4. Biomolecules
Proteins
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In this video we're going to discuss protein structure. And so proteins actually have a hierarchy of structure that's organized into 4 levels that are conveniently labeled primary, secondary, tertiary, and quaternary levels of protein structure. Now notice that each of the four levels of protein structure that we have in our text up above corresponds with the levels of protein structure that we have down below in our image. Now the very first level of protein structure is going to be the primary level of protein structure which can be symbolized with this symbol 1 with the not symbol here. And so the primary level of protein structure specifically refers to the types of amino acids, the quantity of amino acids, and the specific order or the specific sequence of amino acids in the protein chain. And so if we change either the types of amino acids, the quantity of amino acids, or the specific order or the sequence of amino acids in the protein chain, then we will change the primary level of protein structure. Now the primary level of protein structure is arguably the most important level of protein structure and that is because the primary level of protein structure will determine and dictate all of the other levels of protein structure. And so if we change the primary level of protein structure, then we could also potentially change either the secondary, tertiary and or quaternary levels of protein structure as well. And so if we take a look at our image down below on the left hand side, notice we're showing you an image of the primary level of protein structure. And so notice that in this image, each of these circles that you see here represent the amino acids in the protein chain and notice that these amino acids are all covalently linked together via peptide bond. Now, again, the primary level of protein structure as indicated in this image refers specifically to the types of amino acids in the chain, the quantity of amino acids in the chain and or the order or the sequence of amino acids in the chain. And so by changing either the types, quantity or order of amino acids, then we will also change the primary level of protein structure. And again the primary level of protein structure is extremely important because the primary level of protein structure will dictate all other levels of protein structure including the secondary tertiary and quaternary levels of structure. And so the next level of protein structure is going to be the secondary level of protein structure and so we can put a 2 here in this blank and the secondary level of protein structure is the formation of either alpha helices or beta sheets in the protein backbone. And so if we take a look at our image down below, what you'll notice is that this protein chain, can, then fold its backbone into either an alpha helix with as you can see here in this image where the alpha helix is somewhat of a winding staircase type of structure, or the protein backbone could form a beta sheet which is a more elongated extended zigzag type of structure. Up down up down. And so, the secondary protein structure again refers specifically to the formation of either alpha helices, this winding staircase structure, or beta sheets, this zigzag structure, in the protein backbone. Now the next level of protein structure is the tertiary level of protein structure, and so we can go ahead and put a 3 here in this blank and the tertiary level of protein structure is easy to remember because the 3 that's associated with the tertiary can remind you of the 3 in three d. And so the tertiary level of protein refers to the overall three d or three-dimensional shape of the polypeptide chain. And so if we take a look at our image down below what you'll notice is that once the protein backbone folds into alpha helices and beta sheets then the entire protein chain can then take on an overall three-dimensional shape as you see here. And so, this is the tertiary protein structure, when we consider the overall three-dimensional shape of the entire chain. And so within this overall three-dimensional shape, you can see that alpha helices can be embedded in there as well as beta pleated sheets. So these levels of protein structure are building on each other. Now, what's important to note is that all proteins are going to have primary, secondary and tertiary levels of protein structure, but not all proteins will have a quaternary level of protein structure. Only some proteins will have a quaternary level of protein structure. Now the quaternary level of protein structure again can be symbolized with a 4 here, with this symbol and the quaternary level of protein structure, is when a protein has multiple polypeptide chains that are going to associate with each other to form a single functional protein. And so, if we take a look at our image down below what you'll notice is that this one protein chain here with tertiary structure is 1 polypeptide chain. But if this 1 polypeptide chain associates with other polypeptide chains then it would have a quaternary structure. So for example, this one polypeptide chain, could be this one polypeptide chain as you see right here, and it could associate with another polypeptide chain, that's up here in the top right and it could associate with another one over here and a 4th one over here. And so what you'll notice is when you have these separate polypeptide chain each with their own tertiary structure that come together to form a single complex. So all 4 of these separate chains coming together to form a single complex where this would be a complex of 4 separate polypeptide chains, that would be a protein that has quaternary structure. And so once again, a protein that has quaternary protein structure is going to have multiple amino acid chains that complex together or multiple polypeptide chains that complex together And so this specific quaternary, this specific protein that has quaternary level of structure is supposed to represent a protein called hemoglobin which is a protein that is found in our red blood cells and helps to transport oxygen to our tissues. But really, this here concludes our brief lesson on protein structure and how it consists of primary, secondary, tertiary, and quaternary levels of structure and, we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.
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