I'm dying to know "What's the deal with cryo?" Cryoelectron microscopy (CryoEM) has transformed the field of structural biology in recent years and even gained wider fame when Jacques Dubochet, Joachim Frank and Richard Henderson won the Nobel Prize for this technique in 2017. So, what's next? does he act? How does it compare to “traditional” X-ray crystallography? And is crystallography dead? (hint: NO!) Each of these techniques has advantages and disadvantages. So let's take a look at what's going on. Starting with “What is structural biology, anyway?” » (I didn't really hear this term until late in my undergrad and people just expected me to know it and never really explained it, so here's my attempt…)

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“Structural biology” is a subfield of biology that aims to determine what molecules “look like” and how their form relates to their function. We'll talk about that a little later, but for now, just think of a silver fork and spoon. They're made of the same material, so just understanding "hey, they're silver" won't tell you much about what they do (but feel free to pat yourself on the back – I'm all for celebrate small victories!). If you play with them blindly (and carefully) you might be able to figure out that you can eat soup with a spoon but not a fork, but it's easier to stab and eat pasta with your fork. And if you can see them, it's easy to see how their distinct shapes allow them to accomplish these tasks differently.

Structural biology strives to relate "what it does" to "what it looks like" to help determine "how it does it" and uses techniques such as x-ray crystallography and cryo-EM to obtaining appearances, combined with biochemical and biophysical techniques. techniques (activity measurement, binding, etc.) to study in more detail the functional importance of the different parts seen (although sometimes these functional elements are discovered first but make much more sense once you can see them ). It's a bit like if you cut a hole in the spoon part of the spoon, your soup would drip before it hits your lips and if you see the shape of the spoon you can predict where the hole might make you dribble. But if you cut a hole in the handle, you might not know it.

And speaking of things you may not know, let's talk about "cryo" – "cryo" (which my computer insists on un-correcting for crying, so sorry if I don't understand all the "fixes") is l Abbreviation for “cryogenic”. which means really really cold (which we need to keep the molecules safe(ish) and still(ish)). And when structural biologists say "cryo," they're usually referring to single-particle cryo-electron microscopy (cryoEM), as opposed to things like cryo-tomography, which looks at "slabs" like sections of cells or tissues. .

You may sometimes hear this called single *molecule* cryoEM, but the things you're looking at don't have to be individual molecules. A molecule is something where all the atoms (individual carbons, hydrogens, etc.) are joined by strong covalent bonds (like a protein chain) as opposed to "complexes" where multiple molecules can interact with each other by weaker interactions (like multiple protein subunits and pieces of RNA working together to form a ribosome). Thus, “particles” is a broader term that encompasses single proteins, multiprotein complexes, protein/DNA complexes, etc. – any sort of individual “particles” in solution (i.e. each particle surrounded by its own complete layer of water).

Disclaimer: Although I got my PhD just down the hall from a high-end cryo-electron microscope, I personally don't use cryo-EM, but I have done crystallography and will talk more about it at the end. .

So imagine you have a bunch of copies of one of these particles. And you want to know what they look like…

If you want to observe something small, your first thought might be: let's use a microscope. Microscopes magnify things by taking advantage of the wave properties of light (visible light is a form of electromagnetic radiation (EMR) – which can be thought of as packets of energy called photons traveling in waves). When a wave interacts with objects, its trajectory can be changed. So a visible microscope can shine light through something and cause that object to change the path of the light waves. With this modification, you now have a bunch of "out-of-phase" waves (out of phase with each other "peak-wise"). So the signal from your thing is a little confused.

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