Ubiquinones photochemistry

The photochemistry of the reaction center takes place one electron at a time. However, one of the products of the electron transfer process is a reduced ubiquinone, which has taken up two electrons as well as two protons. To form this species, the reaction center must turn over twice, with electrons entering the complex by donation of cytochrome ci with the oxidized special pair. The electrons accumulate in the quinone acceptors and protons are taken up from the surrounding medium. Finally, a fidly reduced ubiquinol is formed, which is released from the complex into the hydrocarbon portion of the membrane. The quinol is subsequently reoxidized at the cytochrome bc complex (described below). 

The identity of a ubiquinone molecule as a primary electron acceptor (A 2 also designated or Q,) is now well established. The most conclusive evidence came following extraction of A 2 from RC of Rp. sphaeroides with hexane containing 0.1% methanol [26] this procedure inhibits the occurrence of D photooxidation, when observed with millisecond spectroscopy since, as stated above, the charge recombination of D," Af is much faster than that of the D] A,A2 state [27]. Primary photochemistry in extracted RC could be reconstituted with pure ubiquinone-10 [26]. The electron transfer to A2 requires 200 ps [28-30], and the transfer from A2 to the secondary acceptor 200 jas this last step can be inhibited by o-phenanthroline [31]. 

Subsequently Halsey and Parson pursued this issue further by removing ubiquinone from Chromatium chromatophores and examining the consequences. The authors found that while the extracted chromato-phores were photochemically active on the first flash, they lost the ability entirely to perform photochemistry on the second flash, even when delayed by several seconds after the first. Reconstitution of the extracted chromatophores with the Mpid extracts restored the photochemical activity. Taking these results together with the chemical identity ofthe extracts allowed these workers to conclude that ubiquinone serves as the secondary electron acceptor, Qb, in such photosynthetic bacteria as Chromatium and Rb. sphaeroides. 

There are totally 11 chromophores in the bacterial photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) virids. Since the excitation process of the reaction center is the primary event of the photo-induced electron transfer in the reaction center, the detailed analysis of the absorption spectrum is one of the key steps for the understanding of photochemistry of the system. The chromophores included in the PSRC are bacteriochlorophyll b dimer (special pair, P), bacteriochlorophyll in L- and M-branches (Bl and B ), bacteriopheophytin in L- and M-branches (Hl and Hm), menaquinone (MQ), ubiquinone (UQ) and four different hemes, c-552, c-554, c-556, and c-559 in c-type cytochrome subunit. 

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