Photosystem II: Water Channels

The Light Reactions

The Light Reactions

Something new under the sun… water channels through Photosystem II. First, some background.

The Photosystem II (PSII)* is enzyme at the beginning of the photosynthetic light reactions that splits water into protons and oxygen. This 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. Here are the highlights for what it does and how it works. 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). This is a very challenging reaction and we’re still not exactly sure how PSII does it.


Photosystem II structure

One thing we’re not sure about is how the water actually gets into and out of the PSII enzyme. In the case of many proteins, water access is restricted to the surface of the protein or just a filler if there is a cavity within the protein. For PSII, water is a reactant or a substrate. This means that water must go into PSII in a very specific way to a specific location in order for the reaction to take place. We have some idea of what PSII looks like, but we don’t have any experimental evidence for where water may travel in and oxygen out. In a paper out today in the Journal of Biological Chemistry (it also got to be the cover art!), my boss Terry Bricker and Laurie Frankel report on experiments to identify what may be water channels. Here’s how they did it:

Observations: PSII uses water as a reactant, so this molecule must have a pathway into the active site. Water molecules can also oxidatively damage the amino acid residues of proteins upon exposure to synchrotron radiation. This protein damage causes a mass change on the protein. These mass changes can be identified and mapped onto a known 3D structure of PSII.

Hypothesis: The protein residues at the PSII surface are exposed to water and should be easily modified by radiation damage. The buried residues will largely be protected from damage. If there is a channel for the entry of water into PSII, then buried residues along this pathway to the active site will be modified. Therefore clusters of damaged residues that are also found on the interior of the PSII structure represent candidates for a water channel.

Experiment:  Isolate PSII-enriched membranes from spinach and treat with increasing radiation (time in the synchrotron electron beam) to induce damage. Identify damaged residues using mass spectrometry. Map the locations of the damaged residues within the PSII structure and look for buried clusters.

Results: The majority of damaged residues were located at the surface of PSII where the water molecules interact with the outer surface of the enzyme. The rest of the residues were found to be buried inside of PSII. A string of damaged sites on buried residues leading out from the surface down into the active site were identified.

Conclusion: This string of buried residues susceptible to oxidative damage upon radiation treatment comprise a possible channel that water uses to access the PSII active site. This region had been predicted to be a water channel based on computer modeling, but this is the first experimental evidence for this kind of channel. A second group of buried residues susceptible to damage were also identified and this may represent a channel for oxygen leaving PSII.

Think Ahead:  More experiments are needed to tell which channel would be the water in channel and which channel would be the oxygen out channel. Knowing these details will help us understand how this important reaction works.

Here’s the reference and the link.**

Laurie K. Frankel, Larry Sallans, Henry Bellamy, Jost S. Goettert, Patrick A. Limbach, and Terry M. Bricke. Radiolytic Mapping of Solvent-Contact Surfaces in Photosystem II of Higher Plants: Experimental Identification of Putative Water Channels within the Photosystem. J. Biol. Chem. 2013 288: 23565-23572.


*Yes, PSII is considered the first in line. There is a PSI, but we’ll talk about it later.

**The full text is freely available for this one. After about noon today, the official full text version will be available and the journal cover can be seen from the JBC website (Aug 9, 2013 issue).

Also, sorry for the lower amount of posts this week. I’ve been traveling and attending a conference dedicated completely to cyanobacteria. More posts on that research coming soon.


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