Photosynthetic reaction centres of purple bacteria

• Prosthetic group features
• Scheme of electron transfer in photosynthetic membranes
• Photosynthetic reaction centres (PRC) in motif databases
• PRC in alignment databases
• PRC in 3­D databases
• References

Centre	Tetrapyrrole group		Metal coordination	Axial metal ligand	Formal metal oxidation states
	Rb. sphaeroides	Rps. viridis
DL DM BA BB	BChl­a	BChl­b	Pentacoordinate	NHis	MgII
A B	BPh­a	BPh­b	-	-	-
	Mononuclear iron centre			Iron ligands	Formal iron oxidation/spin states
Fe	Fe(NHis)4OGlu			4 × NHis; 2­OGlu	FeII (S=2); spin­coupled to the semiquinones QA·¯ and QB·¯
	Quinone				Formal oxidation states
	Rb. sphaeroides		Rps. viridis
QA	Ubiquinone­10 (n=10)		Menaquinone­9 (n=9)		QA; QA·¯ (semiquinone)
QB			Ubiquinone­9 (n=9)		QB; QB·¯ (semiquinone); QBH2 (hydroquinone)
	Carotenoid
	Rb. sphaeroides		Rps. viridis
	Spheroidene		1,2-Dihydroneurosporene		-

Photosynthetic reaction centres (PRCs) are membrane­spanning complexes of polypeptide chains and cofactors that catalyse the first steps in the conversion of light energy to chemical energy during photosynthesis. Two major taxonomic groups of photosynthetic bacteria, purple sulphur bacteria (Chromatiaceae) and purple non­sulphur bacteria (Rhodospirillaceae), contain PRCs of similar structure [1]. PRCs of photosynthetic purple bacteria consist of at least three protein subunits termed L (light), M (medium) and H (heavy); in Rhodopseudomonas viridis and Thiocapsa pfennigii, the four­haem cytochrome c is the fourth (and the largest) protein subunit [2]. (For more details on cytochrome subunit, see Class IV cytochromes c). The subunits L and M bind bacteriochlorophylls (BChl), bacteriopheophytins (BPh), quinones, a ferrous iron ion and a carotenoid as prosthetic groups. PRC from different organisms differs in the types of cofactors used. PRC of Rhodobacter sphaeroides and Rb. capsulatus contain BChl­a and BPh­a, while PRC of Rps. viridis and T. pfennigii contain BChl­b and BPh­b. Rb. sphaeroides has two ubiquinone­10 molecules, whereas Rps. viridis has one menaquinone­9 and one ubiquinone­9 molecule.

The 3­D structures of the PRC from Rps. viridis and Rb. sphaeroides have been determined [2; The Nobel Prize in Chemistry 1988]. The closely associated subunits L and M form the central part of the PRC. The mainly­ subunits L and M share the same fold; the most prominent structural features of each of these subunits are five transmembrane helices. Both the polypeptide backbone of subunits L and M and the attached prosthetic groups show a high degree of local two­fold symmetry, with the symmetry axis perpendicular to the membrane plane. On either side of the membrane­spanning region the L-M complex forms a flat surface parallel to the membrane plane. Subunit H consists of three distinct segments: the N­terminal segment, beginning from formyl­Met, containing the only transmembrane helix of subunit H; a surface segment, which is mostly in contact with the cytoplasmic side of the L-M complex; and a globular segment consisting mainly of ß­sheets. In Rps. viridis, the cytochrome subunit binds at the periplasmic side of the L-M complex. Neither subunit H nor the cytochrome obey the local symmetry possessed by the L-M complex; the cytochrome has internal local symmetry of its own.

With the exception of the carotenoid, the prosthetic groups in the L-M complex are arranged in two approximately symmetric branches, termed A­ and B­branch, each consisting of two Bchl, one BPh and one quinone. In the nomenclature suggested by Deisenhofer and Michel [2], D is used for the special pair of closely associated Bchl, B for the accessory Bchl, for the Bph and Q for the quinones. The branches are denoted by subscripts A and B; since D belongs to both branches, its two Bchl are denoted by subscripts L and M according to the subunit to which their Mg²⁺ is linked. Two His residues each from the subunits L and M are the fifth ligands of the Mg²⁺ of the Bchl. The special pair D_L-D_M is located near the periplasmic membrane surface on the symmetry axis. The A­ and B­branches lead through the membrane to the cytoplasmic site. The high­spin iron is bound near the cytoplasmic membrane surface, between the quinones close to the symmetry axis, and is bound to four His and one chelating (²­)Glu. The role of the ferrous ion is not clear; the iron can be removed [3] or exchanged with several divalent metals [4] without impairing the function of the PRC. The carotenoid is associated with the Bchl of the B­branch. Its possible function is to protect the PRC by quenching the triplet state of D before it can sensitise the formation of singlet oxygen, a powerful oxidising agent [5]. Crystallographic and spectroscopic data show that the carotenoid molecule is not in an all­trans conformation but has a single cis­bond near the centre of the polyene chain (see [5] and references therein).

The local two­fold symmetry is most perfect in the arrangement of the special pair. The two Bchl molecules overlap with their pyrrole rings I nearly parallel to each other and to the symmetry axis. Because of the significant - orbital interaction, the special pair may function as an electron donor. Absorption of a photon or energy transfer from light­harvesting complexes raises the special pair D to its first excited singlet state D^* (1). Then electron transfer progresses sequentially through B_A (2), _A (3), Q_A (4) and Q_B (5).

