Photosynthetic organisms green sulfur bacteria

Interestingly, reaction centers of apparently all photosynthetic organisms may be assigned to one or the other of these two types. For instance, the reaction center of both purple bacteria and the green filamentous bacteria, Chloroflexaceae, and green-plant PS II are of the OQ-type. On the other hand, the green sulfur bacteria, Chlorobiaceae, the Heliobacteria, and photosystem I all have the FeS-type reaction centers. 

Ribulosa b/ phosphate carboxylase, ribulose bisphosphate carboxylase/oxygeiuise, Rubisco, car-boxydismutase, Fraetion-l protein (EC 4.1.1.39) the enzyme responsible for catalysing photosynthetic CO2 fixation in all photosynthetic organisms except green sulfur bacteria (Chlorobiaceae). In plants it occurs in the chloroplast stroma where it constitutes about 50% of the total protein it should be noted. 

The discovery that certain bacteria could carry out photosynthesis opened up a new field of photosynthetic research. It was found that certain green-, red-, purple-, and brown-colored bacteria could produce organic matter from carbon dioxide upon illumination. The formation of organic matter was not accompanied by oxygen evolution. As a result of work with the green sulfur bacteria, van Niel (21) showed that their CO2 assimilation process was in close agreement with the following equation ... 

The only known function of PhQ in cyanobacteria and plants is to function as an electron transfer cofactor in PS I. In spite of its importance in cyanobacteria, the biosynthetic route of PhQ was not previously elucidated. Many prokaryotes contain the metabolic pathway for the biosynthesis of menaquinone (MQ), a PhQ-Hke molecule (Figure 119.1). In certain bacteria, MQ is used during fumarate reduction in anaerobic respiration. - In green sulfur bacteria and in heliobacteria, MQ may function as a loosely bound secondary electron acceptor in the photosynthetic reaction center. The genes encoding enzymes involved in the conversion of chorismate to MQ were cloned in a variety of organisms. MQ differs from PhQ only in the tail portion of the molecule an unsaturated C-40 side chain is present, rather than a mostly saturated C-20 phytyl side chain. Therefore, the synthesis of the naphthalene rings in PhQ and MQ involves similar steps in both pathways. 

A study of photosynthetic organisms other than green plants has revealed that certain bacteria, such as the purple sulfur bacteria, utilize H2S instead of H20 as a reductant in photosynthesis. The product obtained is elemental sulfur instead of oxygen ... 

Fig. 3 Schematic model of light-harvesting compartments in photosynthetic organisms and their position with respect to the membrane and the reaction centers. RC1(2) Photosystem I(II) reaction centre. Peripheral membrane antennas Chlorosome/FMO in green sulfur and nonsulfur bacteria, phycobilisome (PBS) in cyanobacteria and rhodophytes and peridinin-chlorophyll proteins (PCP) in dyno-phytes. Integral membrane accessory antennas LH2 in purple bacteria, LHC family in all eukaryotes. Integral membrane core antennas B808-867 complex in green nonsulfur bacteria, LH1 in purple bacteria, CP43/CP47 (not shown) in cyanobacteria and all eukaryotes.

The RCs in photosynthetic organisms have been classified into two groups, i.e., quinone-type and iron sulfur-type. The quinone-type RCs are present in purple non-sulfur bacteria as well as in the PS II of cyanobacteria and chloroplasts (of algae and higher plants), whereas the iron sulfur-type RCs are present in green and purple sulfur bacteria as well as in the PS I of cyanobacteria and chloroplasts (Blankenship, 1992 HauskaetaL, 1995). 

Fig. 17.15-C(s-carotenoids identified so far in the RCs of various photosynthetic organisms (a) neurosporene, (b) spheroidene and (c) spirilloxanthin in the quinone-type RC of puiple non-sulfur bacteria, Rb. sphaeroides QIC, 2.4.1 and Rs. rubrum SI (d) y-carotene and (e) chlorobactene in the iron sulfiir-type RC of green sulfur bacterium, Cb. tepidum-, and (f) Carotene in the quinone-type PS II RC of spinach chioropiasts as well as in the iron sulfur-type PS I RC of a cyanobacterium. Sc. vulcanus, and spinach chioropiasts.

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