From Atomic-Level Structure to Supramolecular Organization in the Photosynthetic Unit of Purple Bacteria

Abstract

The purple bacterial photosynthetic unit (PSU) is a macromolecular assembly of remarkable simplicity that harvests sunlight with the cooperation of only half a dozen different kinds of proteins. This chapter provides a summary of recent research on the architectural and biophysical aspects of the PSU and its constituents. First, a brief overview is provided of the structure of light-harvesting components. Then the effects of thermal disorder and spectral universality on the light-harvesting function of the pigment-protein complexes is discussed, followed by an account of the physical constraints that shape the evolution of light-harvesting complexes in general. Finally, a summary is provided of recent research on the in silico assembly of an entire PSU in atomic detail. This supramolecular reconstruction of the PSU is made possible by the recent availability of not only the structural data on the individual constituent proteins but also on their global arrangement. The reconstruction is performed by combining data from X-ray crystallography, nuclear magnetic resonance, cryo-electron microscopy, and atomic force microscopy using computational modeling. The architecture of the PSU vesicle that emerges constitutes nearly two hundred light-harvesting proteins, containing around four thousand chlorophylls, which act cooperatively to maintain a very high quantum yield in a pigment array distributed over a pseudo-spherical intracytoplasmic membrane domain with an inner diameter of 60 nm.