The Single Photosystem of Purple Bacteria Generates a Proton Motive Force but No O2

The three-dimensional structures of the photosynthetic reaction centers from two purple bacteria have been determined, permitting scientists to trace the detailed paths of electrons during and after the absorption of light. Similar proteins and pigments compose photosystem II of plants as well, and the conclusions drawn from studies on this simple photosystem have proven applicable to plant systems.

The reaction center of purple bacteria contains three protein subunits (L, M, and H) located in the plasma membrane (Figure 8-35). Bound to these proteins are the prosthetic groups that absorb light and transport electrons during photosynthesis. The prosthetic groups include a "special pair" of bacteriochlorophyll a molecules equivalent to the reaction-center chlorophyll a molecules in plants, as well as several other pigments and two quinones, termed QA and QB, that are structurally similar to mitochondrial ubiquinone.

Initial Charge Separation The mechanism of charge separation in the photosystem of purple bacteria is identical with that in plants outlined earlier; that is, energy from absorbed light is used to strip an electron from a reaction-center bac-teriochlorophyll a molecule and transfer it, via several different pigments, to the primary electron acceptor QB, which is loosely bound to a site on the cytosolic membrane face. The chlorophyll thereby acquires a positive charge, and QB acquires a negative charge. To determine the pathway traversed by electrons through the bacterial reaction center, re-

▲ FIGURE 8-35 Three-dimensional structure of the photo-synthetic reaction center from the purple bacterium Rhodobacter spheroides. (Top) The L subunit (yellow) and M subunit (white) each form five transmembrane a helices and have a very similar structure overall; the H subunit (light blue) is anchored to the membrane by a single transmembrane a helix. A fourth subunit (not shown) is a peripheral protein that binds to the exoplasmic segments of the other subunits. (Bottom) Within each reaction center is a special pair of bacteriochlorophyll a molecules (green), capable of initiating photoelectron transport; two voyeur chlorophylls (purple); two pheophytins (dark blue), and two quinones, QA and QB (orange). QB is the primary electron acceptor during photosynthesis. [After M. H. Stowell et al., 1997, Science 276:812.]

searchers exploited the fact that each pigment absorbs light of only certain wavelengths, and its absorption spectrum changes when it possesses an extra electron. Because these electron movements are completed in less than 1 millisecond (ms), a special technique called picosecond absorption spectroscopy is required to monitor the changes in the absorption spectra of the various pigments as a function of time shortly after the absorption of a light photon.

When a preparation of bacterial membrane vesicles is exposed to an intense pulse of laser light lasting less than 1 ps, each

*A very different type of bacterial photosynthesis, which occurs only in certain archaebacteria, is not discussed here because it is very different from photosynthesis in higher plants. In this type of photosynthesis, the plasma-membrane protein bacteriorhodopsin pumps one proton from the cytosol to the extracellular space for every quantum of light absorbed. This small protein has seven membrane-spanning segments and a cova-lently attached retinal pigment (see Figure 5-13).

reaction center absorbs one photon. Light absorbed by the chlorophyll a molecules in each reaction center converts them to the excited state, and the subsequent electron transfer processes are synchronized in all reaction centers. Within 4 X 10—12 seconds (4 ps), an electron moves to one of the pheophytin molecules (Ph), leaving a positive charge on the chlorophyll a. It takes 200 ps for the electron to move to QA, and then, in the slowest step, 200^s for it to move to QB. This pathway of electron flow is traced in the left portion of Figure 8-36.

Subsequent Electron Flow and Coupled Proton Movement

After the primary electron acceptor, QB, in the bacterial reaction center accepts one electron, forming QB—■, it accepts a second electron from the same reaction-center chlorophyll following its absorption of a second photon. The quinone then binds two protons from the cytosol, forming the reduced quinone (QH2), which is released from the reaction center (see Figure 8-36). QH2 diffuses within the bacterial membrane to the Qo site on the exoplasmic face of a cy-tochrome bc1 complex, where it releases its two protons into the periplasmic space (the space between the plasma membrane and the bacterial cell wall). This process moves protons from the cytosol to the outside of the cell, generating a proton-motive force across the plasma membrane. Simultaneously, QH2 releases its two electrons, which move through the cytochrome bc1 complex exactly as depicted for the mito-

▲ FIGURE 8-36 Cyclic electron flow in the single photosystem of purple bacteria. Blue arrows indicate flow of electrons; red arrows indicate proton movement. (Left) Energy funneled from an associated LHC (not illustrated here) energizes one of the special-pair chlorophylls in the reaction center. Photo-electron transport from the energized chlorophyll, via pheophytin (Ph) and quinone A (QA), to quinone B (QB) forms the semiquinone Q—- and leaves a positive charge on the chlorophyll Following absorption of a second photon and transfer of a second electron to the semiquinone, it rapidly picks up two protons from the cytosol to form QH2. (Center) After diffusing through the membrane and binding to the Qo site on the chondrial CoQH2-cytochrome c reductase complex in Figure 8-21. The Q cycle in the bacterial reaction center, like the Q cycle in mitochondria, pumps additional protons from the cytosol to the intermembrane space, thereby increasing the proton-motive force.

The acceptor for electrons transferred through the cy-tochrome bc1 complex is a soluble cytochrome, a one-electron carrier, in the periplasmic space, which is reduced from the Fe3+ to the Fe2+ state. The reduced cytochrome (analogous to cytochrome c in mitochondria) then diffuses to a reaction center, where it releases its electron to a positively charged chlorophyll a+, returning the chlorophyll to the ground state and the cytochrome to the Fe3+ state. This cyclic electron flow generates no oxygen and no reduced coenzymes.

Electrons also can flow through the single photosystem of purple bacteria via a linear (noncyclic) pathway. In this case, electrons removed from reaction-center chlorophylls ultimately are transferred to NAD+ (rather than NADP+ as in plants), forming NADH. To reduce the oxidized reaction-center chlorophyll a back to its ground state, an electron is transferred from a reduced cytochrome c; the oxidized cytochrome c that is formed is reduced by electrons removed from hydrogen sulfide (H2S), forming elemental sulfur (S), or from hydrogen gas (H2). Since H2O is not the electron donor, no O2 is formed.

Both the cyclic and linear pathways of electron flow in the bacterial photosystem generate a proton-motive force. As exoplasmic face of the cytochrome bc1 complex, QH2 donates two electrons and simultaneously gives up two protons to the external medium, generating a proton-motive force (H+exopiasmic > H+cytosolic). Electrons are transported back to the reaction-center chlorophyll via a soluble cytochrome, which diffuses in the periplasmic space. Operation of a Q cycle in the cytochrome bc1 complex pumps additional protons across the membrane to the external medium, as in mitochondria. The proton-motive force is used by the F0F1 complex to synthesize ATP and, as in other bacteria, to transport molecules in and out of the cell. [Adapted from J. Deisenhofer and H. Michael, 1991, Ann. Rev.. Cell Biol. 7:1.]

Q cycle: additional 2 h+ proton transport

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