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Molecular Organisation and Assembly in Cells

Energy Transfer in a Model Photosystem

Supervisor: Dr Scott Habershon (Chemistry, CSC).

Green sulphur bacteria (GSB) undergoes anoxygenic photosynthesis, which, briefly, involves the absorption of photons by large antennas called chlorosomes, located in lipid monolayer sacs on the surface of the bacterium's membrane [1-3]. This creates an electronic excitation which is transferred to a reaction centre (RC) located within the bacterium's phospholipid bilayer, see Figure 1. A proton gradient is established across the membrane at the RC which provides chemical energy to drive phosphorylation, required to synthesise ATP. Electronic energy transfer (EET) from chlorosome to RC is mediated through a pigment-protein complex which is responsible for the efficiency of EET, providing light to charge conversion efficiencies of >95 % [1]. In GSB, the pigment-protein complex is called the Fenna-Matthews-Olson (FMO) complex. It consists of three identical monomeric subunits, each of which consists of eight Bacteriochlorophylls (BChls). EET is a directed process from the source BChl 'sites' to the sink BChl via a network of many possible pathways.

In this project, we aimed to understand the efficiency and robustness displayed by FMO in its job of efficiently transporting energy from source sites to sink sites using a simple network based analysis of FMO which provided computational tractability allowing for a multitude of simulations to be run. Details of this work can be found in reference 4.

FMO

Figure 1: (left) A schematic of the photosynthesis 'machinery' in green sulphur bacterium. (right) A to-scale schematic of one of the FMO monomers with the Mg2+ ions labelled which are the centre of each BChl. We consider sites 1 and 6 and 8 as the source sites and site 3 as the sink.

[1] Y. Cheng and G. Fleming. Dynamics of light harvesting in photosynthesis. Annu. Rev. Phys. Chem., 60:241-262, 2009.
[2] R. Blankenship. Early evolution of photosynthesis. Plant Physiol., 154:434-438, 2010.
[3] B. Alberts, A. Johnson, J. Lewis, M. Raff, K. Roberts, and P. Walter. Molecular Biology of the Cell. Garland Science, 4th edition, 2002.
[4] Lewis A. Baker and Scott Habershon. Robustness, efficiency, and optimality in the Fenna-Matthews-Olson photosynthetic pigment-protein complex. J. Chem. Phys. 2015, 143, 105101. DOI: 10.1063/1.4930110.