The Crystallographic Structure of the B800-820 LH3 Light-Harvesting Complex from the Purple Bacteria Rhodopseudomonas Acidophila Strain 7050

Abstract

The B800-820, or LH3, complex is a spectroscopic variant of the B800-850 LH2 peripheral light-harvesting complex. LH3 is synthesized by some species and strains of purple bacteria when growing under what are generally classed as "stressed" conditions, such as low intensity illumination and/or low temperature (<30 degrees C). The apoproteins in these complexes modify the absorption properties of the chromophores to ensure that the photosynthetic process is highly efficient. The crystal structure of the B800-820 light-harvesting complex, an integral membrane pigment-protein complex, from the purple bacteria Rhodopseudomonas (Rps.) acidophila strain 7050 has been determined to a resolution of 3.0 A by molecular replacement. The overall structure of the LH3 complex is analogous to that of the LH2 complex from Rps. acidophila strain 10050. LH3 has a nonameric quaternary structure where two concentric cylinders of alpha-helices enclose the pigment molecules bacteriochlorophyll a and carotenoid. The observed spectroscopic differences between LH2 and LH3 can be attributed to differences in the primary structure of the apoproteins. There are changes in hydrogen bonding patterns between the coupled Bchla molecules and the protein that have an effect on the conformation of the C3-acetyl groups of the B820 molecules. The structure of LH3 shows the important role that the protein plays in modulating the characteristics of the light-harvesting system and indicates the mechanisms by which the absorption properties of the complex are altered to produce a more efficient light-harvesting component.

Antenna mixing in photosynthetic membranes from Phaeospirillum molischianum

“…An important question that has not yet been directly addressed is how individual LH2 oligomers vary in cells producing a variety of different light-harvesting polypeptides. In the literature, it is generally assumed that LH2 complexes contain polypeptides from a single puc operon (22,27). However, there is no direct evidence for this.…”

“…In this context it is perhaps pertinent that only a very low resolution (8 Å) x-ray structure was obtained of the Rps. plaustris low light LH3 (22), possibly because these complexes contain a mixture of different polypeptides. However, such mixed systems may be of considerable importance n biology where the properties of mixed complexes can be different from either pure complex and possibly include nonlinear effects.…”

“…Differential expression of multiple different puc operons results in a modification of the spectral properties of the LH2 complexes present. The genes expressed under very low light conditions often give rise to light-harvesting complexes with having near-IR absorption maxima at 800 nm and 820 nm (21,22) as opposed to the more standard complexes with absorption maxima at 800 nm and 850 nm. The molecular origin of the spectral difference between B800-820 and B800-850 type LH2 is well understood.…”

“…The current description of the light-harvesting system during this adaptation to very low light suggests that these complexes with blue-shifted absorption spectra [frequently called LH3 (22), or even LH4 (27)] form an ultraperipheral light-harvesting system that transfers energy to the more standard B800-850 absorbing LH2 and then to LH1 and the reaction center. The energetic funnel represented by this system with four levels (LH3, LH2, LH1, and the reaction center) assures the efficiency of light-harvesting.…”

Crystallization of purple bacterial LH2 antenna complexes

“…3-D structure of 2 [3] compared to 1 [1] has revealed rather subtle differences in the H-bonding pattern but the differences between the HL (NIR-spectra similar to 1) and the LL forms of LH2 from Rps. palustris are expected to be more dramatic, for example, the HL form is thought to be nonameric and the LL form octameric [4].…”