The Central Photochemical Event: Light-Driven Electron Flow: -Bacteria Have One of Two Types of Single Photochemical Reaction Center
One major insight from studies of photosynthetic bacteria came in 1952 when Louis Duysens found that illumination of the photosynthetic membranes of the purple bacterium Rhodospirillum rubrum with a pulse of light of a specific wavelength (870 nm) caused a temporary decrease in the absorption of light at that wavelength; a pigment was “bleached” by 870 nm light. Later studies by Bessel Kok and Horst Witt showed similar bleaching of plant chloroplast pigments by light of 680 and 700 nm. Furthermore, addition of the (nonbiological) electron acceptor [Fe (CN)6]3- (ferricyanide) caused bleaching at these wavelengths without illumination. These findings indicated that bleaching of the pigments was due to the loss of an electron from a photochemical reaction center. The pigments were named for the wavelength of maximum bleaching: P870, P680, and P700. Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical studies have revealed many of the molecular details of reaction centers of bacteria, which therefore serve as prototypes for the more complex phototransduction systems of plants.
The Pheophytin-Quinone Reaction Center (Type II Reaction Center) The photosynthetic machinery in purple bacteria consists of three basic modules (Fig. 19–47a): a single reaction center (P870), a cytochrome bc1 electron transfer complex similar to Complex III of the mitochondrial electron-transfer chain, and an ATP synthase, also similar to that of mitochondria. Illumination drives electrons through pheophytin and a quinone to the cytochrome bc1 complex; after passing through the com plex, electrons flow through cytochrome c2 back to the reaction center, restoring its preillumination state. This light-driven cyclic flow of electrons provides the energy for proton pumping by the cytochrome bc1 complex. Powered by the resulting proton gradient, ATP synthase produces ATP, exactly as in mitochondria. The three-dimensional structures of the reaction centers of purple bacteria (Rhodopseudomonas viridis and Rhodobacter sphaeroides), deduced from x-ray crystallography, shed light on how phototransduction takes place in a pheophytin-quinone reaction center. The R. viridis reaction center (Fig. 19–48a) is a large protein complex containing four polypeptide subunits and 13 cofactors: two pairs of bacterial chlorophylls, a pair of pheophytins, two quinones, a nonheme iron, and four hemes in the associated c-type cytochrome. The extremely rapid sequence of electron transfers shown in Figure 19–48b has been deduced from physical studies of the bacterial pheophytin-quinone centers, using brief flashes of light to trigger phototransduction and a variety of spectroscopic techniques to follow the flow of electrons through several carriers. A pair of bacteriochlorophylls—the “special pair,” designated (Chl)2—is the site of the initial photochemistry in the bacterial reaction center. Energy from a photon absorbed by one of the many antenna chlorophyll molecules sur rounding the reaction center reaches (Chl)2 by exciton transfer. When these two chlorophyll molecules—so close that their bonding orbitals overlap—absorb an exciton, the redox potential of (Chl)2 is shifted, by an amount equivalent to the energy of the photon, con verting the special pair to a very strong electron donor. The (Chl)2 donates an electron that passes through a neighboring chlorophyll monomer to pheophytin (Pheo). This produces two radicals, one positively charged (the special pair of chlorophylls) and one negatively charged (the pheophytin):
(Chl)2+1 exciton → (Chl)2 (excitation)
(Chl)2+Pheo→(Chl)2++Pheo- (charge separation)
The pheophytin radical now passes its electron to a tightly bound molecule of quinone (QA), converting it to a semiquinone radical, which immediately donates its extra electron to a second, loosely bound quinone (QB). Two such electron transfers convert QB to its fully reduced form, QBH2, which is free to diffuse in the mem brane bilayer, away from the reaction center:
2 Pheo-+2H++QB→2 Pheo+QBH2(quinone reduction)
The hydroquinone (QBH2), carrying in its chemical bonds some of the energy of the photons that originally excited P870, enters the pool of reduced quinone (QH2) dissolved in the membrane and moves through the lipid phase of the bilayer to the cytochrome bc1 complex. Like the homologous Complex III in mitochondria, the cytochrome bc1 complex of purple bacteria carries electrons from a quinol donor (QH2) to an electron acceptor, using the energy of electron transfer to pump protons across the membrane, producing a proton motive force. The path of electron flow through this complex is believed to be very similar to that through mitochondrial Complex III, involving a Q cycle (Fig. 19–12) in which protons are consumed on one side of the membrane and released on the other. The ultimate electron acceptor in purple bacteria is the electron depleted form of P870, (Chl)+2(Fig. 19–47a). Electrons move from the cytochrome bc1 complex to P870 via a soluble c-type cytochrome, cytochrome c2. The electron transfer process completes the cycle, returning the reaction center to its unbleached state, ready to absorb another exciton from antenna chlorophyll. A remarkable feature of this system is that all the chemistry occurs in the solid state, with reacting species held close together in the right orientation for reaction. The result is a very fast and efficient series of reactions.
