Structural Origin of High-800 Peripheral Antenna Complexes

Reaction centre and light-harvesting complexes (LH) are the major pigment proteins of the photosynthetic apparatus in the purple non-sulphur bacteria. They are localised on the intracytoplasmic membranes. Two types of light harvesting antenna complexes are present in the majority of purple photosynthetic bacteria, namely the core (LH1) and peripheral (LH2) antennae, with distinct spectral and biochemical properties (1). The LH1 is always present, and is closely associated with the reaction center, it is therefore also referred to as the core or inner antenna. In contrast, the amount of the LH2, and in certain cases its spectral properties, vary according to growth conditions (2). Three different types of LH2 have been described in purple photosynthetic bacteria: the most usual type with an absorption spectrum showing similar sized maxima at 800 and 850 nm; a type in which the long wavelength absorption peak is shifted to 820 nm, such as the B800–820 complex of Rps acidophila; and a third variety in which the absorption at 800 nm is much greater than that at 850 nm, for example the antenna complexes of Rps. palustris grown at low light intensity. The structures of two different 800–850 nm absorbing LH2 complexes have been described at atomic resolution (3,4) allowing the development of an understanding of function of these complexes. The relationship between the 800–850 and 800–820 type LH2 has been elucidated through the combined application of resonance Raman spectroscopy (5), site directed mutagenesis (6) and structure based calculations (7). However currently the relationship between B800–850 type LH2’s and high-800 type LH2’s remains far from clear.