Photosynthetic chain

In contrast to common usage, the distinction between photosynthetic and respiratory Rieske proteins does not seem to make sense. The mitochondrial Rieske protein is closely related to that of photosynthetic purple bacteria, which represent the endosymbiotic ancestors of mitochondria (for a review, see also (99)). Moreover, during its evolution Rieske s protein appears to have existed prior to photosynthesis (100, 101), and the photosynthetic chain was probably built around a preexisting cytochrome be complex (99). The evolution of Rieske proteins from photosynthetic electron transport chains is therefore intricately intertwined with that of respiration, and a discussion of the photosynthetic representatives necessarily has to include excursions into nonphotosynthetic systems. 

Let us pass to other cytochromes. Cytochrome f (or cytochrome C552) (FW = 15 000), the crystal structure of which is known,11 is an electron carrier present in the photosynthetic chain and also possesses a positive overall charge. It exhibits a reversible Fe(III)/Fe(II) reduction at a gold electrode modified with 4,4/-dithiopyridine,12 Figure 10. 

This part of the photosynthetic chain can be mimicked by ruthenium-tyrosinate and manganese-tyrosinate complexes [80-82], In the excited state the ruthenium-polypyridine complex abstracts one electron from the tyrosinate moiety, yielding... 

ET) between the components of the photosynthetic chain. The emergence of an electron from the primary photosynthetic cell, e.g. the transport from the reduced primary acceptor Qa- to the secondary acceptor QB followed by the release of hydroquinone QbH2 was shown to take place only under conditions in which the labels record the mobility of the protein moiety in the membrane with oc > 107 s 1. The rate of another important process, the recombination of the primary product of the charge separation, i.e. reduced primary acceptor (Qa ) and oxidized primary donor, bacteriochlorophyl dimere (P4), fahs from 102 to 103 s 1 when dynamic processes with oc = 103 s 1 occur. 

The presence in chloroplasts and algae of a cytochrome b with the band at 559 nm, and therefore distinct from cyt. was described in the mid 1970s [120]. However, in spite of this early discovery and of the efforts of many laboratories, the actual role of this carrier in the photosynthetic chain is still obscure. 

Fig. 4.6. The proposed arrangement of the RC and h/c, complex in the photosynthetic chain of Rps. sphaeroides. The scheme indicates the reduction of Q to QHj by a pair of RC complexes and the net oxidation of QH2 by two turnovers of the oxidoreductase, as a balance of the oxidation of two quinols and the reduction of one quinone at the site. The proposed sites of proteolytic reactions are also indicated (from Ref. 93).

Fig. 4.7. The Z scheme of the higher plant photosynthetic chain visualized as an arrangement of multiprotein complexes. The two RC of PSII and PSI are arranged in parallel across the membrane and are intereonnected by the b /f complex. The electron transfer pathway within this complex follows a modified Q cycle scheme analogous to that proposed for bacterial photosynthesis. The oxygen-evolving complex is proposed to face the inner thylakoid lumen and to release protons in this compartment. The association of cytochrome 6-559 with PSII-RC and the cyclic role of ferredoxin are also depicted. Proton-binding and proton-releasing sites are illustrated (from Ref. 93).

Many herbicides act by damaging the photosynthetic chain. Atrazine damages tlie quinone binding area on PSII paraquat takes electrons from ferredoxin and generates (lethal) hydrogen peroxide. Uncouplers like DNP (obviously) work in chloroplasts as well as mitochondria. 

The purple sulfur bacteria are strict anaerobes and photoautotrophs, using inorganic compounds as electron donors for a noncyclic photosynthetic chain. The usual donor is sulfide... 

---