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Title: The structure and function of the light-harvesting antenna complexes from purple photosynthetic bacteria
Author: Evans, Mark B.
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1989
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Abstract:
The structure and function of the B800-850 light-harvesting antenna complexes from Rps. palustris, strain French, have been extensively characterised. The B800-850 complexes isolated from high light (HL) grown cells have a normal Type I antenna spectrum. The complexes isolated from low light (LL) grown cells, however, have an 850nm absorption band of much lower intensity than the 800nm absorption band. This implies that there are significant structural differences between the two complexes. A comparative study was therefore undertaken to investigate the relationship between the structure and the near-infrared spectra. This was combined with a functional study to compare the energy transfer efficiencies of the two complexes. The ratio of BChl to carotenoid was determined to be 2:1 for the HL B800-850 complexes and 3:1 for the LL complexes. Linear dichroism measurements detected the presence of an additional BChl chromophore with a different Qx dipole orientation to the other BChls in the complexes. Picosecond energy transfer kinetics measurements on the LL complexes imply the presence of an extra BChl chromophore which absorbs at about 800nm and which acts as an intermediate in the transfer of energy from B800 to B850. The extra BChl does not improve the efficiency of energy transfer from carotenoid to BChl and it is suggested that it acts solely as an additional light-harvesting pigment. The efficiency of energy transfer from carotenoid-to-BChl was measured to be 36% for both the HL and LL complexes. The efficiency is therefore not affected by the structural differences between the complexes. Nine antenna polypeptides have been isolated from both the high light and low light grown cells. Two of these are the B875 complex alpha/beta- polypeptide pair. The N-terminal sequences of these polypeptides have been determined. The sequences of the remaining seven antenna polypeptides, which are almost completed, are also presented. Four of these are B800-850-alpha polypeptides and three are B800-850-beta polypeptides. This is the first report of an antenna complex from a purple bacterium containing more than three different polypeptides. Hydropathy analysis suggests that the polypeptides all contain a central membrane-spanning region. Three of the alpha-polypeptides are 52-54 amino acids long, the fourth is about 10 residues longer. This polypeptide contains an alanine-rich, C-terminal tail which possibly turns and inserts back into the membrane as a short stretch of alpha-helix. Two of the -polypeptides are also 52-54 amino acids long. The third is 5-6 amino acids shorter at the N-terminus. The alpha- and beta-polypeptides contain conserved histidine residues, at positions 31 and 40 respectively, which are believed to ligand the B850 chromophore BChl dimers. Interaction between the BChl molecules and conserved aromatic amino acids may partly determine the position of the near-infrared absorption maxima of the antenna complexes. Both the alpha- and beta-polypeptides contain amino acids which are capable of liganding the additional BChl chromophore in the LL complexes. Liquid chromatography has demonstrated a difference in the stoichiometry of the HL and LL B800-850 complex polypeptides. It remains uncertain, however, whether the complexes contain different combinations of polypeptides or whether they contain the same polypeptides but in a different stoichiometry. Crystals of the LL B800-850 complexes from Rps. palustris have been grown using the vapour diffusion method, with ammonium sulphate as the precipitant, and the small amphiphile benzamidine hydrochloride. The crystals represent the first stage in the determination of the three-dimensional structure of the complexes by x-ray diffraction analysis. A systematic study of the ratios of BChl to carotenoid in a range of antenna complexes is also reported. The ratio of 1:1 for the B875 complexes from Rb. sphaeroides agrees with the published value. However, the ratios for the B800-850 Type I complexes from Rb. sphaeroides, Rps. palustris, Chr. vinosum and Rps. acidophila were all determined to be 2:1 which contradicts the generally accepted value of 3:1 for this complex type. The ratio for the B800-850 Type II complexes from Chr. vinosum was also determined to be 2:1 in contrast to the published value of 3:1. The ratio for the B800-820 complexes of Rps. acidophila, strain 7750, was measured, for the first time, to be 2:1. This contradicts the belief that all B800-820 complexes have BChl-to-carotenoid ratios of 3:1. The ratios for the B800-850 Type II and B800-820 complexes from Rps. acidophila, strain 7050, were determined to be 2.7:1 and 3.5:1 respectively. But because of a possible inaccuracy in the absorption coefficient of cis-rhodopinal, it is proposed that the true ratios for these complexes are 2:1 and 3:1.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.796230  DOI: Not available
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