Pioneering research by scientists at the Indian Institute of Science (IISc) and California Institute of Technology (Caltech) have recently shed new light on Photosystem II (PSII). This key ingredient serves as a fundamental building block in the process of photosynthesis. This protein-pigment complex is absolutely crucial for the process of trapping solar energy. It begins the process of turning light energy into chemical energy, which fuels all life on Earth. Aditya Kumar Mandal, first author of the study, has shed light on the newly discovered complex relationship between structure and function at PSII. Most specifically, it looks in detail at its two branches, D1 and D2.
Photosystem II, the first stage, works by capturing sunlight and splitting water molecules. This reaction releases oxygen and generates energized electrons. These electrons are later passed on to other proteins and molecules, allowing the process of photosynthesis to grow and develop. This study demonstrates functional asymmetry of the D1 and D2 PSII branches. We report how each of these branches contributes to active electron transport.
The Structure of Photosystem II
Photosystem II is unique within the plant kingdom due to its structural symmetry, consisting as it does of two identical structural arms, dubbed D1 and D2. Each arm has four chlorophylls and two pheophytins symmetrically arranged around them. Despite this symmetry, the study highlights a critical distinction: only the D1 branch is functionally active in the process of electron transport.
The researchers utilized advanced methods such as molecular dynamics simulations, quantum mechanical calculations, and Marcus theory to investigate the electron flow within PSII. Their results indicate that electrons can flow unimpeded from chlorophyll to pheophytin and on to plastoquinone via the D1 branch. What’s puzzling is that the same fluid flow does not happen in the D2 branch.
The Functionality Discrepancy
The research team discovered that the D2 branch indeed has a significantly larger energy barrier for electron transport. Therein lies the major distinction between D2 and its D.C. sibling. Specifically, the activation barrier for the third step in D2 is nearly double that of the corresponding step in D1. This new resistance against electron movement is key to understanding why no charge transport happens along the D2 branch.
The data demonstrated that D2 indeed impeded electron flow significantly more than D1. In fact, it turns out that D2’s resistance is two orders of magnitude higher! This large discrepancy prevents the D2 branch from being kinetically active for functional electron transport. It illuminates just how key the D1 branch is in the overall operation of Photosystem II.
Implications of the Research
The ramifications of this research go far beyond academic interest. Learning how to work with the details of Photosystem II should yield deep benefits, particularly in areas like renewable energy and food production. Today, scientists are first understanding the mechanisms behind photosynthesis. Significant discoveries from this research would enable new approaches to improve plant efficiency and develop manmade systems that replicate these processes for clean energy production.
Bill Goddard, a Professor at the California Institute of Technology (Caltech), who is one of the study’s corresponding authors. He illustrated just how meaningful these discoveries are in the larger context of overall scientific research. Researchers are endeavoring to unlock the many remaining mysteries to Photosystem II. That discovery has the potential to inspire entirely new tools, technologies and approaches to meet the world’s increasing and unsustainable energy needs.