Chapter 5: The Light-Harvesting System of Purple Anoxygenic Photosynthetic Bacteria

The anoxygenic purple photosynthetic bacteria synthesize a group of integral membrane pigment–protein complexes that function to absorb the solar energy that allows photosynthetic growth. These complexes, called light-harvesting complex 1 (LH1) and light-harvesting complex 2 (LH2), are circular structures that transfer the absorbed light to the photochemical reaction centre (RC), where charge separation across the membrane occurs. This ultimately results in creation of a proton gradient that is used to drive production of cellular fuel in the form of adenosine triphosphate (ATP). The LH complexes reside in an extensive membrane system, derived from the cytoplasmic membrane, that is synthesized de novo in response to light and low oxygen concentrations. Now is a rich time for researchers interested in the light reactions of the photosynthetic process in purple bacteria, as the 3D structures of all the components involved have been elucidated. This chapter focuses on the purple non-sulfur photosynthetic bacteria and the structure and function of their light-harvesting complexes.