 Structural levels of proteins, the quaternary structure. First the contents, quaternary structures, bonding involved in quaternary structure, examples of proteins having quaternary structures, then the fibrous proteins and the globular proteins. So first the quaternary structures. Quaternary structure is the final structure of protein folding. When the proteins start to fold, the last and final structure of the folding pattern is the quaternary structure. First of all, when the protein is synthesized, that is the primary structure, the linear polypeptide chain. Then there is the folding, the local folding, that is the secondary structure. Then if there is one polypeptide chain, when it folds up, then it becomes a tertiary structure. So quaternary structure basically when there are more than one polypeptide chain, there can be two or more than two polypeptide chains, that they form a quaternary structure. So by definition we will say, quaternary structure is basically the structure of protein, which is formed by folding of two or more than two polypeptide chains. This means that the protein which has one polypeptide chain, that protein will never make quaternary structure. Whereas the one with more than one polypeptide chain will always be the quaternary structure. The final structure will be the quaternary structure. For example, for example, we have hemoglobin. As you can see in the diagram, there are four different polypeptide chains. So there are four different polypeptide chains in hemoglobin, which then form a compact molecule of hemoglobin. So the final structure of hemoglobin is the quaternary structure of hemoglobin. Bondings, what types of bonding are involved in the formation of quaternary structures? Zetter bonds are also involved in the tertiary structure, such as hydrogen bonds, ionic bonds. Van der Waal interactions are basically weak forces that develop in the molecules between the atoms. Sometimes some covalent bonds are also there in the quaternary structures. Examples of proteins with quaternary structures. In the beginning, I told you that the quaternary structure will be of the same protein that has more than two or more than two polypeptide chains. They will form a quaternary structure. So here we have hemoglobin. Hemoglobin has four polypeptide chains. These are pepsin. It is also having more than one polypeptide chain, creatin. Creatin is basically the creatin that makes our nails and hair. So these are all the examples of the quaternary structure of proteins. In the diagram, you are looking at the sum of the four structural levels. First of all, when the protein is formed, it is in a linear form. What is the name of the linear chain? Primary structure of proteins. Basically, it is a polypeptide chain. The second structure above its second step is a polypeptide chain that you fold. What we will say is that it is the secondary structure of proteins. Then, on the third level, the secondary structure which has a secondary structure, a secondary structure will be a further fold-up and it will become a tertiary structure. And then, when you have more than two or more polypeptide chains, they will form a quaternary structure. This is the primary structure. This is the secondary structure. This is the tertiary structure. And this is the quaternary structure. The quaternary structure is a final structure of proteins. The proteins of which have more than two polypeptide chains. The polypeptide chains of which have a tertiary structure are the final structure. Students, if we want to classify the protein based on the tertiary or quaternary structure, then the number of proteins is huge. There are thousands of proteins in any organism. It is very difficult to classify those thousands of proteins based on the tertiary or quaternary structure. Because each protein has its own shape. Each protein has its own folding pattern. It has its own arrangement. So, in this way, you cannot classify the proteins based on the tertiary or quaternary structure. Is it difficult or not? Then, the scientists have divided the proteins into four categories. The first category is the fibrous proteins and the second one is the globular proteins. Fibrous proteins are the ones that make the folding pattern in a fibrous arrangement. They are made of a thread-like structure. Fibrous proteins, because they are made of a thread-like structure, they are having a structural role. The structures of our body are mostly made of fibrous proteins. If we look at our hair, there are fibrous proteins in our hair. There are collagen in our bones. There is silk in our hair. There are silk fibroids in our hair. All these are fibrous proteins. Since fibrous proteins are made of structures, their important property is that they are water insoluble. Because if the structure is made of water insoluble, the structure will dissolve as soon as the water comes. So, you will see the most properties that have a structural pattern. Those things that are made of structures are always water insoluble. Globular proteins. Globular proteins are the category of their overall appearance. They are not thread-like. They are made of a rounded form. The functional role of proteins in the cell is the globular proteins. There are fibrous proteins that play a structural role. That is, they make a structure. When the function is performed, like the function of hemoglobin oxygen is being carried, enzymes are playing a role in different biochemical reactions. All these functional proteins are mostly globular proteins. Globular proteins are water insoluble. They can interact with water. Secondly, if we take its examples, all the enzymes are globular proteins. Hemoglobin, myoglobin or succubus-type proteins are globular in nature.