The Fe-S Reaction Center (Type I Reaction Center) Photo synthesis 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 bc1 complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP syn thesis, just as in purple bacteria and in mitochondria.
However, in contrast to the cyclic flow of electrons in purple bacteria, some electrons flow from the reaction center to an iron-sulfur protein, ferredoxin, which then passes electrons via ferredoxin: AD reductase to NAD+, producing NADH. The electrons taken from the reaction center to reduce NAD are replaced by the oxidation of H2S to elemental S, then to SO42-, in the re action that defines the green sulfur bacteria. This oxidation of H2S by bacteria is chemically analogous to the oxidation of H2O by oxygenic plants.
FIGURE 19–47 Functional modules of photosynthetic machinery in purple bacteria and green sulfur bacteria. (a) In purple bacteria, light energy drives electrons from the reaction center P870 through pheo phytin (Pheo), a quinone (Q), and the cytochrome bc1 complex, then through cytochrome c2 back to the reaction center. Electron flow through the cytochrome bc1 complex causes proton pumping, creating an electrochemical potential that powers ATP synthesis. (b) Green sulfur bacteria have two routes for electrons driven by excitation of P840: a cyclic route passes through a quinone to the cytochrome bc1 complex and back to the reaction center via cytochrome c, and a noncyclic route from the reaction center through the iron-sulfur protein ferredoxin (Fd), then to NAD+ in a reaction catalyzed by ferredoxin: NAD reductase.
FIGURE 19–48 Photoreaction center of the purple bacterium Rhodopseudomonas viridis. (PDB ID 1PRC) (a) The system has four components: three subunits, H, M, and L (brown, blue, and gray, re spectively), with a total of 11 transmembrane helical segments, and a fourth protein, cytochrome c (yellow), associated with the complex at the membrane surface. Subunits L and M are paired transmembrane proteins that together form a cylindrical structure with roughly bilateral symmetry about its long axis. Shown as space-filling models (and in (b) as ball-and-stick structures) are the prosthetic groups that participate in the photochemical events. Bound to the L and M chains are two pairs of bacteriochlorophyll molecules (green); one of the pairs (the “special pair,” (Chl)2) is the site of the first photochemical changes after light absorption. Also incorporated in the system are a pair of pheophytin a (Pheo a) molecules (blue); two quinones, menaquinone (QA) and ubiquinone (QB) (orange and yellow), also arranged with bilateral symmetry; and a single nonheme Fe (red) located approximately on the axis of symmetry between the quinones. Shown at the top of the figure are four heme groups (red) associated with the c-type cytochrome of the reaction center. The reaction center of another purple bacterium, Rhodobacter sphaeroides, is very similar, except that cytochrome c is not part of the crystalline complex. (b) Sequence of events following excitation of the special pair of bacteriochlorophylls (all components colored as in (a)), with the time scale of the electron transfers in parentheses. 1 The excited special pair passes an electron to pheophytin, 2 from which the electron moves rapidly to the tightly bound menaquinone, QA. 3 This quinone passes electrons much more slowly to the diffusible ubiquinone, QB, through the nonheme Fe. Meanwhile, 4 the “electron hole” in the special pair is filled by an electron from a heme of cytochrome c.
