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 denaturation, the Anfinsen experiment, protein folding, and chaperone 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 4th and final level of protein structure, quaternary protein structure. So quaternary protein structure is just referring to a single protein complex consisting of multiple polypeptide chains. And so each of the polypeptide chains that are part of this larger protein complex is referred to as a subunit. And so a subunit is really just any polypeptide chain that assembles with other polypeptide chains. And so, when they assemble with other polypeptide chains, that automatically forms quaternary structure. And so subunits can either be identical or homo, or they could be different or hetero. And so, what you'll see is that the terms dimers, trimers, and tetramers consist of respectively 2, 3, and 4 subunits. And so in our example below, we'll 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 dimers. And the reason that we know that these are dimers is because we can see that there are 2 polypeptide chains or 2 subunits that are complexing or assembling together to create a single unit. So this is a single complex that has 2 polypeptide chains or 2 subunits, and this is a single complex that has 2 subunits as well. Now notice that these 2 subunits that are over here, that they are identical, and because they're identical, that means that they are going to be homodimers. These are homodimers. And these 2 subunits over here, because they are not identical, they're different from one another, that makes them heterodimers. And so, over here in our middle block, what you'll see is that we've got 3 subunits, And so with this single protein complex, we've got 3 subunits that makes it a trimer. So this is a trimer. And again, because all 3 of these subunits are not identical, we've got 3 different subunits, that technically makes it a heterotrimer. And so over here in our 4th and final block, what we have is, a single protein complex, but it has 4 subunits in it. And so because it has 4 subunits, that makes it a tetramer, Tetra meaning 4. And so with this tetramer, because not all 4 of the subunits are identical, that technically makes it a heterotetramer. So you only call it homo if it has all identical subunits. If it has at least one subunit that is different, it's automatically going to be a hetero structure. And so, this concludes our lesson on quaternary structure and these terminologies, and we'll be able to apply these concepts in our practice video. So, I'll see you guys in that practice video.
Quaternary Structure - Video Tutorials & Practice Problems
Quaternary Structure
Video transcript
Hemoglobin, a four-subunit protein, contains only two different types of subunits and is therefore a:
Quaternary Structure
Video transcript
Now that we've introduced quaternary structure, we could talk about quaternary structure interactions, or the interactions that take place between subunits and allow the subunits to stick together in a protein with quaternary structure. And it turns out that subunits mainly interact with each other via non-covalent interactions, such as, for instance, the hydrophobic effect. Now, although they mainly interact via non-covalent interactions, there are some exceptions and of course, disulfide bridges can covalently link subunits together. But recall that disulfide bridges form between 2 cysteine residues, specifically the R groups of the cysteine residues. So you could have 2 cysteine residues on separate subunits and then those subunits are linked together via the R groups of the cysteines in the disulfide bridge. But it's really important to keep in mind that subunits are not linked via their backbones. So backbones of subunits are not covalently linked and that's very important to keep in mind for subunits. Now because these subunits are so closely associated with one another, they're literally right up on each other, a conformational change in one subunit can actually alter the other subunits. And so, if one subunit makes a conformational change, that might induce the other subunit to also make a conformational change even though they are not linked via their backbones. And so, we'll see examples of that when we talk about hemoglobin in more 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 heterotetramer, which means that it's got 4 subunits that are not identical. And so down here, what we can say is that it's got 4 subunits. And notice that these 4 subunits are color-coded with 4 different colors in this image right here. And so these 4 subunits, they actually only complex with each other and interact with each other via non-covalent interactions. It turns out that there are 0 disulfide bonds holding these separate subunits together. And so, hemoglobin is a classic example of showing how most of the interactions between subunits are non-covalent. Now over here with insulin, what you'll see is that we've got 2 separate polypeptide chains or 2 separate subunits. We've got the alpha 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 alpha chain or the A chain up here has a total of 21 amino acids residues and the B chain has a total of 30 amino acid residues. And what you'll see is that these 2, subunits of insulin, notice that they have disulfide bridges. So they are highlighted here. So, you can see that there is a disulfide bridge here, but this one forms between 2 cysteine residues on the same A chain. And so this one's not linking the 2 subunits, but these other disulfide bridges that are formed here and here in light blue, these 2 are forming between cysteine residues on the 2 separate subunits, and so they are linking the 2 subunits covalently via the R groups. And again, the backbones are not covalently linked together. So they both still have their free amino and carboxyl groups of both subunits. And so over here, what we have is a different depiction of the same insulin molecule. So you can see this blue portion here corresponds with this blue alpha 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 disulfide bridges. So there are a total of 3 disulfide bridges. One of them forms between the same chain, but 2 of them form between the separate subunits. And so, for the insulin molecule, what we'll see is that it's got a total of 2 subunits and it's got a total of 3 disulfide bonds. 2 of them are linking the subunits together whereas one of them is an intra-chain disulfide bond forming within the same subunit. And so, 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.
Which of the following statements about protein structure is correct?
Match each level of protein structure to the appropriate real-world description.
_____ Primary Structure. _____ Secondary structure. _____ Tertiary structure. _____ Quaternary structure.