7.2 Photosynthetic Electron Transport
In plants the photosynthetic process occurs inside chloroplasts, which are small organelles (5-10 microns across) found inside specialized cells. The chloroplast consists of three membranes, an outer envelope membrane, an inner envelope membrane, and an internal membrane system, known as the photosynthetic or thylakoid membrane, which absorbs light, transfers electrons and protons, and produces ATP (see Chapter 1 for a more complete description of the structure of the chloroplast). The photosynthetic membrane is composed mostly of glyceral lipids in the form of a bilayer, which is heavily embedded with the protein complexes that make up the photosynthetic apparatus. To transduce light energy the photosynthetic membrane functions as a vesicle, with an inner (lumen) and outer (stromal) water phase. The protein complexes are asymmetrically arranged in the photosynthetic membrane, enabling electron transport to create a proton gradient and an electric potential across the membrane. The energy stored in the proton electrochemical gradient drives a membrane bound ATP synthase that produces ATP from ADP and inorganic phosphate. The enzymes required for the fixation and reduction of CO2 are located outside the photosynthetic membrane in the surrounding aqueous phase of the chloroplast.
The photosynthetic membrane is arranged in circular stacks (grana) that are interconnected by non-stacked membranes (stromal membranes) (see Chapter 1). To further complicate this picture, the protein complexes in the membrane are deployed unevenly between grana and stroma membranes. Fortunately, to understand the fundamentals of photosynthetic electron transport and energy transduction, we can ignore the complexity of the design and treat the photosynthetic membrane as a simple vesicle with an inner and outer aqueous space. The structure of the photosynthetic membrane and the consequences of separating electron transport complexes between grana and stroma membranes are discussed in Section 7.6.
Electron transfer from water to NADP+ involves three integral membrane protein complexes operating in series: the PS II reaction center, the cytochrome bf complex, and the PS I reaction center (Fig. 7.1). The two reaction centers are the site of primary charge separation, in which light energy or exciton energy is transformed into redox free energy. Photosynthetic electron transport consists of a series of electron transfer steps from one electron carrier to another over relatively short distances - typically less than 20 Å. Most of the electron carriers are metal ion complexes bound within proteins, although a few of the carriers are aromatic groups. The only non-protein carriers involved in photosynthetic electron transport are plastoquinone and NADPH. The transfer of an electron from one site to another is known as an oxidation/reduction or redox reaction (for a discussion of the thermodynamics of redox reactions see Cramer and Knaff 1991) In photosynthetic electron transport an oxidation reaction always is coupled to a reduction reaction, so that each component acts as an electron acceptor and as an electron donor. Furthermore, some redox reactions involve the loss or gain of protons along with electrons. This coupling of proton transfer to electron transport reactions is essential for energy transduction by the photosynthetic membrane.
The energy for this uphill electron transfer reaction is provided by light that is either absorbed by the reaction center directly or transferred to it as an exciton from the light harvesting antenna system (light absorption and the antenna system are described in Chapter 2). The primary photochemical reaction of photosynthesis is charge transfer from an excited electronic state of a specialized electron donor to an electron acceptor. This reaction occurs in each reaction center. In the case of PS II, the primary electron donor is reduced by electrons removed from water. The oxidation of two water molecules by PS II results in the release of molecular oxygen into the atmosphere. In this section we will ignore the pathway of electrons and protons within PS II and view it as a protein complex that extracts electrons from water molecules and transfers them to plastoquinone. The internal workings of PS II and the other protein complexes will be discussed in section 7.4.
As shown in Fig. 7.1, electrons are transferred from PS II to the cytochrome bf complex by plastoquinone, which functions as a mobile electron carrier within the hydrophobic core of the photosynthetic membrane. Plastoquinone is a key player in energy transduction because it links electron transport to proton transfer across the photosynthetic membrane. The reduction of plastoquinone by PS II requires two electrons and two protons, creating PQH2. The reduced plastoquinone molecule unbinds from PS II and diffuses in the photosynthetic membrane until it encounters a specific binding site on the cytochrome bf complex, which is a membrane bound protein complex containing four electron carriers. In a complicated reaction sequence that is not fully understood, the cytochrome bf complex removes two electrons from PQH2 and releases protons into the inner aqueous space. The electrons extracted from plastoquinone are transferred to plastocyanin, a small copper-containing protein that operates in the inner aqueous space of the photosynthetic membrane.
The next step in electron transfer is driven by light and occurs in the PS I reaction center. As in PS II, the primary photochemical reaction is charge separation between the primary donor of PS I, P700, and the primary acceptor, a chlorophyll a molecule. Plastocyanin serves as the electron donor to P700+. Ferredoxin, a small FeS protein located in the stromal aqueous space of the chloroplast, serves as the electron acceptor. Electrons are transferred from ferredoxin to NADP+ by ferredoxin-NADP oxidoreductase (FNR), a peripheral flavoprotein located on the stromal surface of the photosynthetic membrane. The final product of light driven linear electron transport is NADPH, a small mobile electron carrier operating in the stromal phase of the chloroplast. The transfer of a single electron from water to NADP+ involves about 29 metal ions, including Fe, Mg, Mn and Cu, and approximately seven nonmetal carriers, including quinones, pheophytin, NADPH, tyrosine, and flavin.
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