Photosystem 2 and the oxygen evolving complex: a brief overview

Coupling and Slips in Photosynthetic Reactions-From Femtoseconds to Eons

Photosynthesis stands as a unique biological phenomenon that can be comprehensively explored across a wide spectrum, from femtoseconds to eons. Across each timespan, a delicate interplay exists between coupling and inherent deviations that are essential for sustaining the overall efficiency of the system. Both quantum mechanics and thermodynamics act as guiding principles for the diverse processes occurring from femtoseconds to eons. Processes such as excitation energy transfer and the accumulation of oxygen in the atmosphere, along with the proliferation of organic matter on the Earth's surface, are all governed by the coupling-slip principle. This article will delve into select time points along this expansive scale. It will highlight the interconnections between photosynthesis, the global population, disorder, and the issue of global warming.

Structure of Chlorella ohadii Photosystem II Reveals Protective Mechanisms against Environmental Stress

Green alga Chlorella ohadii is known for its ability to carry out photosynthesis under harsh conditions. Using cryogenic electron microscopy (cryoEM), we obtained a high-resolution structure of PSII at 2.72 Å. This structure revealed 64 subunits, which encompassed 386 chlorophylls, 86 carotenoids, four plastoquinones, and several structural lipids. At the luminal side of PSII, a unique subunit arrangement was observed to protect the oxygen-evolving complex. This arrangement involved PsbO (OEE1), PsbP (OEE2), PsbB, and PsbU (a homolog of plant OEE3). PsbU interacted with PsbO, PsbC, and PsbP, thereby stabilizing the shield of the oxygen-evolving complex. Significant changes were also observed at the stromal electron acceptor side. PsbY, identified as a transmembrane helix, was situated alongside PsbF and PsbE, which enclosed cytochrome b559. Supported by the adjacent C-terminal helix of Psb10, these four transmembrane helices formed a bundle that shielded cytochrome b559 from the surrounding solvent. Moreover, the bulk of Psb10 formed a protective cap, which safeguarded the quinone site and likely contributed to the stacking of PSII complexes. Based on our findings, we propose a protective mechanism that prevents QB (plastoquinone B) from becoming fully reduced. This mechanism offers insights into the regulation of electron transfer within PSII.

CryoEM PSII structure reveals adaptation mechanisms to environmental stress in Chlorella ohadii

Performing photosynthesis in the desert is a challenging task since it requires a fast adaptation to extreme illumination and temperature changes. To understand adaptive mechanisms, we purified Photosystem II (PSII) from Chlorella ohadii , a green alga from the desert soil surface, and identified structural elements that might enable the photosystem functioning under harsh conditions. The 2.72 Å cryogenic electron-microscopy (cryoEM) structure of PSII exhibited 64 subunits, encompassing 386 chlorophylls, 86 carotenoids, four plastoquinones, and several structural lipids. At the luminal side of PSII, the oxygen evolving complex was protected by a unique subunit arrangement - PsbO (OEE1), PsbP (OEE2), CP47, and PsbU (plant OEE3 homolog). PsbU interacted with PsbO, CP43, and PsbP, thus stabilising the oxygen evolving shield. Substantial changes were observed on the stromal electron acceptor side - PsbY was identified as a transmembrane helix situated alongside PsbF and PsbE enclosing cytochrome b559, supported by the adjacent C-terminal helix of Psb10. These four transmembrane helices bundled jointly, shielding cytochrome b559 from the solvent. The bulk of Psb10 formed a cap protecting the quinone site and probably contributed to the PSII stacking. So far, the C. ohadii PSII structure is the most complete description of the complex, suggesting numerous future experiments. A protective mechanism that prevented Q B from rendering itself fully reduced is proposed.