The Medicine/Physiology and Physics prizes went to the popular topics of body clocks and gravity waves. The subject of the Chemistry prize is perhaps a little more obscure yet very important for those interested in the molecules of life. The three chemistry prize winners, Jacques Dubochet, Joachim Frank and Richard Henderson all work in the field of cryo-electron microscopy which enables large biological molecules to be observed in solution in water.
Looking at molecules
Even the largest protein molecules are far too small to be seen under microscopes that use ordinary visible light. One way to obtain images of these molecules is to use very short wavelength X-rays in the method known as X-ray crystallography. Many previous Nobel prizes have been awarded for developments in and the use of this technique, not least the 1963 award for the discovery of the structure of DNA by Crick, Watson, Wilkins and, of course, Rosalind Franklin who actually made the images. X-ray crystallography, as it name suggests, requires crystals of the material being studied. Crystals contain lots of the molecules in a fixed, regular arrangement. Not all biological molecules form crystals easily. Indeed, it was Franklin’s exceptional skill in making DNA crystals that allowed the images to be obtained at all. In life, that is, in cells, biological molecules float around in solution in water, constantly moving and rotating. X-ray crystallography cannot be used to take pictures in those circumstances.
Another way of looking at tiny objects is electron microscopy. A beam of electrons is bounced off or passed through the object, and collected by a sensor in the same way that a camera collects light reflected from an object. However, electrons have an effective wavelength much smaller than visible light and so can produce images of objects less than a micrometre in length.
Ernst Ruska, a German, was belatedly awarded the Nobel Prize for Physics in 1986 for developing the first electron microscope in 1933. Electron microscopes of various types have since found uses in many scientific disciplines. They could not be used for looking at molecules in solutions however. They require a total vacuum so the water would evaporate. Also, the energy of the electrons destroys fragile proteins and other biological molecules. Those disadvantages did not stop this year’s Nobel Prize winners.
Richard Henderson was born in Edinburgh, Scotland just after the end of the Second World War in 1945. He studied Physics at Edinburgh University but after obtaining his degree went to the Laboratory of Molecular Biology at Cambridge University to work on X-ray crystallography. Having completed his doctorate and a few years of research at Yale in the USA he returned to Cambridge in 1973. He was joined by Nigel Unwin to investigate the structure of paryticular protein molecules.
The molecule they chose was bacteriorhodopsin, a light sensitive molecule found packed together in membranes on the surface of some bacteria. This protein could not be obtained in a form suitable for X ray crystallography so they turned to electron microscopy as an alternative. They covered the protein sample with a glucose solution that would not dry out completely in the vacuum of the electron microscope. The normal beam strength would destroy their sample so they used a very weak beam. The molecules in the sample scattered the electrons in a pattern from which they could deduce a rough shape of the molecule. Henderson saw that his method had possibilities.
He changed the angle of the beam to obtain diffraction patterns in different directions and gradually succeeded in improving the resolution of his images. By 1990 he had improved his technique to build a picture that showed the arrangement of all the atoms in the molecule of the protein. The method worked for proteins that occurred in neat arrangements but did it have wider uses?
In New York, Joachim Frank was working on just this problem. Frank was born in Siegen in western Germany in 1940. He studied in Freiburg and Munich and did research in the USA and UK before settling in New York in 1975. In 1987 he spent a short time with Richard Henderson at Cambridge.
Back in 1975 Frank had come up with the idea of using a low intensity electron beam to produce an image of protein molecules in solution. The image showed the 2-dimensional shadows of the molecules floating in a thin film of water. Frank developed a computer programme to analyse the shadow cast by each molecule to build a 3-dimensional model of the molecule. In the late 1980s he had managed to produce an image of ribosomes, the protein building machines in cells.
A problem that remained was stopping the water in a protein solution form being evaporated by the vacuum of an electron microscope. Henderson’s method using glucose solution only worked for certain proteins but Jacques Dubochet provided the answer. Dubochet was born in 1942 in Switzerland. He obtained his PhD from the University of Geneva and worked at the European Molecular Biology Laboratory at Heidelberg in Germany. He was a professor at the University of Lausanne from 1987 to 2007.
Freezing the water would prevent it evaporating away in the electron microscope. However, ice crystals could block the electron beam or break up the protein molecules like they destroy soft fruit put in a freezer. Dubochet thought that if the sample could be cooled fast enough there wouldn’t be time for ice crystals to grow and the water would be turned into a glass-like, vitrified state. His team had success in 1982 when they cooled a drop of water to below -190oC in liquid ethane itself cooled by liquid nitrogen. Now Dubochet was able to develop cryo-electron microscopy. A thin film of the protein sample in water is spread over a fine metal mesh. The water is vitrified in liquid ethane and then exposed to the electron beam. In 1984 Dubochet obtained his first sharp images of viruses which are simply packets made of protein.
Cryo-e.m. – a new tool
A combination of Dubochet’s vitrification method and Frank’s computer programme allows cryo-electron microscopy to be used to obtain images of proteins showing the position of each individual atom in the molecule.
Now scientists can use cryo-electron microscopy to explore large proteins in action in and on cells. For example: the way salmonella bacteria attack cells; the molecules responsible for regulating our body clocks; and the pressure sensitive molecules in our ears that allow us to hear sounds.
The epidemic of the Zika virus in South America, particularly in Brazil during the 2016 Olympic Games, provided another case for cryo-e.m. The method was used to determine the structure of the proteins on the surface of the virus and hence suggest a vaccine for the disease. The work of Henderson, Frank and Dubochet has provided another valuable tool to look in to the workings of the molecules of life.
- Why is cryo-e.m. more useful than X ray crystallography for determining the structure of many proteins?
- Why are normal electron microscopes not suitable for imaging protein molecules?
- Why is discovering the structure of proteins important?
- What does the “cryo” part of the name cryo-electron microscopy mean?
- What are the countries of birth and the countries where they work of the three winners of the 2017 Nobel Prize for Chemistry?
- It is more than 25 years since cryo-e.m. was developed by the three Nobel winners. Why do you think it has taken that long for them to receive the award?
- This year’s award winners are white males in their 70s. What are your opinions on this fact?
- Follow the news in the media and online about the Nobel Prize and find out more about the winners.