Photosynthesis 481

Photosynthesis 481 C3 plants, which regulate the opening of stomatal in the dark by using respiration to utilize organic pores for gas exchange in leaves, also lack rubisco compounds from the environment. They are ther- and apparently use PEP carboxylase exclusively to mophilic bacteria found in hot springs around the fix CO2. world. They also distinguish themselves among the Contributions of the late Martin Gibbs to this arti- photosynthetic bacteria by possessing mobility. An cleare acknowledged. example is Chloroflexus aurantiacus. Gerald A. Berkowitz; Archie R. Portis, Jr.; Govindjee 4. Heliobacteria (Heliobacteriaceae). These are strictly anaerobic bacteria that contain bacterio- Bacterial Photosynthesis chlorophyll g. They grow primarily using organic Certain bacteria have the ability to perform photo- substrates and have not been shown to carry out synthesis. This was first noticed by Sergey Vinograd- autotrophic growth using only light and inorganic sky in 1889 and was later extensively investigated substrates. An example is Heliobacterium chlorum. by Cornelis B. Van Niel, who gave a general equa- Like plants, algae, and cyanobacteria, anoxygenic tion for bacterial photosynthesis. This is shown in photosynthetic bacteria are capable of photophos- reaction (9). phorylation, which is the production of adenosine triphosphate (ATP) from adenosine diphosphate bacteriochlorophyll 2H2A + CO2 + light −−−−−−−−−→ {CH2O}+2A + H2O (ADP) and inorganic phosphate (Pi) using light as the enzymes (9) primary energy source. Several investigators have suggested that the sole function of the light reaction where A represents any one of a number of reduc- in bacteria is to make ATP from ADP and Pi. The hy- tants, most commonly S (sulfur). drolysis energy of ATP (or the proton-motive force Photosynthetic bacteria cannot use water as the that precedes ATP formation) can then be used to hydrogen donor and are incapable of evolving oxy- drive the reduction of CO2 to carbohydrate by H2A gen. They are therefore called anoxygenic photo- in reaction (9). synthetic bacteria. The prokaryotic cyanobacteria Photochemical apparatus. Photosynthetic bacteria (formerly called blue-green algae) are excluded in do not have specialized organelles such as the chloro- this discussion of bacterial photosynthesis, since plasts of green plants. Electron micrographs of cer- their photosynthetic system closely resembles that tain photosynthetic bacteria show tiny spherical found in eukaryotic algae and higher plants discussed sacs, with double-layered walls, as a result of invagi- above. Anoxygenic photosynthetic bacteria can be nations which form stacks of membranes (Fig. 12a). classified in four major groups: Other photosynthetic bacteria have invaginations 1. Proteobacteria.Two groups with somewhat which form thylakoids (Fig. 12b). These intracyto- different properties are known. plasmic membranes, often called chromatophores, (A) Nonsulfur purple bacteria (Rhodospirillaceae). contain the photosynthetic apparatus and can be In these bacteria, H2Aisusually an organic H2 donor, isolated easily by mechanical disruption of bacte- such as succinate or malate; however, these bacteria riafollowed by differential centrifugation. Isolated can be adapted to use hydrogen gas as the reductant. chromatophores are often used for biochemical and They require vitamins for their growth and usually biophysical studies of bacterial photosynthesis. grow anaerobically in light, but they can also grow Reaction centers. The pigment bacteriochloro- aerobically in the dark by using respiration to utilize phyll (BChl) is a necessary component for bacterial organic compounds from the environment. They are thus facultative photoheterotrophs. Examples of this µ group are Rhodospirillum rubrum and Rhodobac- 0.25 m ter sphaeroides. (B) Sulfur purple bacteria (Chromatiaceae). These cannot grow aerobically, and H2Aisaninorganic sulfur compound, such as hydrogen sulfide, H2S; the carbon source can be CO2. These bacteria are called (a) obligate photoautotrophic anaerobes. An example is Chromatium vinosum (alternate name: Allochro- matium vinosum). 2. Green sulfur bacteria (Chlorobiaceae). These bacteria are capable of using the same chemicals as Chromatiaceae but, in addition, use other organic H2 donors. They may then be called photoautotrophic and photoheterotrophic obligate anaerobes. An ex- (b) ample of the green sulfur bacteria is Chlorobium tepidum. Fig. 12. Photosynthetic bacteria. (a) Electron micrograph of Rhodobacter sphaeroides with vesicle-like invaginations 3. Green gliding bacteria (Chloroflexaceae) [also (from T. W. Goodwin, ed., Biochemistry of Chloroplasts, known as filamentous anoxygenic phototrophs, vol. 1, Academic Press, 1966). (b) Pictorial representation of FAP]. These are primarily photoorganotrophic bac- a stacked invagination in a photosynthetic bacterium; at left is a longitudinal section and at right is a transverse teria which can grow under anaerobic conditions section (after R. Whittenbury and A. G. McLee, Archiv. fur ¨ in light by photosynthesis or in aerobic conditions Mikrobiologie, 59:324–334, 1967). 