Green filamentous bacteria secondary electron acceptor

C. Secondary Electron Acceptor in Green Filamentous Bacteria Menaquinone. 170... 

Photosynthetic eubacteria are classified as filamentous, green sulfur, gram-positive linked, purple, and cyanobacteria. All contain membrane-bound RCs in which (B)Chl serves as the primary electron donor. The RCs may be divided into two main types RC-1, in which the initial electron acceptor is a (B)Chl molecule and the secondary acceptor is an Fe-S center, and RC-2, in which the initial acceptor is a (B)Ph molecule and the secondary acceptor is a quinone. RC-1 centers are found in green sulfur and gram-positive linked bacteria, while RC-2 centers are found in filamentous bacteria and purple bacteria. Cyanobacteria contain both RC-1 and RC-2 centers in which the chlorophyll is Chi a. BChl a is found in filamentous, green sulfur and purple bacteria, while BChl g is characteristic of the grampositive line. BChl b is found in certain purple bacteria instead of BChl a. 

Those photosynthetic eubacteria with RC-2 centers (filamentous and purple bacteria) reduce NAD" for CO2 fixation by reverse electron flow from the quinone pool, whereas the green sulfur bacteria (RC-1 center) reduce ferredoxin and NAD directly from the secondary acceptor (Fe-S center) of the RC. In both cases an external reductant such as H2S is required. The mechanism of NAD reduction in the gram-positive line has not yet been investigated, but H. chlorum is a het-erotroph rather than an autotroph, and may not need to fix CO2. 

When electron transfer to the secondary acceptor is disrupted, the separated charges recombine in a few nanoseconds, via the radical pair mechanism, to form the spin-polarized triplet state of the primary donor, P. As shown in Fig. 11, the decay time of P865 in the green filamentous bacterium Cf. aurantiacus is 6 //s at ambient temperature. At 1.2 K it is 75 /js. Reaction centers of Cf. aurantiacus contain two menaquinone molecules, MQa and MQg, which behave the same way as a pair of analogous quinones in purple bacteria and photosystem II. Under non-physiological conditions, MQa recombines with P865 in 60 ms and MQb in 1 s. 

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