So, from our last lesson video, we already know that antibodies are Y-shaped proteins. But in this video, we're going to talk more details about antibody structure. Antibodies actually consist of 4 polypeptide chains, 2 of which are identical light chains or L chains for short, and the other 2 are identical heavy chains or H chains for short. The light chains are actually much lighter than the heavy chains, which are much much heavier and larger. These 4 chains are covalently linked together via disulfide bonds. If we take a look down below at our image, notice we have our Y-shaped antibody. It has this light chain that is identical to this light chain over here and then it has this heavy chain, right here that is identical to this heavy chain. The heavier chains, highlighted in green, are much larger and therefore much heavier than the lighter chains, which are much smaller and lighter in mass. Also notice that these four chains are covalently linked together via these disulfide bonds that exist between the R groups of cysteine residues.
What's also important to note is that each of these light and heavy chains has a variable region also known as a V domain, as well as a constant region also known as a C domain. The variable region or the V domain is going to be located at the tip of each of the prongs of the Y and it contains the N-terminal end of each of the polypeptide chains. If we take a look down below at our image, notice that the V or variable domain is highlighted with a green background right here at the tips of the prongs of the Y. It also contains the N-terminal end or the free amino groups of each of the 4 polypeptide chains just like what we said above. The V domain contains the antigen binding site. This is where the antibody is going to bind to the antigen at these two potential positions indicated by the arrows. The reason it's called the V or variable domain is because this region will actually vary between different antibodies. The C domain, on the other hand, being the constant domain, is not going to vary. It's going to remain constant even between different antibodies. The C domain is, of course, going to be the rest of the antibody, located at the hinge and the stem of the Y. So if we look down below, the C domain is going to be the rest of the antibody here. The C domain is important because it's actually recognized by immune system cells. Essentially, what happens is the V domain will bind to the antigen at these positions and then an immune system cell can bind to the C domain. The antibody can act as an intermediate between the immune system cell and the antigen.
What you'll notice in our image here, we have the V domain, with the V's on them and whether or not they're light or heavy chains is indicated by the L and the H. Both the heavy chain and the light chain have a constant domain as well. What's important to note is that we can further break up the structure of this antibody. If we imagine breaking the antibody at the hinge of the Y. Here in a dotted red line, what we have is an imaginary line if we were to break our antibody right at the hinge. This would leave us with the top portion here, which we would refer to as the FAB region and this has the fragment that has the antigen binding sites. We have the antigen binding site in this region. We're also left with the bottom half of the antibody down below, which would be the FC region or the fragment that contains the constant region. This would be the FC region, this bracket right here. This is the structure of a typical antibody and we'll be able to get some practice applying the concepts that we've learned as we move forward in our course. So I'll see you guys in our next video.
