Frozen Images: how cryo-electron microscopy won the 2017 Nobel Prize for chemistry
At the chemical level, all life is built upon the interplay of complex molecules; a tiny, intricate universe whose inhabitants remain mostly invisible to us. However, a technique called cryo-electron microscopy has enabled scientists to extract images of biological molecules at an unprecedented level of detail, for which Jacques Dubochet, Joachim Frank and Richard Henderson were recently awarded the 2017 Nobel Prize for chemistry.
Previously, the standard method used to decipher the shapes of large molecules had been the same one in use for many decades: x-ray crystallography, in which a crystallised substance is bombarded with x-rays, and the resulting diffraction pattern used to deduce the underlying structure. This was, for example, the same technique used by Rosalind Franklin to generate the images which led to the discovery of the structure of DNA in the early 1950s. However, while a familiar and reliable tool, x-ray crystallography has significant limitations – mainly its requirement for large quantities of pure material in a crystallized state (where individual molecules pack together in a regular, repeating pattern), something which is in practice very difficult to accomplish for many biologically important proteins, and impossible for some.
Left: Composite image of a low-resolution cryo-electron microscopy (cryo-EM) density map, 2.2 Å resolution map, and fitted atomic coordinates for the enzyme beta-galactosidase, demonstrating the gradual increase in quality of the cryo-EM structures from low to high resolution. Credit: V. Falconieri, S. Subramaniam
In contrast, cryo-electron microscopy (cryo-EM) can produce high-resolution structures of molecules without requiring crystallization. Instead, the substance in question is frozen at extremely low temperatures (hence the “cryo” part of the name), and a beam of subatomic particles – electrons – is fired through it. These electrons are scattered by the material which they pass through, allowing a detector on the other side to record images from which, ultimately, three-dimensional structures can be reconstructed.
The special freezing process, pioneered by Jacques Dubochet, was a crucial advance; samples analysed by electron scattering must be cooled as much as possible to slow the movement of the constituent atoms while they are barraged with electrons, but this leads to a problem: because most biological chemistry takes place in water, the majority of proteins must be in aqueous solution to maintain their natural shape. Typically, cooling down a water-based solution creates ice crystals, which both damage the protein and diffract electrons themselves, thus interfering with the signal one is trying to detect. Using liquid ethane, Dubochet perfected a way to flash-freeze protein solutions so quickly that the water molecules do not have time to line up to form ice crystals and instead form a glassy, or vitreous, state in which the protein structure is preserved like a tiny ice sculpture. Meanwhile, Joachim Frank was advancing the computational side, developing image-processing software to take the complex, fuzzy electron scattering data into three-dimensional molecular structures. Richard Henderson, making use of these and other advances, published the first atomic-resolution structure of a protein (bacteriorhodopsin) by cryo-EM in 1990.
Since then, the field has exploded, with more than 100 different molecular structures determined by cryo-EM in 2015 alone. Significant targets have included cancer-associated molecules such as p97, the Zika virus, and γ-secretase, a molecule implicated in Alzheimer’s disease; detailed knowledge of the structures of such molecules is can inform the design of drugs to modify their activities. Even famous names such as Roderick MacKinnon and Venki Ramakrishnan (2003 and 2009 Chemistry Nobel winners respectively, both for work done using x-ray crystallography) have now adopted the cryo-EM method.
This year’s prize is sure to bring a new level of attention to the technique and to the contributions made by the winning trio of Dubochet, Frank and Henderson – frozen images which bring the invisible world of biological chemistry closer to our view.