Structural Changes in a Photosynthetic Protein Demonstrated in Four Dimensions

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Biophysics researchers used a free-electron laser to detect light-induced structural changes with the utmost in temporal and spatial precision

No 250/2019 from Sep 04, 2019

The process by which light energy translates into a change in protein structure plays a crucial role in many areas of life, from vision to photosynthesis. A team of researchers from the Department of Physics at Freie Universität Berlin has used a free-electron laser and femtosecond laser systems to study the first processes of a light-active protein. The experiments, which were conducted under the leadership of Professor Ilme Schlichting, Director of the Department of Biomolecular Mechanisms, Max-Planck Institute for Medical Research, received key support from researchers affiliated with the Collaborative Research Center (Sonderforschungsbereich, SFB) 1078 "Protonation Dynamics in Protein Function" at Freie Universität Berlin. Dr. Ramona Schlesinger’s working group provided the extremely large volumes of purified membrane protein. Electronic and oscillatory transitions were studied by the working group headed by Dr. Karsten Heyne in femtosecond experiments. Dr. Joachim Heberle’s working group contributed expertise in spectroscopy, helping to ensure that this extremely complex experiment was a success.

The use of light energy is vital to many organisms - from perceiving their surroundings to controlling growth processes and obtaining energy, for example in photosynthesis in plants. As soon as a photon hits the light-absorbing center, the energy is transferred to a protein as a whole in the form of a spatial change in structure. This principle of absorbing energy from light and then passing it along to the surrounding protein is fundamentally important, not just in terms of basic research, but also to the development of future biomimetic systems for energy conversion.

Tracing light-driven processes requires special methods, as these processes take place within femtoseconds (10-15 seconds = one-millionth of one-billionth of a second). To achieve this, the researchers used a device known as a free-electron laser, which emits very short, highly intense X-ray pulses. These ultra-short X-ray pulses make it possible to visualize the structural changes that take place. To perform these experiments, a research team traveled to the Linac Coherent Light Source (LCLS) at Stanford University in California, where this type of laser is available for use by the scientific community. A similar, even more powerful X-ray laser (EU-FEL) recently went into operation in Hamburg. Since intense light pulses generate both linear and nonlinear effects, the team of researchers used femtosecond laser systems at Freie Universität Berlin in the visible and infrared spectral range with different excitation intensities in order to distinguish between the structural changes measured and nonlinear effects.

Within the scope of this interdisciplinary cooperation, the researchers succeeded in clarifying the atomic structure of the ultra-short-lived intermediate states of the light-driven proton pump bacteriorhodopsin at different points in time after photoactivation. They saw how retinal, a molecule that forms the photosensitive center of the protein, changes in structure after absorbing a photon, going from the stretched-out all-trans configuration to a curved 13-cis configuration. The researchers also demonstrated correlated oscillatory movements in the electronically stimulated retinal, the surrounding amino acids and water molecules and their hydrogen bonding network. These results can be applied to other retinal proteins, such as the light-absorbing pigment rhodopsin; they will contribute to the understanding of efficient light absorption.

The work is receiving funding from the German Research Foundation (DFG) within the scope of SFB 1078, of which Joachim Heberle is the spokesperson. Within this research alliance, the role of protonation dynamics in protein functioning is being studied at the atomic level. In methodological terms, a combination of new experiments in biophysics with molecular simulations and quantum chemistry calculations is being used. Although the research program concentrates on fundamental questions, it is hoped that it will provide impetus for new approaches in energy sciences (light-controlled water oxidation) and support the development of new custom-tailored tools in biomedicine (optogenetics).