Computational Visualization of the QA Binding Site through Potential Energy Surface Mapping in Rhodobacter sphaeroides

The photochemical reaction center of purple bacteria serves as a key model for understanding the primary processes of photosynthesis. In this study, we investigate the structural arrangement of the Q_A binding site in the reaction center of Rhodobacter sphaeroides to gain insights into the stabilization and interaction mechanisms of the primary quinone acceptor, ubiquinone (Q_A). The potential energy surface (PES) was constructed within a 10 Å sphere surrounding the Q_A binding pocket to visualize its local structural environment. The ubiquinone molecule contains two carbonyl oxygen atoms (O₁ and O₄) in its benzoquinone ring, which form hydrogen bonds with the backbone nitrogen of Ala260 (O₁ ≈ 2.84 Å) and the imidazole nitrogen of His219 (O₄ ≈ 2.78 Å), respectively. These interactions contribute significantly to the stabilization of the Q_A molecule within the hydrophobic core of the protein. The remaining portion of the molecule is surrounded by predominantly nonpolar residues forming van der Waals contacts, while positively charged residues located near the membrane surface assist in orienting and stabilizing the ubiquinone head group to facilitate electron transfer. The results highlight the intricate hydrogen-bonding and hydrophobic interactions that define the geometry of the Q_A binding pocket, providing a structural basis for understanding quinone-mediated electron transfer in bacterial photosynthesis.