• Prosthetic group features
• Light­harvesting complex II (LH­II) in motif databases
• LH­II in alignment databases
• LH­II in 3­D databases
• References

Centre	Bacteriochlorophyll	Metal coordination	Axial metal ligand	Formal metal oxidation states
B800	BChl­a	Rs. molischianum		MgII
		Pentacoordinate	OAsp
		Rps. acidophila
		Pentacoordinate	OformylfMet
B850		Pentacoordinate	NHis
	Carotenoid
	Lycopene
	Rhodopin glucoside

The photosynthetic antenna systems collect and deliver excited­state energy through excitation transfer to the reaction centre [1, 2]. Photosynthetic purple bacteria have up to three types of antenna, or light­harvesting, complexes (LHs) besides the reaction centre. LH­I complexes are closely associated with the reaction centre and form a `core' complex, while LH­II and LH­III complexes are peripheral to the `core'. The peripheral complexes have absorption maxima at shorter wavelengths than LH­I and the reaction centre. Thus, the antenna complexes broaden the spectral region of the light which can be used for photosynthesis and transfer the energy from one to another and to the reaction centre [3].

The LHs from purple bacteria are oligomers with the ß heterodimer as a basic unit. In LH­II, the ß heterodimers bind three bacteriochlorophyll a (BChl­a) molecules and at least one molecule of carotenoid. LH­II is often referred to as the B800­850 complex since one BChl­a has absorption maximum at around 800 nm (B800) and other two absorb at around 850 nm (B850). Thus, inside LH­II a first energy transfer (B800  B850) can take place [3]. The role of the carotenoids in LH­II is twofold: (i) they absorb light in the visible region of the spectrum and transfer it to BChl­a (accessory light­harvesting) and (ii) they protect the LH­II from damage by singlet oxygen species [2].

The 3­D structures of the LH­II from Rhodospirillum molischianum [3] and Rhodopseudomonas acidophila [4] have been determined. In both structures, the ß heterodimers arranged in a ring­shaped aggregate with strict rotational symmetry. The and ß subunits each possess single transmembrane helices which are arranged in two concentric rings, where the subunits form the inner circle and ß subunits form the outer one. However, Rs. molischianum LH­II exists as an (ß)₈ complex with eight­fold symmetry (Figure 1LGH c, d) whereas Rps. acidophila LH­II is an (ß)₉ complex with nine­fold symmetry (Figure 1KZU a, b). In Rs. molischianum LH­II, B850 molecules are more tightly and evenly distributed in the ring. The carotenoid molecules are also different: lycopene in Rs. molischianum and rhodopin glucoside in Rps. acidophila. B850 molecules are sandwiched between and ß subunits, with the plane of BChl­a perpendicular to the membrane plane. Each protein subunit coordinates B850 through a conserved His residue. In Rps. acidophila, B800 molecules are oriented parallel to the membrane with their Mg coordinated by the formyl group of fMet­1 of the ß subunit Figure 1KZU c). In Rs. molischianum, B800 is bound to the subunit via O of Asp­6 and is tilted away from the membrane plane by 38° (Figure 1LGH e). The transfer of energy between B800 and B850 presumably occurs through Förster induced dipole-dipole resonance [3, 5].

LH­II in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
LIGHTHARVSTA	PR00673	ANTENNA_COMP_ALPHA	PS00968	BL00968
LIGHTHARVSTB	PR00674	ANTENNA_COMP_BETA	PS00969	BL00969

LH­II proteins in alignment databases

Protein Family	Pfam	LPFC 3­D alignment
01944; light­harvesting protein chain	-	-
01945; light­harvesting protein ß chain	-	-

LH­II in 3­D databases

All LH­II contain three molecules of bacteriochlorophyll a and at least one molecule of carotenoid per ß heterodimer (see Figures

PDB	MSD	scop	BSM	RELI Base	Header	¹
1kzu	1kzu	1kzu	1kzu	1kzu	Light­harvesting complex II (B800­850); Rhodopseudomonas acidophila strain 10050	MS6PT1
1lgh	1lgh	1lgh	1lgh	1lgh	Light­harvesting complex II (B800­850); Rhodospirillum molischianum	MS7RRC1

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

References

1. Blankenship, R.E. (1996) Photosynthetic antennas and reaction centers: Current understanding and prospects for improvement. In Nozik, A.J., Ed. Research Opportunities in Photochemical Sciences. Also published online at http://photoscience.la.asu.edu/photosyn/education/antenna.html.
2. Isaacs, N.W., Cogdell, R.J., Freer, A.A. and Prince, S.M. (1995) Light­harvesting mechanisms in purple photosynthetic bacteria. Curr. Opin. Struct. Biol. 5, 794-797.
3. Koepke, J., Hu, X., Muenke, C., Schulten, K. and Michel, H. (1996) The crystal structure of the light­harvesting complex II (B800­850) from Rhodospirillum molischianum. Structure 4, 581-597.
4. McDermott, G., Prince, S.M., Freer, A.A., Hawthornthwaite­Lawless, A.M., Papiz, M.Z., Cogdell, R.J. and Isaacs, N.W. (1995) Crystal structure of an integral membrane light­harvesting complex from photosynthetic bacteria. Nature 374, 517-521.
5. Freer, A.A., Prince, S.M., Sauer, K., Papiz, M.Z., Hawthornthwaite­Lawless, A.M., McDermott, G., Cogdell, R.J. and Isaacs, N.W. (1996) Pigment-pigment interactions and energy transfer in the antenna complex of the photosynthetic bacterium Rhodopseudomonas acidophila. Structure 4, 449-462.

Bibliography on structural studies of light­harvesting complex II of purple bacteria

Reviews on light­harvesting complex II of purple bacteria

Light Harvesting in Bacterial Photosynthesis

ASU Photosynthesis Center