Transcription
In this slide, we're going to briefly talk about the nature of human hair and what that has to do with, uh, proteins and denaturation.
So, um, human hair is made of a protein called keratin. And keratin, this is a model of keratin, a primary structure. And you can see I made it with lots of yellow beads, more yellows than I would usually normally use. Those yellows represent cysteine amino acids, one of the amino acids called cysteine. Keratin has a lot of cysteines in it, more than a general protein might. And if you remember back, we learned that cysteines have a sulfhydryl R group, and that allows cysteines to make a disulfide bridge if they come up, they fold up next to each other. And, uh, there's a lot of disulfide bridges in the tertiary and quaternary structure of hair, and that explains how hair is so tough. You know, like your hair is pretty strong, and it's undigestible. Uh, cats cough up furballs because they can't digest their hair. Um, and we can also talk a little bit about styling hair and what that has to do with protein structure.
So, first of all, um, if you were to have straight hair and you wanted to curl it, one thing that you can do in the morning is use something like a curling iron. And that applies high, you, you wrap the hair around the curling iron, you put high heat, and then the, the hair is curly. Or the opposite for a straightener. If your hair is curly, you can use a straightener and you can strip, pull it straight with a flat iron, and then it will stay flat. Now, what you've done there is you've applied heat. So you have temporarily disrupted some of the R group interactions, like the, um, ionic bonds and the hydrogen bonds. But that much heat probably didn't break the covalent bonds between the disulfide bridges. So you was able to take your hair, which let's say is normally straight, and you were able to temporarily denature it, make it into a different shape, which was a curl, like this. But the disulfide bonds didn't break. You only broke the other interactions between the R groups. And so over the day, your curly hair will gradually fall back into shape, or the straight shape that it was before. That's an example of denaturation followed by renaturation. As the hair cools and over time, and the weight of gravity, it's going to pull itself back to its regular shape because you didn't completely break all those disulfide bridges. And there's so many of them in hair that it just sprung back to its old shape. So it was able to renature. And we talked before about how renaturation isn't very common, but in hair, this is an example. So, okay, that's a, that's a temporary situation where you're denaturing and then renaturing using heat, and then it cools off.
If you want your hair to be permanently change shape, like from straight to curly or from curly to straight, you have to have a perm, which means permanent. And a perm doesn't use heat, it uses chemicals to re-change, to change the molecular structure of your hair. So who knew that, who knew that perming was so, um, chemically complicated? So here's what happens. If you look down here, let me get my slide here, at, um, this image right here. This is a picture of, um, two chains of keratin. So here's one chain along here, here's another chain here, and here's the disulfide bridges that I was linking. This person has straight hair currently. And so you can see that their two, their hair is lying very straight, and the disulfide bridges are helping hold it straight. So that's an example here. The hair, the molecules are lying side by side, and every time you see two yellow ones, there's a bridge between them holding this together.
So if I want to make this straight hair curly, the first chemical that you apply for a perm is called ammonium thio glycolate. This is the one that smells really bad. And what this does is it actually, um, breaks the disulfide bridges. So you're disrupting covalent bonds. So for this, it would be the same as taking this piece of hair and like separating it out like that. So it's now, they're now separate, separated like this. So this is one of the reasons why perming your hair over and over or straightening your hair over and over is actually quite damaging to your hair because you're molecularly weakening the hair by breaking the covalent bonds. Alright, so you break it apart. Then you take the hair and you put it into whatever shape you want. So if it's straight, you want to wrap it around curlers. If it's curly, you want to pull it straight or brush it straight. And that's what we see in this next step here. The hair has been bent around a curler, and you can see the R groups from the amino acids are still sticking off. Sh, sh, I'm showing the R groups right here. All right, um, but because the hair has been bent around a roller, or straightened in, in the care of straining case of straightening hair, they're not lining up with who they, with the R groups they lined up with before. So what I mean by that is that if we look over here, we can see that this one is lining up with this one, this one's lining up with this one, this one's lining up with, they line up like that, right? But once it's been, once the hair has been, um, curved, this sulfhydryl used to line up with this one, but now it's lining up with that one. And this one used to line up with that one, but now it's lining up with that one. And then this one used to line up over there, but now it's lining up over here. So we're changing which cysteines are lying next to each other, so which ones are going to make disulfide bridges by stretching the hair around the curlers or straightening it.
Next, the last step is to then put this last chemical on, the hydrogen peroxide. And what that does is it washes away the first chemical and allows the disulfide bridges to form back. But so you can see here, they're forming back, but they're falling back with different partners. This one is forming to this one instead of where it used to be, which is there. And this one's forming to that one instead of there. And this one's forming to that one instead of there. And that has now introduced a permanent kink in the hair by realigning these disulfide bridges. And the opposite is true for straightening. You break the disulfide bridges, pull the hair straight, and then allow the disulfide bridges to form again, but with different pairs, and then you get the straightened hair. So it's kind of fascinating. Who knew that perms were so dependent on protein folding? Um, but it, this explains a little bit why perming your hair and straightening your hair a lot is damaging because you are molecularly weakening the hair by destroying the disulfide bridges. And it also explains why they tell you, like, once you've had a perm or straightening, they tell you to not wash your hair for a couple of days because they don't want, they want those, they want the disulfide bridges to form back, uh, in the shape they're supposed to be. And if you wet your hair and make it hang straight, then the curls might not form back correctly if you're having a perm for curls, um, or if you wash away the chemicals, it's not going to form back as it's supposed to. So a couple of reasons for why they tell you to do those things after a perm. And kind of some fascinating details of how 3D structure of proteins has everyday life implications.