Transcription
All right, our last level of protein folding is quaternary, or the fourth level. Um, not every functional protein is a quaternary structured protein. Everything goes at least at level three, um, but many do have a quaternary structure. And so I'm gonna, uh, a quaternary structure is very simple to demonstrate again using my, um, handy dandy protein model. So here's my tertiary folded protein. This is quaternary structure. You ready? That's a quaternary protein. I'm gonna make it a bit more complicated. There you go.
Okay, so what a quaternary protein is, is a, is a molecule that's built up of separate individual subunits. So each of these are their own separate chains. They all fold up separately using primary, secondary, tertiary rules, and then they stick together to make like a complex with like different subunits. What holds together the subunits are the same interactions that we had at the tertiary level, except for the disulfide bridge. So that's what the diagram here is is showing you.
On this diagram here, you can see the different kinds of interactions. So on the left here is like the protein on the left, and on the right over here is the protein on the on the right. And then when they come together like this, you can see how the beads from the two sides are starting to touch each other. What really touches is not the beads, but the R groups that are sticking out from the beads. And that's what we're seeing here. Here's an R group sticking out, here's an R group sticking out, here's an R group sticking out. So this is an ionic bond right here because you've got a positive and a negative. So this is an ionic interaction. Here is our friend, the hydrogen bond. All right. And then here's, um, this is supposed to be showing a hydrophobic interaction because these are both carbon rings. So there's no charges. So this is a hydrogen bond, hydrogen bond, and this is a hydrophobic interaction.
Now, the only thing you don't have here is a disulfide bridge, which is the fourth one from the third level. Uh, you don't have disulfide bridges because if you had a disulfide bridge between the one on the left and the one on the right, there would be a permanent link between them two. Remember, the disulfide bridge is an actual covalent bond, and you want this quaternary structure to be able to fit together and then come apart. Sometimes it can be like a temporary arrangement.
So separately, these two protein molecules aren't functional in your cells. So your cell can just be going along to do with these two proteins, but as soon as it needs the function of the tool to happen, it can put them together and it can be ready immediately to go do its thing. And then when your cell doesn't need that to be done anymore, it can take these apart and then it won't be wasting time doing jobs it doesn't need to do. So this is a way for your cell to like temporarily put a tool together when it needs it and then take it apart when it doesn't need it anymore. Kind of like what you do with a like a wrench and socket set in your garage. You have the handle, you have the socket. They don't work separately. When you put them together, they work, and then you can store them apart again, right? That's sort of how this, um, quaternary structure works.
And it doesn't just have to be two parts stuck together. Proteins can have many sub-proteins can get ginormous, really complicated. And so like you could have one, two, multiple subunits all piled together to make big complicated molecules. Um, for example, hemoglobin in our blood cells, that's made of four separate chains that all kind of stick together to make the the functional hemoglobin protein.
Um, how do you tell quaternary structure? So you tell quaternary structure pretty easily in diagrams, basically by color coding. Like it's, it's really hard. I mean, I guess you could go in and count the ends, and if you see like just two ends, you've got one strand, and if you see more than ends, you've got multiple chains. But generally, to make it easy to spot, they color code. So like here's a dark blue and a light blue chain, and they're stuck together to create this quaternary protein.
Here's a couple other examples. So on the left, this is not quaternary. This one is tertiary because it's all one color, but you can see some really nice alpha helix and some beta sheets here, see? Um, and then you can see the tertiary folds because the whole thing's kind of jumbled together. This is an enzyme that works in your nervous system, which is a protein. And then here is a, um, [Music] another protein from a from SARS virus, which is very closely related to the COVID virus. And this is a quaternary protein because you see it's got two different chains, a green, they've color-coded green and red in here, and you can see how the chains are kind of stuck together down here, some with some interactions going on here. And you can also see alpha helix, you can see beta sheets, you know. So this is another ribbon diagram, uh, showing, uh, quaternary. So this is showing quaternary structure. This is showing tertiary structure. Both ribbon diagrams.