LIGHT-HARVESTING COMPLEXES

INTRODUCTION

The primary source of energy that makes all life possible is through the trapping of light energy from the sun. This trapped solar energy is then converted into chemical energy. The chemical changes are driven by electrons and protons flowing across the cell membrane of photosynthetic organisms. A large number of complicated steps must then take place before one ATP molecule, the fundamental biological unit of chemical energy, can be created.

Photosynthesis is defined as the process required to generate chemical energy from light. But we are primarily interested in the first step of light trapping by photosynthetic pigments within light harvesting complexes followed by the transportation of excited state energy to a special pair of pigments in the Reaction Centre where free electrons are created.

Light-Harvesting Complex II (LH2) are the main complexes involved in trapping light. They are produced in purple bacteria, along with all the other photosynthetic complexes, as a response to anoxic conditions and in the presence of light. In most purple bacteria a core complex, comprising LH1 and Reaction Centre, and a single kind of LH2 complex is produced. In some species of purple bacteria a variety of LH2 complexes are expressed under different light conditions. The predominent form of LH2 is the B800-850 complex which is made when solar radiation is plentiful. Under low-light conditions some species produces a complex which has its main absorption peak at 820 nm rather than 850 nm. Despite a different absorption spectrum this B800-820 LH2 complex is very similar in atomic structure to B800-850 LH2.

A novel low-light adapted LH2 complex has been isolated from the purple bacterium Rps. palustris. We have named it B800 LH2 due to the presence of only one near infra-red absorption peak at 800 nm. The B800 LH2 complex is produced under low-light conditions (< 300 lx) and replaces the majority of B800-850 LH2. The presence of additional bacteriochlorophyll a pigments also indicates that this complex is markedly different to all the others. Through onging structural work and through collaboration with Prof. T. Aartsma and co-workers at Leiden University, who are undertaking single molecule spectroscopy of this complex, we aim to establish the properties of this low-light adapted LH2 complex and to establish how it confers evolutionary advantage to the organism.