 another fun amino acid I want to show you is this one it's cysteine so it has a sulfur here and a hydrogen there in isolation that is nothing special at all but what happens if I take two of them yellow is a good color for this so if I have a chain here and then I have a cysteine here that sulfur would normally have a hydrogen but I'm deliberately not drawing that one now I'm having a second chain here that also has an ac what can happen is that this can for they can form a bond and this means that the entire chain on the left here is now bound with a covalent bond on the chain to the right whether this happens or not depends if I'm oxidizing or reducing this whole reaction but if I under normal conditions if I have two cysteines very close to each other this type of bond typically will form if they're within a few well eight angstroms or so the solvers do you see that this is creating a very strong global constraints there's completely different chains of the protein and once I brought them together they now form this bond this turns out to be what happened to cysteine alfinsen when those ribonuclease form incorrect structures so in the correct structures we have would have these so-called disulfide bridges disulfide bridge formed in the right place but when he denaturated them they like to form pairs so the structure that was in just a big blobby chain they formed disulfide bridges in the wrong place and then we need some help to break those and that is broken by a small protein called protein disulfide isomerase so that would go in break these bonds and then give the protein a second chance to form them again do you see how I draw this with many chains I told you in one of the initial lectures might have been lecture one that it's rare to use NMR spectroscopy to determine structures but for these small structures NMR can actually work quite well so this is an NMR structure and it's cool no pun intended because it's a structure that we don't have a 100 Kelvin but in room temperature insolvent and that's typical that we see a bit of the motion when we have this entire ensemble of structures so these disulfide bridges can really help stabilize structures that are otherwise a bit chaotic which are going to be useful in some cases let me show you so this might initially look like a piece of an unfolded structure it's just a random coil but trust me it's not let me show you that in slightly more detail what if I show you all the side chains too no it doesn't make more sense it's just still just a random blob of things but what if I show you the cysteine residues do you see them in yellow here here and there was something in there so there are at least three places where this molecule has formed these disulfide bridges so at first sight it might look like a piece of yarn randomly thrown on the floor but because of these three disulfide bridges this is not being a very rigid structure although it doesn't really have any normal secondary structure so this is a stable protein the structure which we've been able to determine this particular one is called Hanna toxin and there's actually a fun story about that the Kenton Schwartz working with voltage gated channels his daughter was called Hanna so he named this particular toxin when they discovered it after her I have no idea what his wife said but I remember Kenton saying at one conference that you know when when she was a child and everything was a bit awkward but when you're a 20 year old it's pretty cool to have a toxin named after you Hanna toxin is special that if you are say a spider or some other organisms and you want to be able to inject a protein in your prey it is actually very convenient to have something that is super stable and it's difficult for the prey to break down or defend against so it makes a lot of sense that for toxins you want to add things that are small and very very hard to unfold