Chapter 11 Structure and function of light-harvesting pigment-protein complexes

Abstract

This chapter describes the structure and function of light-harvesting pigment-protein complexes. The antenna complexes of photosynthetic organisms are multi-molecular energy transport systems whose function is to funnel excited-state energy to the photochemical reaction center (RC). These light-harvesting pigment-protein complexes and the special pair of the RC thus form a cooperative, highly regulated energy transfer and energy trapping system. This system is the starting point for a light-induced redox reaction leading ultimately to the production of adenosine triphosphate (ATP) and the generation of reducing power. Light-harvesting antennae of the various organisms are composed generally of a number of pigment-protein (antenna) complexes of varying absorption maxima. Such complexes form systems for heterogeneous and directed energy transfer to the RC. The structures of antenna complexes acting within the same heterogeneous energy transfer system can differ greatly. Pigment molecules (Chl, BChl, bilins, carotenoids) are highly ordered within the antenna complexes, and their position and orientation follow defined laws of symmetry. The antenna system of photosynthetic organisms is, therefore, very efficient, and energy flows with little energy dissipation to the RC. Antennae contain a large number of pigment molecules, approximately 25–1000 pigments per RC.