The biological effects of light

Lights, camera ... reaction: Charting the biological effects of light in billionths of a second

March 20, 1998

Scientists using extremely high powered X-rays and a pulsed laser have succeeded in taking the first "snapshots" of a photoactive protein molecule as it converts light energy into chemical energy, a process that takes less than one billionth of a second.

The finding, reported in today's issue of Science, provides the first direct structural evidence of how light can be converted into chemical energy--the initial stage of processes as different, and as fundamental, as photosynthesis and vision. It also may suggest a powerful new mechanism for the development of optical computers.

The ability to record this ultra-rapid conversion results from a recent, million-fold improvement in time resolution of X-ray measurements that can now record changes in the shape of the working protein that occur in billionths of a second.

The experiments, only the second of their type, were performed by an international team at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. About one year ago, a related team reported the first nanosecond movie of the muscle protein myoglobin as it opened up to release carbon monoxide. Today's report emphasizes the wider applicability of this new type of measurement.

Keith Moffat, PhD, professor of biochemistry and molecular biology and director of the Consortium for Advanced Radiation Sources at the University of Chicago, who led both research teams, and colleagues from the University of Chicago, ESRF, Lund University and the E.C. Slater Institute of the University of Amsterdam, Netherlands, focused on a blue light photo-reactive protein called xanthopsin, found in the eubacterium Ectothiorhodospira halophila. These bacteria respond to the absorption of light by altering their swimming behavior.

This bacterium, so far found only in a few high arid lake beds in Oregon and salt depressions in the Egyptian desert, is "rather obscure," says Moffat, but this simple organism appears to be quickly moving towards center stage as a subject of scientific inquiry.

"The protein we studied is exquisitely sensitive to light," explains Moffat. "It is comparatively small, simple, water-soluble and extremely robust. If handled correctly it can tolerate intense repetitive stimulation from lasers, x-rays or light, all the qualifications one would choose for an optical storage mechanism."

All of the previously examined light harvesting complexes are very complicated, water-insoluble, membrane protein assemblies, notes Moffat, involving many different proteins, different chromophores, different and often quite complex ways of responding to light.

"This system provides a simpler way to study how light energy is transformed into signal energy. Here we have a single, polypeptide chain with a chemically simple chromophore, which undergoes a rather simple initial structural transformation--namely this trans to cis isomerization."

"In the dark, the system is cocked and ready for structural changes," explains Moffat. A single photon provides enough energy to pull the trigger.

Although this paper focuses on only the first nanosecond after light exposure, the research team has been gathering information on a series of subsequent changes in molecular structure of the protein after the first impulse.

This time-resolved crystallography is the type of experiment that will soon be performed at the Advanced Photon Source at Argonne National Laboratory in Illinois. Seeing biomolecules in their true dynamic state would be a boon to drug developers, explained Moffat, since "it would reveal more directly how the drugs look as they interact with the biomolecules."

Other authors are Benjamin Perman, Vukica Srajer, Zhong Ren, Tsu-yi Teng, and Claude Pradervand from the University of Chicago; Dominique Bourgeois, Friederich Shotte and Michael Wulff from ESRF; and Remco Kort and Klaas Hellingwerf from the E.C. Slater Institute in Amsterdam, Netherlands. The research was funded by the National Institutes of Health and was initiated by a grant from the Keck Foundation.

Summary:
Scientists have taken the first "snapshots" of a photoactive protein molecule converting light into chemical energy, the first direct structural evidence of the initial stage--less than 1 billionth of a second--of processes such as photosynthesis and vision. The finding suggests a new mechanism for optical computers.

For more background on molecular movies, see also "Watching biology in action in billionths of a second."

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