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FIGURE 19-58 Comparison of the topology of proton movement and ATP synthase orientation in the membranes of mitochondria, chloroplasts, and the bacterium E. coli. In each case, orientation of the proton gradient relative to ATP synthase activity is the same.

chloroplasts by division, during which their DNA is replicated and divided between daughter chloroplasts. The machinery and mechanism for light capture, electron flow, and ATP synthesis in photosynthetic bacteria are similar in many respects to those in the chloroplasts of plants. These observations led to the now widely accepted hypothesis that the evolutionary progenitors of modern plant cells were primitive eukaryotes that engulfed photosynthetic bacteria and established stable endosymbiotic relationships with them (see Fig. 1-36).

Diverse Photosynthetic Organisms Use Hydrogen Donors Other Than Water

At least half of the photosynthetic activity on Earth occurs in microorganisms—algae, other photosynthetic eukaryotes, and photosynthetic bacteria. Cyanobacteria have PSII and PSI in tandem, and the PSII has an associated water-splitting activity resembling that of plants. However, the other groups of photosynthetic bacteria have single reaction centers and do not split H2O or produce O2. Many are obligate anaerobes and cannot tolerate O2; they must use some compound other than H2O as electron donor. Some photosynthetic bacteria use inorganic compounds as electron (and hydrogen) donors. For example, green sulfur bacteria use hydrogen sulfide:

These bacteria, instead of producing molecular O2, form elemental sulfur as the oxidation product of H2S. (They further oxidize the S to SO|-.) Other photosynthetic bacteria use organic compounds such as lactate as electron donors:

2 Lactate + CO2 ''ght > (CH2O) + H2O + 2 pyruvate

The fundamental similarity of photosynthesis in plants and bacteria, despite the differences in the electron donors they employ, becomes more obvious when the equation of photosynthesis is written in the more general form

in which H2D is an electron (and hydrogen) donor and D is its oxidized form. H2D may be water, hydrogen sulfide, lactate, or some other organic compound, depending on the species. Most likely, the bacteria that first developed photosynthetic ability used H2S as their electron source, and only after the later development of oxygenic photosynthesis (about 2.3 billion years ago) did oxygen become a significant proportion of the earth's atmosphere. With that development, the evolution of electron-transfer systems that used O2 as their ultimate electron acceptor became possible, leading to the highly efficient energy extraction of oxidative phos-phorylation.

In Halophilic Bacteria, a Single Protein Absorbs Light and Pumps Protons to Drive ATP Synthesis

The halophilic ("salt-loving") bacterium Halobacterium salinarum, an archaebacterium derived from very ancient evolutionary progenitors, traps the energy of sunlight in a process very different from the photosynthetic mechanisms we have described so far. This bacterium lives only in brine ponds and salt lakes (Great Salt Lake and the Dead Sea, for example), where the high salt concentration—which can exceed 4 M—results from water loss by evaporation; indeed, halobacteria cannot live in NaCl concentrations lower than 3 M. These organisms are aerobes and normally use O2 to oxidize organic fuel molecules. However, the solubility of O2 is so low in brine ponds that sometimes oxidative metabolism must be supplemented by sunlight as an alternative source of energy.

The plasma membrane of H. salinarum contains patches of the light-absorbing pigment bacteriorho-dopsin, which contains retinal (the aldehyde derivative of vitamin A; see Fig. 10-21) as a prosthetic group. When the cells are illuminated, all-trans-retinal bound to the bacteriorhodopsin absorbs a photon and undergoes photoisomerization to 13-cis-retinal. The restoration of all-trans-retinal is accompanied by the outward movement of protons through the plasma membrane. Bacteriorhodopsin, with only 247 amino acid residues, is the simplest light-driven proton pump known. The difference in the three-dimensional structure of bacteri-orhodopsin in the dark and after illumination (Fig. 19-59a) suggests a pathway by which a concerted series of proton "hops" could effectively move a proton across the membrane. The chromophore retinal is bound through a Schiff-base linkage to the e-amino group of a Lys residue. In the dark, the N of this Schiff base is pro-tonated; in the light, photoisomerization of retinal lowers the pKa of this group and it releases its proton to a nearby Asp residue, triggering a series of proton hops that ultimately result in the release of a proton at the outer surface of the membrane (Fig. 19-59b).

The electrochemical potential across the membrane drives protons back into the cell through a membrane ATP synthase complex very similar to that of mitochondria and chloroplasts. Thus, when O2 is limited, halobac-teria can use light to supplement the ATP synthesized by oxidative phosphorylation. Halobacteria do not evolve O2, nor do they carry out photoreduction of NADP+; their phototransducing machinery is therefore much simpler than that of cyanobacteria or plants. Nevertheless, the proton-pumping mechanism used by this simple protein may prove to be prototypical for the many other, more complex, ion pumps. ^ Bacteriorhodopsin ■

Dark

Retinal

Retinal

Thr89—OH

Thr89—OH

Low pKa O

Protonated Schiff base (high pKa)

Proton-release complex (protonated; high p^"a)

O mil HO O

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