Three Step Dissociation and Covalent Stabilization of Phycobilisome

Abstract

Phycobilisomes are large, light harvesting complexes that extend the spectral range of photosynthesis of cyanobacteria by capturing visible light (470–660 nm) and transfer the energy to Photosystem II and Photosystem I. These complexes are soluble complexes that attach to the thylakoid membrane and act as antennae for both PSII and PSI. Phycobilisomes utilize up to 1,500 linear tetrapyrrole chromophores or bilins that are covalently attached to the a and β subunits found in either the rod subunits or the core complexes. Within these enormous structures are pigment-containing linker proteins that facilitate the very rapid and efficient downhill energy transfer from PE → PC → APC → Chl. Interestingly, efficient energy transfer in vitro has been observed only in the presence of very high phosphate (0.7–1.0 mol). We have investigated the mechanism and kinetics of how these complexes dissociate during dilution from the high phosphate buffer. This disassembly process has been followed using fluorescence spectroscopy, differential scanning calorimetry, circular dichroism, density gradient centrifugation, and Western blotting. This analysis has lead to a 3-step model of how the PBS disassembles. To facilitate the use of these large light harvesting complexes in applied photosynthesis, we have also explored covalent method for stabilization of the phycobilisome subunit interactions in aqueous, low salt conditions. Using the stabilization condition we are now beginning to determine if the light harvesting capabilities can be used to drive charge separation in Photosystem I for either photovoltaic or hydrogen evolution.