Photosynthetic reaction center algae

Figure 23-18 Schematic view of photosynthetic reaction centers and the cytochrome //complex embedded in a thylakoid membrane. Plastocyanin (or cytochrome c6 in some algae and cyanobacteria) carries electrons to the PSI core.

In green plants and certain algae, the photosynthetic machinery is elaborated over those found in purple bacteria and is now able to reach the high potentials needed to oxidize water to dioxygen. This oxygenic photosystem is comprised of two photosynthetic reaction centers (sensitizer assemblies), photosystem I (PS I) and... 

Zuber, H. Structure of Light-Harvesting Antenna Complexes of Photosynthetic Bacteria, Cyanobacteria and Red Algae. Trends Biochem. Sci. 11, 414-419 (1986). [Concentrates on the protein portion of the photosynthetic reaction center.]... 

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers.118 120 Cytochrome c2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure.121 Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter 23).120,122 124 Some bacterial membranes as well as those of mitochondria contain a cytochrome bct complex whose structure is shown in Fig. 18-8.125,126... 

In contrast, the reaction centers of green sulfur bacteria resemble PSI of chloroplasts. Their reaction centers also receive electrons from a reduced quinone via a cytochrome be complex.245 However, the reduced form of the reaction center bacteriochlorophyll donates electrons to iron-sulfur proteins as in PSI (Fig. 23-17). The latter can reduce a quinone to provide cyclic photophosphorylation. Cyanobacteria have a photosynthetic apparatus very similar to that of green algae and higher plants. 

The electron donor to Chl+ in PSI of chloroplasts is the copper protein plastocyanin (Fig. 2-16). However, in some algae either plastocyanin or a cytochrome c can serve, depending upon the availability of copper or iron.345 Both QA and QB of PSI are phylloquinone in cyanobacteria but are plastoquinone-9 in chloroplasts. Mutant cyanobacteria, in which the pathway of phylloquinone synthesis is blocked, incorporate plasto-quinone-9 into the A-site.345a Plastoquinone has the structure shown in Fig. 15-24 with nine isoprenoid units in the side chain. Spinach chloroplasts also contain at least six other plastoquinones. Plastoquino-nes C, which are hydroxylated in side-chain positions, are widely distributed. In plastoquinones B these hydroxyl groups are acylated. Many other modifications exist including variations in the number of iso-prene units in the side chains.358 359 There are about five molecules of plastoquinone for each reaction center, and plastoquinones may serve as a kind of electron buffer between the two photosynthetic systems. 

JP Thornber and JM Oison (1971) Chlorophyll-proteins and reaction center preparations from photosynthetic bacteria, algae and higher plants. Photochem Photobiol 14 329-341... 

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