The reaction center of green sulfur bacteria1Dedicated to the memory of Jan Amesz.1

Abstract

The composition of the P840-reaction center complex (RC), energy and electron transfer within the RC, as well as its topographical organization and interaction with other components in the membrane of green sulfur bacteria are presented, and compared to the FeS-type reaction centers of Photosystem I and of Heliobacteria. The core of the RC is homodimeric, since pscA is the only gene found in the genome of Chlorobium tepidum which resembles the genes psaA and -B for the heterodimeric core of Photosystem I. Functionally intact RC can be isolated from several species of green sulfur bacteria. It is generally composed of five subunits, PscA-D plus the BChl a-protein FMO. Functional cores, with PscA and PscB only, can be isolated from Prostecochloris aestuarii. The PscA-dimer binds P840, a special pair of BChl a-molecules, the primary electron acceptor A(0), which is a Chl a-derivative and FeS-center F(X). An equivalent to the electron acceptor A(1) in Photosystem I, which is tightly bound phylloquinone acting between A(0) and F(X), is not required for forward electron transfer in the RC of green sulfur bacteria. This difference is reflected by different rates of electron transfer between A(0) and F(X) in the two systems. The subunit PscB contains the two FeS-centers F(A) and F(B). STEM particle analysis suggests that the core of the RC with PscA and PscB resembles the PsaAB/PsaC-core of the P700-reaction center in Photosystem I. PscB may form a protrusion into the cytoplasmic space where reduction of ferredoxin occurs, with FMO trimers bound on both sides of this protrusion. Thus the subunit composition of the RC in vivo should be 2(FMO)(3)(PscA)(2)PscB(PscC)(2)PscD. Only 16 BChl a-, four Chl a-molecules and two carotenoids are bound to the RC-core, which is substantially less than its counterpart of Photosystem I, with 85 Chl a-molecules and 22 carotenoids. A total of 58 BChl a/RC are present in the membranes of green sulfur bacteria outside the chlorosomes, corresponding to two trimers of FMO (42 Bchl a) per RC (16 BChl a). The question whether the homodimeric RC is totally symmetric is still open. Furthermore, it is still unclear which cytochrome c is the physiological electron donor to P840(+). Also the way of NAD(+)-reduction is unknown, since a gene equivalent to ferredoxin-NADP(+) reductase is not present in the genome.

Function of a PscD Subunit in a Homodimeric Reaction Center Complex of the Photosynthetic Green Sulfur Bacterium Chlorobium tepidum Studied by Insertional Gene Inactivation

“…The RC complex of the green sulfur bacterium Chlorobium tepidum consists of four subunits, PscA, PscB, PscC, and PscD with an antenna size of about 30 bacteriochlorophyll (BChl) molecules, whereas the heterodimeric photosystem I complex is made of 12 polypeptides with a much larger antenna size of nearly 100 chlorophyll (Chl) molecules (4,5). The functions of PscA, PscB, and PscC have been studied intensively using biochemical and spectroscopic methods.…”

Biochemical Studies of asoxF-Encoded Monomeric Flavoprotein Purified from the Green Sulfur BacteriumChlorobaculum tepidumThat Stimulatesin VitroThiosulfate Oxidation

“…Of the multiple sulfide oxidation electron transport pathways with photooxidized P840 as the terminal electron acceptor in C. tepidum, the one involving SQR appears to be more efficient in terms of energy conversion than the others, because it should generate a proton-motive force through a cyt b-Riesketype iron-sulfur protein complex during the oxidation of sulfide by photooxidized P840, and the others do not. 30) Although the FCSD and SoxF pathways are energetically inefficient as compared with the SQR pathway and the affinities for sulfide did not differ very much, retention of these activities is likely favored under certain environmental conditions, because sulfide is a diffusible substrate required by many bacterial species occupying the same ecological niche. 6) Oxidation of sulfide by FCSD or SoxF, or even by core TOMES to elemental sulfur, allows an individual bacterium to utilize sulfide and subsequently generate six electrons as electron donors for further reactions.…”

“…29) The primary donor of the reaction center (RC) is a special pair of bacteriochlorophylls called P840, and its immediate electron donor is RC-bound cyt c-551. 30) Itoh et al 31) found that a soluble mono-heme cyt c-554 of about 10 kDa (the CT0075 protein, 10) sometimes called a small soluble cytochrome c) donates electrons to bound cyt c-551 rather than directly to oxidized P840. In the reconstituted core TOMES (SoxAXK, SoxB, SoxYZ) of this bacterium, cyt c-554 functioned as an efficient electron acceptor.…”

Identification of a Gene Essential for the First Committed Step in the Biosynthesis of Bacteriochlorophyll c

“…Similar results were observed by Frigaard et al (28) for the bchK mutant. The absorption band near 800 nm was derived from a combination of BChl a-binding proteins, including CsmA, Fenna-Matthews-Olson protein, and the reaction centers (28,31,32). Fig.…”