Antenna Complexes from Green Photosynthetic Bacteria

Abstract

SummaryGreen photosynthetic bacteria contain antenna complexes known as chlorosomes. These complexes are appressed to the cytoplasmic side of the inner cell membrane and function to absorb light and transfer the energy to the photochemical reaction center, where photochemical energy storage takes place. Chlorosomes differ from all other known photosynthetic antenna complexes in that the geometrical arrangement of pigments is determined primarily by pigment-pigment interactions instead of pigment-protein interactions. The functional role of the proteins found in chlorosomes is not well understood. The bacteriochlorophyll c, d or e pigments found in chlorosomes form large oligomers with characteristic spectral properties significantly perturbed from those exhibited by monomeric pigments. Because of their close spatial interaction, the pigments are thought to be strongly coupled electronically, and many of the optical properties result from exciton interactions.In addition to chlorosomes, the green sulfur bacteria contain another unique antenna complex known as the bacteriochlorophyll a protein, or the Fenna-Matthews-Olson (FMO) protein. This complex was the first pigment-containing photosynthetic complex to have its high resolution structure determined. It has been intensely studied by spectroscopic and theoretical methods.This review summarizes existing knowledge on the chemical composition and properties of chlorosomes, the evidence for the oligomeric nature of chlorosome pigment organization and proposed structures for the oligomers, the properties of and possible functions of chlorosome proteins, the kinetics and mechanisms of energy transfer in chlorosomes, and the structure and spectroscopic properties of the FMO protein.