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A Slow-Motion Film of the Changes in a Membrane Protein

​Bacteriorhodopsin is a protein that transports protons through cell membranes—an essential function of biological systems. An international consortium involving IBS has "filmed" this transfer of protons.

Published on 19 January 2017

Bacteriorhodopsin harvests the energy content from light and transports protons through cell membranes in order to create a charge difference used to generate the energy necessary for the cell to function. The scientists have long sought to explain the mechanism of proton expulsion, which takes place unidirectionally, from the inside to outside of the cell. To discover this mechanism, an international consortium of scientists involving IBS scientist Antoine Royant has used the SACLA free-electron laser in Japan, which produces an X-ray beam one million times more intense than those of synchrotron sources, which themselves are already very intense. What makes SACLA's X-rays special is that they are generated for an extremely short period of time: one hundredth of a billionth of a second (about ten femtoseconds).

The researchers used a technique called time-resolved serial femtosecond crystallography to record tens of thousands of images of bacteriorhodopsin, after a period of time varying between one nanosecond and one millisecond. By analyzing the data, they were able to decrypt the mechanism of the expulsion of protons outside of the cell, in a more positively charged environment. Like in a battery, this charge difference fuels the chemical reactions responsible for the life of the cell.

"With this experiment, we were able to confirm the hypotheses formulated in the early 2000s on the first stages of the mechanism, and above all, to visualize the different atomic movements within the bacteriorhodopsin in real time, and to understand how they unfold," said Royant. The light-induced excitation causes a change in the configuration of retinal (a form of vitamin A), the colored molecule located at the heart of the protein. This change pushes away a molecule of water and then a set of rearrangements in the protein structure triggers the expulsion of a proton towards the extracellular side of the protein.

"We finally understood how the changes in the vicinity of the retinal prevent the proton from crossing the membrane once again. This result creates opportunities for understanding the mechanism of proteins with a high level of details, and consequently, to use them to our own benefit" he said.

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