The Photosystem II (PSII) is enzyme at the beginning of the photosynthetic light reactions that splits water into protons and oxygen. Refer to the figure above to orient yourself and PSII within the photosynthetic electron transfer chain.
PSII is by far my favorite enzyme and I have spent most of my research career focused on how it works and how this complex molecular machine gets put together. Light energy is absorbed by antenna complexes and funneled into the PSII complex reaction center P680. The reaction center is a special pair of chlorophyll molecules buried in the heart of this multisubunit membrane protein complex. These chlorophylls are poised in just the right environment relative to other cofactors such that when energy is absorbed by them, charge separation occurs. This means that light causes the movement of electrons (as opposed to energy transfer along antenna systems flowing energetically downhill). When an electron has been separated from the reaction center, it travels through several other cofactors within PSII and finally onto a plastoquinone molecule (PQ). PQ is a small organic molecule that can move within the thylakoid membrane and carry its electrons over to the next complex in the light reactions (cytochrome b6f). With the light energy causing electrons to be displaced from the reaction center, PSII must replace those electrons from somewhere. It does this by pulling electrons from water molecules. In order to balance the reaction for products and reactants, PSII sequentially pulls four electrons from two water molecules to make oxygen, release 4 protons and make 2 PQH2 molecules (that’s just PQ with 2 electrons). PSII performs this water-splitting reaction using an inorganic manganese-calcium-chloride cluster buried on the lumenal face of the enzyme.
Here are the highlights of what we still don’t know:
PSII Structure: There are structural models of PSII, but they have only been solved for the cyanobacterial enzyme. See figure below for an example. This structure represents a highly uniform complex, which may not tell the entire story of what the PSII complexes really looks like. Also, there are some differences in the PSII components between cyanobacteria and plants making it difficult to infer what may be going on in the more complex plant system.
Water Oxidation Mechanism: Splitting water to form oxygen is a very challenging reaction and we’re still not exactly sure how PSII does it. Structural data as well as sophisticated spectroscopy data are getting us closer to understanding how water splitting works, but it is technically challenging to probe the details of this enzymatic reaction.
PSII Assembly: PSII is made up of more than twenty different protein subunits plus cofactors. All of these must come together precisely to create the highways for electron transfer within the enzyme. We don’t know all of the details as to how this works either. Also, there are a handful of other proteins that only act as chaperones to facilitate assembly of the complex, but do not actually become part of PSII. Understanding the details of PSII assembly is more than just an intellectual pursuit because of the topic listed below.
PSII Damage-Repair Cycle: PSII is at the heart of the ‘light problem’ for photosynthetic organisms. During the normal course of PSII function, its proteins become damaged by the electrons zipping through the complex. A sophisticated mechanism exists for recognizing the damaged protein, removing it and replacing it with a freshly made protein. There are lots of details still to be determined as far as how his process works and how it may be regulated. Researchers are also working to tease apart the differences and/or overlap between the de novo assembly pathway and the damage-repair cycle in the Life Cycle of PSII (see figure above).
PSII Regulation: PSII function is also finely tuned according to environmental conditions, and its regulation in response to light and the redox balance within the thylakoid membranes on a wide-ranging timescale represents a frontier in photosynthesis research. Some general components and strategies are known, but many new discoveries are waiting on the horizon in this area.