The ocean is considered as the largest ecosystem in the world, harboring two photosynthetic organisms which generate almost half of the oxygen on Earth. Compared to the net global primary production of the agricultural industry, the cyanobacterium Prochlorococcus fixes about 4 gigatons of carbon each year.
(Photo : Wikimedia Commons/ Luke Thompson from Chisholm Lab and Nikki Watson from Whitehead, MIT)
What is Prochlorococcus?
Prochlorococcus is a unicellular marine cyanobacterium which serves as the foundation of the aquatic food chain. Its size ranging from 0.5 to 0.7 micrometer in diameter makes Prochlorococcus the smallest known photosynthetic organism.
Photosynthesis depends on iron, the supply of which is limited in the ocean. The remarkable success of Prochlorococcus to carry out this process is based on its ability to thrive in low-nutrient waters.
Redox Molecular Switch
At the University of Southampton, Ivo Tews investigated the Prochlorococcus iron-binding protein FutA using complementary structural biology techniques. These include serial crystallography at I24 and at SACLA.
In a recent study, Tews and his colleagues revealed that FutA can accommodate iron in two various oxidation states, an ability which is believed to make Prochlorococcus more efficient in photosynthesis. The details of their research is described in the paper "A redox switch allows binding of Fe(II) and Fe(III) ions in the cyanobacterial iron-binding protein FutA from Prochlorococcus."
As part of the work, neutrons, X-rays and visible light were all used to help understand iron binding in FutA. Hydrogen atoms around the iron binding site were located using neutron crystallography, allowing scientists to determine the charges of amino acid side chains and the charge state of iron. Meanwhile, optical spectroscopic measurements were used to observe the rate of change of oxidation state from rust-red ferric iron to colorless ferrous iron when irradiated by X-rays.
The I24 beamline team at Diamond Light Source helped in designing two X-ray experiments to expose the iron binding protein to particular X-ray doses. The researchers used a method called serial crystallography which exposes thousands of crystals briefly to the X-ray beam. The single crystal measurements are then merged to create a complete, high-quality dataset.
According to Tews, their work included many different types of experiments and sources, but the one that stands out is serial synchrotron crystallography at Diamond Light Source. It has enabled them to follow changes in structure of FutA, in real time, under ambient conditions.
I24 has created hardware for fixed target serial crystallography which can be used at both Diamond Light Source and at other sources like the X-ray Free Electron Laser (XFEL) SACLA in Japan. To enable studies at SACLA, I24 fixed target hardware was delivered to Japan and built up at the XFEL beamline. The experiments performed at this facility enabled exposure to multiple, extremely brief X-ray pulses, while the next set of experiments conducted at beamline I24 allowed multiple structures to be determined at accumulating dose.
The study explores how bacteria utilize iron by using a protein which specializes in iron binding. Surprisingly, two different forms or oxidation states of iron can be bound by FutA. This implies two essential functions, such as the uptake of ferric iron from the environment, and the protection of the bacterial photosystems as ferrous iron.
As cyanobacteria usually have two types of proteins for such functions, scientists speculate that the presence of a single protein in the cyanobacterium Prochlorococcus that can carry out both tasks is a vital factor in its ecological success.
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