A Study of the Energetic Properties of LH2 Complexes from RPS. Acidophila 10050 Containing Modified (Bacterio) Chlorin Molecules

The primary events in bacterial photosynthesis involve the capture of light energy by light harvesting (LH) antenna complexes, followed by rapid and efficient energy transfer to a reaction centre (RC), where the absorbed energy is “trapped”. In most species of purple bacteria, there are two types of light harvesting complex. The first type, LH1, is intimately associated with the RC to form a “core” complex. Arranged more peripherally to this and present in variable amounts is the second type of complex called LH2. Both types of LH complex are built on a similar modular principle, in which the light-absorbing pigments bacteriochlorophyll a (Bchla) and carotenoid molecules are non-covalently bound to two low Mr hydrophobic apoproteins α and β. The native complexes are oligomers of these monomeric units. The crystal structure of the LH2 complex from Rps. acidophila 10050 has recently been determined to a resolution of 2.5 A° by McDermott et al1. The complex was observed to be an α9β9 oligomer, with each monomeric unit containing an α and β apoprotein, 3 Bchla molecules and 1 (or 2?) carotenoid (rhodopin-glucoside) molecules. The 9 α apoproteins form a hollow cylinder with the 9 β apoproteins arranged radially outside them. Eighteen Bchla molecules are sandwiched between the α and β apoprotein helices and form a continous overlapping ring. These molecules absorb at approximately 850 nm (B850) and have their bacteriochlorin rings perpendicular to the membrane surface. A further nine Bchla molecules are positioned more peripherally in the complex, situated between the β apoproteins.