D		D*	(1)
D* + BA		D·+ + BA¯	(2)
BA·¯ + A		BA + A·¯	(3)
A·¯ + QA		A + QA·¯	(4)
QA·¯ + QB + 2H+		QA + QBH2	(5)

The product of reaction (2), the positively charged oxidised dimer D·⁺, can be re­reduced by the nearest of the four haems of cytochrome c in Rps. viridis or by a soluble cytochrome c₂ in Rb. sphaeroides. Q_B or its reduced form, the hydroquinone Q_BH₂, is not tightly bound to the protein but exchanges with quinones in the quinone pool of the membrane so that electrons are transported outside the protein. The electron gradient results in a coupled H⁺ gradient which is used for ATP synthesis.

PRC proteins in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
REACTNCENTRE	PR00256	REACTION_CENTER	PS00244	BL00244

PRC proteins in alignment databases

Protein Family	Pfam	LPFC 3­D alignment
01947; reaction center protein (chains L and M)	PF00124; photoRC	-
33127; reaction center protein chain H	-	-

PDB	scop	BSM	RELI Base	Header	¹
1aig	1aig	1aig	1aig	Photosynthetic reaction centre (charge­separated D+QB¯ state) (complex with ubiquinone­10); Rhodobacter sphaeroides	-
1aij	1aij	1aij	1aij	Photosynthetic reaction centre (charge­neutral DQAQB state) (complex with ubiquinone­10 and lauryl dimethylamine­oxide); Rhodobacter sphaeroides	-
1mps	-	1mps	1mps	Photosynthetic reaction centre (M subunit Y177F, F197R mutant) (complex with ubiquinone­10, spheroidenone, lauryl dimethylamine­oxide and phosphate); Rhodobacter sphaeroides	-
1pcr	1pcr	1pcr	1pcr	Photosynthetic reaction centre (complex with ubiquinone­10, spheroidene and lauryl dimethylamine­oxide); Rhodobacter sphaeroides	MS5AA1
1prc	1prc	1prc	1prc	Photosynthetic reaction centre (complex with ubiquinone­1, menaquinone­7, dihydroneurosporene and lauryl dimethylamine­oxide); Rhodopseudomonas viridis	-
1pss	1pss	1pss	1pss	Photosynthetic reaction centre (wild­type) (complex with ubiquinone­10 and spirilloxanthin); Rhodobacter sphaeroides, strain 2.4.1	-
1pst	1pst	1pst	1pst	Photosynthetic reaction centre (M subunit H202L mutant) (complex with ubiquinone­10 and spirilloxanthin); Rhodobacter sphaeroides, strain 2.4.1	-
1yst	1yst	1yst	1yst	Photosynthetic reaction centre (wild­type) (Mn2+ substituted for Fe2+) (complex with ubiquinone­10 and spheroidene); Rhodobacter sphaeroides, strain Y	-
2prc	-	2prc	-	Photosynthetic reaction centre (complex with ubiquinone­2, menaquinone­7, dihydroneurosporene, lauryl dimethylamine­oxide and sulphate); Rhodopseudomonas viridis	-
2rcr	2rcr	2rcr	2rcr	Photosynthetic reaction centre (complex with ubiquinone­10); Rhodobacter sphaeroides, strain R­26	-
3prc*	-	3prc*	-	Photosynthetic reaction centre (QB depleted) (complex with menaquinone­7, dihydroneurosporene, lauryl dimethylamine­oxide and sulphate); Rhodopseudomonas viridis	-
4prc	-	4prc	-	Photosynthetic reaction centre (complex with stigmatellin, menaquinone­7, dihydroneurosporene, lauryl dimethylamine­oxide and sulphate); Rhodopseudomonas viridis	-
4rcr	4rcr	4rcr	4rcr	Photosynthetic reaction centre (complex with ubiquinone­10 and B­octylglucoside); Rhodobacter sphaeroides, strain R­26	-

¹ Macromolecular Structures abstract. Full text is available to BioMedNet Members

References

1. Pierson, B.K. and Olson, J.M. (1987) Photosynthetic bacteria. In Amesz, J., Ed. Photosynthesis. Elsevier, Amsterdam, pp. 21-42.
2. Deisenhofer, J. and Michel, H. (1992) High­resolution crystal structures of bacterial photosynthetic reaction centers. In Ernster, L., Ed. Molecular Mechanisms in Bioenergetics. Elsevier, Amsterdam, pp. 103-120.
3. Blankenship, R.E. and Parson, W.W. (1979) The involvement of iron and ubiquinone in electron transfer reactions mediated by reaction centers from photosynthetic bacteria. Biochim. Biophys. Acta 545, 429-444.
4. Debus, R.J., Feher, G. and Okamura, M.Y. (1986) Iron­depleted reaction centers from Rhodopseudomonas sphaeroides R­26.1: Characterization and reconstitution with Fe²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺. Biochemistry 25, 2276-2287.
5. Frank, H.A. (1993) Carotenoids in photosynthetic bacterial reaction centers: Structure, spectroscopy, and photochemistry. In Deisenhofer, J. and Norris, J.R., Eds. The Photosynthetic Reaction Center, vol. II. Academic Press, San Diego, pp. 221-237.

Bibliography on structural studies of photosynthetic reaction centres of purple bacteria

Reviews on photosynthetic reaction centres of purple bacteria

Bibliography on structural studies of Class IV cytochromes c

ASU Photosynthesis Center

Photosynthesis Directory