X-ray snapshots of proteins in motion.

X-ray snapshots of proteins in motion

Proteins are intimately involved in just about every biological system. Elucidating exactly how they perform their functions during chemical reactions has been a major goal for biochemists. Now a British team of scientists has demonstrated the potential of using the reflections of X-rays from a protein crystal to track the changes in the protein's form during a reaction. Their technique makes it possible to record three-dimensional motion pictures of a protein's chemical behavior and overcomes the problem of long data-collection times normally associated with X-ray diffraction experiments.

"Despite its limitations,' says chemist Dagmar Ringe of the Massachusetts Institute of Technology, "the technique of millisecond protein crystallography promises to extend the range of biochemical problems that can be studied by X-ray diffraction.'

In a conventional X-ray diffraction experiment, an X-ray beam of a single wavelength shines on a crystal. Planes of atoms within the crystal reflect the X-ray beam and generate a pattern of spots on a nearby piece of photographic film. To ensure a complete sampling, many such exposures at different crystal angles are needed. The collection of sufficient data to deduce the crystal's atomic arrangement can take hours or even days. As a result, the diffraction pattern represents an average atomic arrangement rather than the arrangement at a particular instant.

To avoid lengthy collection times, Janos Hajdu of Oxford University and his colleagues went back to an X-ray technique called Laue diffraction, first used 75 years ago. Instead of X-rays of a single wavelength, they used X-rays of extremely high intensity showing a broad spectrum between 0.2 and 2.1 angstroms. The wide range of X-ray wavelengths allowed the researchers to collect data quickly.

In their experiment, reported in the Sept. 10 NATURE, Hajdu and his team photographed the patterns produced by a crystal of the enzyme phosphorylase as it was bathed with maltoheptase, a molecule that binds with the protein enzyme. They were able to gather the necessary data in 3 seconds, and from the results they could detect structural changes in the protein.

The researchers conclude that there is no reason why consecutive X-ray images could not be taken at intervals of a few milliseconds. Furthermore, although the present technique depends on knowing the protein's crystal structure beforehand, a similar method can be used when the specimen's atomic structure is unknown.