An artist's impression of an X-ray laser hitting a biological sample, photographing the result. The purple and orange inset shows what the photo would look like.
Credit: Terry Anderson/SLAC
SYDNEY: A new technique to accurately measure the structure of membrane proteins has been developed using powerful X-ray lasers – a discovery that will help fast track research into targeted drugs.
To unravel their structures, Harry Quiney and Keith Nugent of the University of Melbourne used X-ray Free-Electron Lasers (XFEL) to produce such a bright light, that it is possible to see the X-rays bouncing off a single molecule.
“The problem is that 70% of the drugs we take need to be transported across the surface of the cell, and that surface is made of membrane proteins. Only about 30 structures of 10,000 we currently know,” said Quiney. “If you really know the form of the molecules, you can model in great detail their function and how they respond to the drugs.”
Making the damage work for you
A major problem in this field of research is that radiation damage in molecules is an unavoidable, necessary aspect of X-ray diffraction experiments, and occurs within a femtosecond from when the X-rays first hit the sample.
A femtosecond is to 1 second, what 1 second is to 31.7 million years, or one millionth of one billionth of a second.
“The brighter the light, the greater the damage. You can’t have one without the other,” said Quiney. “At the same time as reconstructing what the molecule looks like, the molecule will be damaged. You have to find a point where this competition works for you.”
In order to solve this, Quiney and Nugent showed how high-resolution molecular structures can be obtained from X-ray scattering data using a few-femtosecond pulse from an XFEL, despite significant damage to the sample.
Producing a structural match
Using as an example the well-studied bacteriorhodopsin molecule, which transports protons into and out of bacterial cells, Quiney and Nugent simulated the form of data that would be obtained after a successful alignment of actual data from an XFEL interaction.
Comparing the known structure of this molecule obtained from the Protein Data Bank (a repository of three-dimensional structural data of large biological molecules submitted by biologists and biochemists from around the world) to both 2- and 3-D reconstructions from their simulated data, the researchers successfully produced a matching structure.
“What we’ve done is to achieve a conceptual breakthrough, which cleared the way and got rid of an important obstacle to achieving this goal of trying to view the structure of a single molecule,” said Quiney.
