Green filamentous bacteria reaction center

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. 

A. Preparation and Properties of Reaction-Center Complexes of Green Filamentous Bacteria...168... 

Pigment and cofactor compositions as well as the light-induced absorbance changes in the reaction centers of green filamentous bacteria are similar to those of purple bacteria. Fig. 8 (A) shows room-temperature absorption spectra for the reaction center of Cf aurantiacus reduced by Asc (solid line) in one case and in the other oxidized by FeCy (dashed line). The far-red absorption maxima of the reduced sample are at 865, 815 and 756 nm while in the oxidized sample, most ofthe 865-nm band and some of the 605-nm band are bleached and the 815- and 756-nm bands are shifted to 806 and 757 nm, respectively. A distinct shoulder at 793 nm also disappears upon oxidation. The BO-a absorption band at 534... 

Fig. 11. Electron-transfer schemes for the reaction centers of the green sulfur bacteria (A) and green filamentous bacteria (B). The reaction-center components of the green sulfur bacteria are compared to green-plant photosystem I and those of the green filamentous bacteria are compared to green-plant photosystem II or purple bacteria. The decay times and redox potentials are for Prosihecochloris aestuariiand Chloroflexus aurantiacus. See text lor discussion. Figure adapted from Amesz (1987)...

The resemblance between PS II and the reaction centers of purple bacteria and filamentous green bacteria has been established by extensive biochemical and spectroscopic studies. In addition to the analogous electron carriers (bacterio)pheophytin and quinones present in purple bacteria and PS II, their reaction-center-core protein subunits, the L and M subunits in purple bacteria and the DI and D2 subunits in PS II, show significant amino-acid sequence homologies. 

Fig. 4. Structural model for the chlorosome-reaction center complex of filamentous green bacteria. BChl molecules In chlorosome and cytoplasmic membranes are represented by squares. Direction of the transition dipole moment of the lowest excited singlet state of the respective pigment complex or molecules is represented by a straight arrow. Curved arrows show direction of energy transfer. See text for other details. Figure source same as Figure 2 in this chapter.

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. 

The reaction center (RC) has been obtained completely free of antenna pigments and other extraneous material from most purple bacteria (Feher, Okamura 1978 Gingras, 1978) and from one green filamentous bacterium, Chloroflexus aurantiacus which is thougjit to be the evolutionary precursor of all photosynthetic organisms (cf. Pierson, Thornber 1983). Other green bacteria have not yet yielded their RCs to isolation. 

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