Researchers at the Paul Scherrer Institute PSI have taken a molecular energy machine into action with the Swiss light source SLS and thus demonstrated how energy production works on cell membranes. For this purpose, they developed a new method of investigation, with which the analysis of cellular processes can be carried out much more effectively than before. They have now published their findings in the journal Science .
Structural changes in proteins are responsible for many biochemically controlled functions in all living things, for example for cell membrane energy production. The protein Bacteriorhodopsin is found in microorganisms that live on the surface of lakes, streams and other waters. This sunlight-activated molecule pumps positively charged particles, protons, from the inside out through the cell membrane. It constantly changes its structure.
PSI researchers have already been able to elucidate part of this process using free-electron X-ray lasers (FELs) such as SwissFEL. Now they have managed to capture the as yet unknown part of the process in a kind of molecular film. For this they have chosen a method that was previously only applicable to FELs and further developed for use in the Swiss Light Source SLS. The study highlights the synergy between the analytical capabilities of these two major research facilities at PSI. "With the new method at SLS, we can now track the last part of the bacteriorhodopsin movement, where the steps are in the millisecond range," explains Tobias Weinert, first author of the paper. "When measuring at FELs in the US and Japan, we had already measured the first two subprocesses before SwissFEL went online," says Weinert. "These happen very quickly within femtoseconds to microseconds." A femtosecond is a trillionth of a second.
In order to observe such processes, the researchers use the so-called "pump-probe" crystallography. With this method, they can take snapshots of protein movements, which can then be put together into films. For the experiments, proteins are brought into crystal form. A laser beam that mimics the sunlight triggers the movement process in the protein. X-rays that subsequently hit the sample produce diffraction images that are picked up by a high-resolution detector. Computers generate a picture of the protein structure at all times.
The film obtained from the SLS measurements shows how the structure of the bacteriorhodopsin molecule changes in the next 200 milliseconds after its activation by light. This is now a complete so-called "photocycle" of the molecule elucidated.
Bacteriorhodopsin acts as a biological machine that pumps protons out of the cell interior through the membrane. This creates a concentration gradient at the cell membrane. There are more protons on its outside than on its inside. The cell uses this gradient to gain energy for its metabolism by allowing protons elsewhere to compensate for the externally and internally different concentrations. The cell produces ATP, a universal source of energy in living things. Subsequently, bacteriorhodopsin restores the concentration gradient.
"In the new study, we have now been able to determine the largest real-time structural changes in a molecule" – large means the scientist nine Angstrom, that is one-millionth of the thickness of a human hair. These structural changes open a gap in the protein that forms a chain of water molecules responsible for transporting protons through the cell membrane. "Before us, nobody had directly observed this water chain," says the biochemist happily.
These observations were only made possible by the modification of the method previously used at SwissFEL for use with SLS and thanks to the new high and faster "Eiger" detector at SLS. Weinert is confident that the new research methodology with synchrotrons such as SLS will inspire research worldwide. "Researchers can apply the new method and become more efficient, because there are many more synchrotrons in the world than free-electron lasers, and less protein crystals than are required for experiments on FELs," adds Weinert.
Researchers rely on SwissFEL for the very fast molecular processes and for particularly sharp images and precise results. "The processes at the beginning of the photocycle terminate in a few femtoseconds, and such rapid chemical reactions can only be observed in FELs." In addition, higher resolution structures can be recorded on FELs. Since so many photons hit the sample simultaneously with the linear accelerator, the detector can take an extremely sharp image.
Weinert emphasizes the synergy between the two major research institutes: "At SwissFEL, only a short beam time is required The measurements at the SLS enable us to ensure in advance that our experiment at SwissFEL is successful."
Die Results of the study, the researchers now in the journal Science [19459011veröffentlicht].
Biological light sensor filmed in action
Tobias Weinert et al. Proton uptake mechanism in bacteriorhodopsin, obtained by serial synchrotron crystallography, Science (2019). DOI: 10.1126 / science.aaw8634
Molecular energy machine as a movie star (2019, 5 July)
retrieved on 5 July 2019
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