Reaction Center-Light-Harvesting Core Complexes of Purple Bacteria

Abstract

Reaction center-light-harvesting 1 (RC-LH1) complexes, the fundamental photosynthetic units, are fascinatingly diverse in their aggregation state, with both monomers and dimers found in membranes. Some complexes also contain the PufX polypeptide in addition to the RC and LH1 complexes. Many structural techniques have been applied to RC-LH1 complexes, in order to analyze their shape, variation in conformation, the individual polypeptide structures, and the overall organization in the membrane. Thus, the unique capabilities of cryo- and negative stain electron microscopy (EM), X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy and atomic force microscopy (AFM) have all complemented one another in the analysis of membranes and purified protein complexes, from both wild type and mutated strains of photosynthetic bacteria. However, many unsolved problems remain; for example, the ways in which quinones and quinols exchange at the RC QB site despite the apparent barrier arising from the encircling LH1 complex are still not resolved, and this hints at still unexplored flexibility and dynamics associated with the LH1 complex and with the photochemistry of the RC. Other controversies currently involve the structure, location and function of the PufX polypeptide, and most basic of all, there is still a need for high-resolution structures of core complexes. Finally, the assembly pathways of core complexes are not known. This chapter attempts to convey the diversity of experimental approaches used to investigate core complexes and it summarizes the current state of progress in this evolving field.