Quaternary Structure

So at this point, we've talked about primary protein structure all the way up through tertiary protein structure. And we've also talked about Dina Toray Shin, the infants and experiment protein folding and chaperon proteins. So you guys know a lot about protein structure, and in this video we're going to continue to talk about protein structure as we talk about our fourth and final level of protein structure. Quaternary protein structure. So Quaternary protein structure is just referring to a single protein complex consisting of multiple poly peptide chains. And so each of the poly peptide chains that are part of this larger protein complex is referred to as a sub unit. And so a sub unit is really just any poly peptide chain that assembles with other poly peptide chains. And so when they assemble with other poly peptide chains that automatically forms Quaternary structure, and so sub units can either be identical or homo or they could be header, they could be different or hetero. And so what you'll see is that the terms Dime er's trimmers and te tremors consist of respectively 23 and four subunits and so in our example below will be able to distinguish between these terms. And what you'll see is that in our first block over here on the left, what we have are die MERS. And the reason that we know that these are die MERS is because we can see that there are two poly peptide chains or to sub units that are complex ing or assembling together to create a single unit. So this is a single complex that has to polly peptide chains or to sub units and this is a single complex that has two subunits as well. Now notice that these two sub units that air over here, that they are identical and because they're identical, that means that they're going to be homo dimmers, these air homo dimmers and these two sub units over here because they are not identical, they're different from one another. That makes them hetero dimmers. And so over here in our middle block, which will see, is that we've got three sub units. And so with this single protein complex, we've got three sub units That makes it a try. Mur. So this is a try mur and again because all three of these sub units are not identical. We've got three different subunits that technically makes it a hetero, a hetero trimmer. And so over here in our fourth and final block, what we have is, ah, single protein complex. But it has four subunits in it. And so because it has four sub units, that makes it a te trimmer tetra, Meaning for And so with this tetra, more because not all four of the sub units are identical. That technically makes it a hetero te trimmer. So you only call it Homo if it has all identical subunits. If it has at least one sub unit that is different, it's automatically gonna be a hetero, uh, structure. And so this concludes our lesson on Quaternary structure and these terminology, and we'll be able to to apply these concepts in our practice video. So I'll see you guys in that practice video

So now that we've introduced Quaternary Structure, we could talk about quaternary structure interactions or the interactions that take place between sub units and allow the sub units to stick together and a protein with Quaternary structure. And it turns out that sub units mainly interact with each other via non covalin interactions such as, for instance, the hydrophobic effect. Now, although they mainly interact Vietnam Covalin interactions, there are some exceptions. And, of course, die sulfide bridges can co violently link sub units together but recall that die sulfide bridges formed between two Sistine residues, specifically the are groups of the Sistine Residues, so you could have to Sistine residues on separate sub units. And then those subunits are linked together via the are groups of the Sistine in the dye sulfide bridge. But it's really important to keep in mind that sub units are not linked via their backbones, so backbones of sub units are not co violently linked. And that's very important to keep in mind for sub units now, because these subunits are so closely associated with one another, they're literally right up on each other. Ah, confirmation. I'll change in one sub unit can actually alter the other sub units. And so if one sub unit makes a confirmation, I'll change. That might induce the other sub unit toe. Also make a confirmation will change, even though they are not linked via their backbones. And so we'll see examples of that when we talk about hemoglobin and mawr detail later down the line in our course. Now, in our example below, what you'll see is that we've got hemoglobin over here on the left, and we've got insulin on the right. And we already know that hemoglobin is a hetero te trimmer, which means that it's got four sub units that are not identical and so down here. What we can say is that it's got four sub units and notice that these four subunits are color coded with four different colors in this image right here. And so these four sub units, they actually Onley complex with each other in a, um, interact with each other via non covalin interactions. It turns out that there are zero dy sulfide bonds holding these separate sub units together, and so hemoglobin is a classic example of showing how most of the interactions between sub units are non co violent. Now over here with insulin. What you'll see is that we've got to separate poly peptide chains or two separate sub units. We've got the Alfa chain, which is this chain here in purple. And then we've got the beta chain or the B chain, which is this chain over here in pink. And so the Alfa chain or the A chain up here has a total of amino acid residues, and the B chain has a total of 30 amino acid residues. And what you'll see is that these two, uh, sub units of insulin notice that they have die sulfide bridges, so they are highlighted here. So you can see that there is a dye sulfide bridge here. But this one forms between, um to Sistine residues on the same a chain. And so this one's not linking the two sub units, but these other die sulfide bridges. They're formed here and here in light blue. These two are forming between 16 residues on the two separate sub units. And so they are linking the two subunits co violently via the are groups. And again, the backbones are not co violently linked together. so they both still have their free amino and car boxing groups or both sub units. And so over here, what we have is a different depiction of the same insulin molecules. So you can see this blue portion here, correspondence with this blue Alfa chain over here. And then this red chain over here corresponds with the red beta chain down here. And so you can see that these yellow bonds here are the dye sulfide bridges. So there are a total of three die sulfide Bridge is one of them forms between the same chain, but two of them formed between the separate sub units. And so for the insulin molecule, what we'll see is that it's got a total of two sub units and it's got a total of three die sulfide bonds. Two of them are linking the sub units together, where, as one of them is an intra chain die sulfide bond forming within the same sub unit. And so, uh, this concludes our lesson on Quaternary structure interactions and we'll be able to get some practice on all of these concepts in our practice video. So I'll see you guys there