Lights, camera, . . . reaction

By John Easton
Medical Center Public Affairs

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 the March 20 issue of the journal 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 is the result of a recent, million-fold improvement in time resolution of X-ray measurements.

The experiments, only the second of their type, were led by Keith Moffat, Professor in Biochemistry & Molecular Biology and Director of the Consortium for Advanced Radiation Sources, and 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.

Moffat and colleagues at Chicago, ESRF, Lund University and the E.C. Slater Institute of the University of Amsterdam 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," said Moffat, but this simple organism appears to be quickly moving toward center stage as a subject of scientific inquiry.

"The protein we studied is exquisitely sensitive to light," Moffat explained. "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, noted Moffat, involving many different proteins and chromophores, and 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.

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

Although this paper focuses on only the first nano-second 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. Seeing biomolecules in their true dynamic state would be a boon to drug developers, Moffat said, since "it would reveal more directly how the drugs look as they interact with the biomolecules."

In addition to Moffat, the authors of the study are Chicago researchers Benjamin Perman, Vukica Srajer, Zhong Ren, Tsu-yi Teng, and Claude Pradervand; Dominique Bourgeois, Friederich Shotte and Michael Wulff from ESRF; and Remco Kort and Klaas Hellingwerf from the E.C. Slater Institute.