate the electron-transfer processes in green-plant photosynthesis, but their basic features have not deviated from that shown in Fig. 15 (B). For instance, we show a currently accepted, concise Z-scheme in Fig. 15 (C) it includes many more individual components than were originally envisioned, plus a representation of the operation of the so-called Q-cycle in the Cyi-b(,f complex. 

Kuroda FI, Kobashi K, Kaseyama H et al. Possible involvement of a low redox potential component(s) downstream of photosystem I in the translational regulation of the D1 subunit of the photosystem II reaction center in isolated pea chloroplasts. Plant Cell Physiol 1996 37 754-761. 

Fig. 4. (A) Scheme of possible molecular arrangement of a chloroplast extract membrane containing chlorophyll, carotenoid and phospholipid as essential components. Ch, chromophore portion of chlorophyll Pht, phytol tail of chlorophyll C, carotenoids P, phospholipid. (B) The corresponding energy barrier diagram for the membrane (M)-water (W) system. The broken line in the membrane represents the reduced energy barrier level due to carotene. The solid lines in the potential wells represent the ground and first excited states of chlorophyll. The dotted line at the right of the membrane solution energy barrier represents the lowering of the energy barrier due to excited biliprotein (phy-cocyanin). The 4 A width is the distance of closest approach between the redox species in water and the chromophore in the membrane.

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