Cryo-electron microscopy, also known as Cryo-EM, is a technique for visualizing macromolecular structures which has been developed over the last two decades. It is a form of transmission electron microscopy which permits samples to be studied at cryogenic temperatures. Biological structures such as ribosomes and mitochondria from pathogens, ion channels and super-coiled DNA have all been defined in atomic detail via Cryo-EM. Thoroughly visualizing and understanding biological structures is an essential first step for the development of new drugs and treatments. For this reason, the added advantages of Cryo-EM mean the technique is gaining popularity in structural biology.
Imaging biological structures via Cryo-EM
In Cryo-EM, the biological material in a sample is flash frozen by putting thin films of sample into ethane baths that are cooled to the temperature of liquid nitrogen (-180°C). The sample is retained in the frozen solution and bombarded with electrons. The electrons pass through a lens creating a magnified image on the detector, from which the structure of the sample can be analyzed.
There are two main types of Cryo-EM. The first is single particle analysis Cryo-EM where three-dimensional structures are constructed from two-dimensional projections. Two – dimensional images of the same kind of biological object within the frozen structure are captured first and are then organized into three-dimensional structures by image processing algorithms. The second type of Cryo-EM is Cryo-electron tomography. This method involves capturing multiple images of a single biological object by tilting the object at various angles for the electron beam to penetrate, creating a three-dimensional structure.
Image: An example of electron cryo-tomography. The image shows a central slice through a tomographic reconstruction of an intact Bdellovibrio bacteriovorus cell. Scale bar 200 nm.©Eikosi/ en.wikipedia.org. Shared under the license.
Advantages of Cryo-EM in structural biology
Previous structural biology techniques included X-ray crystallography and nuclear magnetic resonance spectroscopy. Both methods have had limited application because of the need for large sample sizes. X-ray crystallography also necessitates the crystallization of specimens, a difficult process that changes the environment to one that is non-physiological. Cryo-EM does not require large sample sizes or crystallization and is therefore suited to…