How a cyanobacterium causes far red light to mean “go”
âWe knew by isolating and characterizing the complexes that photosystem I contains 7 to 8 molecules of chlorophyll f, and that photosystem II contains one molecule of chlorophyll d and 4 to 5 molecules of chlorophyll f, as well as about 90% of the original chlorophyll. wanted to know where these changes occurred in the complexes, âsaid Bryant. “One way to figure this out is to determine the structure of the complexes, but because they’re so big and complex – and the chemical differences are so minor – it was extremely difficult.”
Photosystem I and II complexes are very difficult to crystallize – because they are very large membrane-bound complexes – so x-ray crystallography, a standard laboratory method for determining the three-dimensional structures of molecules, was not likely to work. The researchers then turned to cryo-EM, but the tiny differences in the shapes of the chlorophyll molecules pushed the limits of the cryo-EM resolution to be detected. Chlorophylls differ in only a few atoms of similar mass.
âMy collaborator, Chris Gisriel, who is a postdoctoral fellow in Gary Brudvig’s lab at Yale, was fortunate enough to obtain a very high resolution structure for the photosystem II complex – 2.25 angstrom (Ã ) – allowing him to visualize the differences in some of the chlorophylls directly, âsaid Bryant. âThe extent of the difference between chlorophyll a and f is that two hydrogen atoms are replaced by one oxygen atom in a molecule with the composition of C55H72MgN4O5. In a complex like Photosystem I which contains almost 100 pigment molecules and 11 protein subunits or Photosystem II with 35 chlorophylls and 20 protein subunits, these small changes are like looking for a few needles for two very large haystacks. Because these chlorophylls impart the special properties that allow the use of far red light, it is very important to understand exactly how these molecules are arranged.
Most of the time, oxygen atoms are linked by hydrogen bonds, so researchers can look for hydrogen bond donors near the right places in chlorophyll molecules. By applying this method and others to structures determined using cryo-EM, they were able to identify the locations of chlorophyll f molecules in the two photosystem complexes and the position of the single chlorophyll d molecule in the photosystem. II also.
âIdentifying the structural basis of how this far-red light absorption occurs in nature is an important step forward,â said Gisriel, first author of the two studies. âIdentifying the precise locations in photosystem I and II complexes where alternative forms of chlorophyll are incorporated could open the door to exciting future applications. For example, crops could potentially be designed to harvest light beyond the visible spectrum. Additionally, two crops could potentially be grown together, with shorter crops, using the far red light filtered from their shaded locations under taller crops. Alternatively, the plants could be grown closer to each other due to better light capture in the leaves under the canopy.
In addition to Bryant and Gisriel, the research team at the first paper, titled “Structure of a photosystem I-ferredoxin complex from a marine cyanobacterium provides information on photo-acclimatization to far red light,” includes David A. Flesher, Gaozhong Shen, Jimin Wang, Ming-Yang Ho, and Gary W. Brudvig. Funding was provided by the United States National Science Foundation and the United States Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.
The research team of the second paper, titled “Structure of a monomeric photosystem II core complex from a cyanobacterium acclimated to far-red light reveals the functions of chlorophylls d and f”, includes Gaozhong Shen, Ming-Yang Ho, Vasily Kurashov, David A. Flesher, Jimin Wang, William H. Armstrong, John H. Golbeck, MR Gunner, David J. Vinyard, Richard J. Debus, and Gary W. Brudvig. The research was supported by the National Science Foundation of the United States; US Department of Energy, Bureau of Basic Energy Sciences, Division of Chemical Sciences; US Department of Energy, Division of Chemical, Geoscience, and Bioscience, Photosynthetic Systems; and the National Institute of General Medical Sciences of the United States National Institutes of Health.