482 Photosynthesis photosynthesis. There are specialized BChl absorbed photon) must be very high (close to 1.0). molecules in bacteria which engage in the primary (3) The primary light reaction should occur at very ◦ ◦ chemical reactions of photosynthesis. In addition low temperatures, down to 1 K (−460 For−273 C). to these specialized molecules, there are 40– (4) The above photochemical reaction should be ex- 50 BChl molecules referred to as antenna pigments, tremely fast, that is, in the picosecond range. whose sole function is to harvest light energy and All the above criteria are fulfilled by P870, and thus transfer it to reaction center molecules. This is it is the reaction center of bacterial photosynthesis similar to the photosynthetic unit of plants, algae, in Rhodobacter sphaeroides. Among other reaction and cyanobacteria. Each reaction center contains a centers that have been identified and studied exten- special pair (dimer) of BChl molecules that engage sively are P890 in Chromatium vinosum, P960 in in chemical reactions after they trap the absorbed Rhodopseudomonas viridis (also called Blastochlo- light energy. They are also called the energy traps of ris viridis), P840 in Chlorobium limicola and P798 bacterial photosynthesis. Helicobacterium chlorum. Each species of bacteria The energy trap in Rhodobacter sphaeroides has has only one type of reaction center, unlike plants, been identified as P870. Such identification is carried algae, and cyanobacteria, which utilize two types out with a difference (absorption) spectrophotome- of reaction centers, PSI and PSII, that include P680 ter. In this instrument a weak monochromatic mea- and P700, respectively. The reaction centers from suring beam monitors the absorption of the sample; oxygenic photosynthetic organisms have been iden- abrief but bright actinic light given at right angles to tified by means similar to those used for bacterial the measuring beam initiates photosynthesis. When reaction centers. Reaction centers have been iso- photosynthesis occurs, changes in absorption take lated as pure proteins, which has served the im- place. Figure 13a shows the absorption spectrum of portant function of providing a well-defined system reaction centers isolated from R. sphaeroides. These in which primary reactions of photosynthesis can changes are measured as a function of the wave- be studied. A milestone in bacterial photosynthesis length of measuring light. A plot of the change in- wasreached in the early 1980s by the crystallization duced in R. sphaeroides reaction centers by an ac- and determination of the three-dimensional struc- tinic light flash, as a function of the wavelength of ture of Rhodopseudomonas viridis reaction centers measuring light, is the difference absorption spec- by Hartmut Michel, Johann Deisenhofer, and Robert trum (Fig. 13b). This spectrum is due largely to the Huber, who received the 1988 Nobel Prize in Chem- photooxidation of the BChl dimer, P870. istry for their work. These crystals enabled an atomic If P870 is the energy trap, then the following cri- resolution of the molecular structure of the reaction teria must be met: (1) It must undergo a reduction center to be obtained. or oxidation reaction, since this is the essential reac- Although isolated reaction centers are able to ab- tion of photosynthesis. The decrease in absorption at sorb light and convert it to chemical energy, the 870 nm (Fig. 13) is an oxidation reaction since chem- antenna pigment system in chromatophores (or in ical oxidants cause a similar change. (2) The quan- whole cells) absorbs most (>90%) of the light. The tum yield (number of trap molecules oxidized per antenna transfers this energy to the reaction center. Antenna BChl molecules are bound to protein in a specific manner; this binding and pigment-pigment 400 Bchl, Bpheo interactions modify the properties of the pigment Bchl 1 and define the absorption maxima and the width of − the absorption band. An example is B800 (B repre- cm 1 200 − Bpheo Bchl sents BChl, and the number indicates the wavelength of one of the absorption peaks in nanometers) found mM Bchl , ∋ Bpheo in Rhodobacter sphaeroides (Fig. 14). 0 Components of photosynthetic bacteria. These bac- (a) 400 600 800 1000 teria contain the usual components of living ma- terial: proteins, lipids, carbohydrates, deoxyribonu- 1 50 − cleic acid (DNA), ribonucleic acid (RNA), and cm various metals. However,

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