Studying photosynthetic antenna complexes in vivo and in vitro using time-resolved fluorescence spectroscopy

Abstract

Chapter 1 Introduction 7 Chapter 2 Picosecond excitation energy transfer of allophycocyanin studied in solution and in crystals 33 Chapter 3 State transitions in the cyanobacterium Synechococcus elongatus 7942 involve reversible quenching of the photosystem II core 57 Chapter 4 Light harvesting dynamics in desert crust cyanobacteria: Changes in aggregation state as a mechanism for modulating energy transfer 93 Chapter 5 Efficiency of excitation-energy trapping in the reaction center of the low-light adapted green photosynthetic bacterium Chlorobaculum tepidum 129 Chapter 6 General discussion 163 Chapter 7 Summary 175 Acknowledgements 179 Photosynthesis, cyanobacteria, and state transitions Photosynthetic organisms convert light energy into chemical energy to sustain their metabolism, but which can also be used by other living organisms. Photosynthetic reactions are divided into light-dependent and light-independent reactions (Blankenship 2008a). Plants and cyanobacteria produce Adenosine Triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) during the light-dependent reactions. In the light-independent reactions, ATP and NADPH are used to fix carbon dioxide into carbohydrates. The light-dependent reactions start with the absorption of light by the photosynthetic antennas. Cyanobacteria use phycobilisomes (PBSs) and photosystems I and II (PSI and PSII) to absorb light (Figure 1). PBSs are the main light-harvesting antennas in cyanobacteria which mainly absorb green-orange light. They are composed of rodlike proteins and core proteins. (MacColl 2004; Mirkovic et al. 2016). The rods are built of C-phycocyanins (CPCs) and in various organisms they also have phycoerythrins (PEs). The rods are attached to allophycocyanins (APCs), which contain APC trimers and three other protein-pigments, namely LCM, β 16 , and α B. The excitation energy absorbed by PEs and PCs travels along the rods and reaches APCs, which are attached to the rods by linker proteins, and from there it is delivered in cyan. The dotted line separates one monomer. The codes in the protein data bank are 1jbo (CPC) and 4rmp (APC). PSII is another multi-subunit complex present in cyanobacteria, which is generally believed to form dimers in vivo (Nelson and Ben-Shem 2004; Nickelsen and Rengstl 2013; Zlenko et al. 2017) (Figure 3). The PSII reaction center complex largely consists of two proteins, D1 and D2 and binds six chlorophylls a, two pheophytins a a, two pheophytin and two quinone molecules as described as follows. PD1 and PD2, which are Chls a, are shown in red and blue and next to them are two other Chls a in green, which are called ChlD1 and ChlD2. The pheophytin molecules (PheD1 and PheD2) are represented in black and the quinones in purple. The bottom of the figure shows two different pathways that result in the same chargeseparated state. Only one branch is active in charge separation. The star shows the excited state and the rectangles show the charge-separated states. it takes up one proton from the stroma (two in t...