Image courtesy of Terry M. Bricker, Louisiana State University
Modified amino acid residues leave a tell-tale “trail” of water’s passage through the protein and to the active site of the photosynthetic protein complex known as Photosystem II (PS II). Proposed pathways for the substrate (water) and the product (molecular oxygen) are shown superimposed on a model of PS II.
Short bursts of high-energy radiation were used to generate highly reactive compounds from water. These compounds modified amino acids in close proximity to the water within Photosystem II (PS II), the key enzyme complex that splits water molecules in photosynthesis. The modified amino acids were “mapped” to the known protein structure using sophisticated mass spectroscopic techniques, suggesting that at least one channel directs water to the PS II active site.
This research provides the first direct experimental evidence of the specific amino acids that may line the channel delivering water molecules to the active site of PS II. Identification of the pathways used by the substrate (water) and the product (oxygen) can provide new insights into the function of PS II and may suggest new approaches for solar fuels production using natural or artificial photosynthesis.
Photosynthesis makes use of large multi-protein complexes called photosystems (PS) to capture solar energy and convert it to chemical energy. PS II specifically uses light energy from the sun to extract electrons from water, resulting in the oxidation or “splitting” of water to form oxygen and hydrogen. Putative water “channels” have been proposed as an explanation for the way that surface water gets to the active site of PS II, which is “buried” deep within the protein complex. Researchers at Louisiana State University used a combination of induced water radiolysis followed by mass spectrometry to identify possible water and oxygen pathways within PS II. Their direct experimental results were compared to earlier computational studies conducted by other researchers suggesting possible water/oxygen channels in PS II. While this study examined only a subset of the proteins in PS II, the results were consistent with one of the computational models. Identification of the paths taken by the substrate, water, as well as its product, oxygen, are providing important new knowledge on how natural photosystems work. Such insights may lead to new strategies for developing artificial photosynthetic systems for solar fuels production.
Louisiana State University
DOE Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Photosynthetic Systems Program
Frankel, L.K., Salans, L., Bellamy, H., Goettert, J.S., Limbach, P.A., and T.M. Bricker, “Radiolytic Mapping of Solvent-Contact Surfaces in Photosystem II of Higher Plants.”J. Biol. Chem 288, 23565 (2013). [DOI: 10.1074/jbc.M113.487033]