Journal of Energy Bioscience 2024, Vol.15, No.4, 255-266 http://bioscipublisher.com/index.php/jeb 256 This study explores the current understanding of the structural and functional aspects of photosynthetic protein complexes, with a focus on their optimization for improved light energy conversion efficiency. We will examine the latest structural data on PSI and PSII, highlighting the key features that contribute to their high efficiency. Additionally, we will discuss recent advances in synthetic biology and bioengineering that have enabled the creation of novel photosystems with enhanced energy harvesting capabilities. By synthesizing findings from multiple studies, this study seeks to provide a comprehensive overview of the strategies for optimizing photosynthetic protein complexes and their potential applications in renewable energy technologies. 2 Photosynthetic Protein Complexes: Structure and Function 2.1 Description of key photosynthetic protein complexes Photosynthesis in plants, algae, and cyanobacteria is driven by four major multi-subunit membrane-protein complexes: Photosystem I (PSI), Photosystem II (PSII), the cytochrome b6f complex, and ATP synthase. Photosystem I (PSI): PSI is a large pigment-protein complex that plays a crucial role in the light-dependent reactions of photosynthesis. It is responsible for generating the most negative redox potential in nature, which is essential for the conversion of light energy into chemical energy. PSI is composed of multiple subunits and cofactors, including chlorophylls, carotenoids, and iron-sulfur clusters, which facilitate efficient light capture and electron transfer (Jordan et al., 2001; Nelson and Yocum, 2006; Qin et al., 2015). Photosystem II (PSII): PSII is another essential complex that catalyzes the photo-oxidation of water, producing oxygen as a byproduct. It consists of a core complex surrounded by light-harvesting complexes (LHCs) that capture and transfer light energy to the reaction center. The PSII core includes several subunits and cofactors, such as chlorophylls, pheophytins, and a manganese-calcium cluster, which are involved in the water-splitting reaction (Nelson and Yocum, 2006; Croce and Amerongen, 2011; Suga et al., 2014; Chukhutsina et al., 2015). Cytochrome b6f Complex: This complex acts as an intermediary in the electron transport chain, facilitating the transfer of electrons between PSII and PSI. It also contributes to the generation of a proton gradient across the thylakoid membrane, which is used to drive ATP synthesis (Nelson and Yocum, 2006; Yadav et al., 2017). ATP Synthase: ATP synthase is responsible for the synthesis of ATP from ADP and inorganic phosphate, utilizing the proton gradient generated by the electron transport chain. This enzyme complex is crucial for providing the energy required for various cellular processes (Nelson and Yocum, 2006). 2.2 Structural components and their roles in light energy conversion The structural components of these photosynthetic protein complexes are intricately designed to optimize light energy conversion: Chlorophylls and Carotenoids: These pigments are essential for capturing light energy. Chlorophylls absorb light primarily in the blue and red regions of the spectrum, while carotenoids absorb in the blue-green region, extending the range of light that can be utilized for photosynthesis. The arrangement of these pigments within the protein complexes ensures efficient energy transfer to the reaction centers (Jordan et al., 2001; Qin et al., 2015). Reaction Centers: The reaction centers of PSI and PSII are specialized protein complexes where primary charge separation occurs. In PSII, the reaction center includes the D1 and D2 proteins, which bind the primary electron donor (P680) and the primary electron acceptor (pheophytin). In PSI, the reaction center includes the PsaA and PsaB proteins, which bind the primary electron donor (P700) and the primary electron acceptors (A0, A1, and FX) (Nelson and Yocum, 2006; Croce and Amerongen, 2011; Suga et al., 2014). Iron-Sulfur Clusters and Quinones: These cofactors play critical roles in electron transport. In PSI, iron-sulfur clusters (FX, FA, and FB) facilitate the transfer of electrons from the reaction center to ferredoxin. In PSII, plastoquinone (QA and QB) accepts electrons from pheophytin and transfers them to the cytochrome b6f complex (Jordan et al., 2001; Suga et al., 2014).
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