hi guys in this video we'll be looking at globular proteins hemoglobin insulin pepsin and then we'll finish with a summary so globular proteins are a specific category of proteins which have a variety of functions so the overall 3d structure of a protein is obviously determined by the primary structure and the 3d shape determines which type of protein we end up with and we can have two types they can either be fibrous or they can be globular so here's a brief sort of diagram describing some of the differences between them fibrous proteins tend to be of a specific type and they look longer and thinner globular proteins are more like this like hemoglobin and they look more like a globule so we need to distinguish a few important properties that globular proteins have roughly they are spherical in shape so they look like a blob or a ball and they have hydrophobic r groups on the inside and hydrophilic r groups on the outside so remember some r groups of amino acids can be either hydrophilic which means they love water and they're attracted to water or they can be hydrophobic where they repel water so this purple chain represents the polypeptide or the protein and usually proteins are surrounded by water because our cells are aqueous and we have lots of water in our body so imagine water surrounds this protein the hydrophobic residues like these orange ones here go to the center and they sort of cluster away from the water as if they're trying to hide so the hydrophobic residues kind of collapse into the center and then on the other hand the hydrophilic residues like these which are attracted to the water and happy to face them tend to face the outside where they can bind to the water so this gives them this spherical or blob like shape and because the hydrophilic residues face the water they're therefore soluble in the water anything soluble can make a bond with the waters and so all of these hydrophobic residues on the outside can interact with the water and therefore it becomes dissolved globular proteins also tend to have very specific shapes or 3d shapes and we know that all proteins have their own specific 3d shape for their function but for globular proteins they can be very very specific this allows them to carry out very specific functions for example enzymes are an example of globular proteins and they have to have the exact shape active site with very specific arrangements of amino acids to interact with the substrate and carry out particular reactions so all of these properties allow globular proteins to carry out many important roles in the body including transport so some proteins are called transport proteins for example hemoglobin some globular proteins are hormones flowing through the blood dissolved in the blood plasma for example insulin and some of them are enzymes again dissolved in the cells or the outer cells environment and one example is pepsin and we'll be going through all of these so a globular protein is a protein with a spherical shape which is soluble in water globular proteins typically have metabolic roles so all of these examples we'll be going through dissolve in water for example in the cell or in the blood plasma and they all have a role involved in this so an important example of a globular protein is hemoglobin and this is found in red blood cells so hemoglobin is the particular protein which we sometimes write as big h little b and this hemoglobin protein is packed into red blood cells which are the most abundant cells found in the blood the purpose of hemoglobin is very very important it has an important role in transporting oxygen from the lungs to the body tissues so at the lungs we breathe oxygen into the airways and we breathe co2 out when the oxygen comes into the airways it needs to be delivered into the blood and around the body the reason for this is because at the tissues they use oxygen to carry out respiration so it needs a way of being transported from a to b the hemoglobin does this and the structure of hemoglobin is that it's made up of four polypeptide chains meaning the protein has a quarternary structure so you can see we've got two purple polypeptides and two blue polypeptides these two are each called alpha globin so there's two of these and the blue polypeptides are called beta globin and again there are two of those they're obviously not this color in real life this is just to show you that there are four polypeptides interacting with each other here this is why it has a quarternary structure hemoglobin is also described as being a conjugated protein and by definition when we have this we have a prosthetic group attached to each polypeptide chain in this case this is what we have so here's one of those alpha globin chains and for each chain that there is there's a prosthetic group which you can see here added in and associating with that polypeptide and this interaction between a polypeptide and a prosthetic group means that the whole of hemoglobin is conjugated so a prosthetic group which has been added is a non-protein component which is necessary for the protein to carry out its function if it was a protein this would just be simply another polypeptide interacting so it'd just be another polypeptide on top of the four we already have the prosthetic group is normally not a protein and it has to interact with other methods so in hemoglobin each polypeptide gets one prosthetic group so there are four prosthetic groups in hemoglobin and each of them is called a heme group and they contain an iron ion which is fe2 plus so the symbol for iron is fe and it always has a two plus charge and you can see this is a heme group here so this is one of those prosthetic groups and each polypeptide and hemoglobin will have one of these there's the iron ion in the center and it's got this general structure which you don't need to worry about too much but it's basically non-protein and essentially how it works is that one oxygen molecule binds to each heme group so each hemoglobin molecule overall can transport four oxygen molecules so we've got four polypeptides each one of them has its own prosthetic group so we have four prosthetic groups and they're called heme and each heme binds to one oxygen molecule so this one will have one oxygen this one will take one this one and finally the fourth one so overall it can take four oxygen molecules with it and this maximizes the amount of oxygen that can be carried through the blood another example of a globular protein is insulin and it's a very important globular protein because it's a hormone and the hormone insulin is made and secreted by the pancreas so the pancreas is an organ as part of the digestive system and it's found near the small intestine it looks like a kind of sweet corn shape and the pancreas makes and secretes i.e sends out the hormone insulin which itself is the globular protein the role of insulin is to help maintain blood glucose concentration in the blood glucose travels around as it's soluble and it's delivered to all of our tissues which use glucose for respiration but the glucose level in the blood has to be very tightly controlled because if it's too high or too low it can be dangerous so insulin travels in the blood to help control this looking at the structure of insulin is composed of two polypeptide chains this time one of them has an alpha helix on its end and the other has a beta-pleated sheet so on this side we have an alpha helix and this is called the a chain and then on this side we have another chain where there's a b to pleated sheet and this is called the b chain so if you get mixed up between which chain is which remember that alpha goes with a b2 goes with b and you can see that they're linked together the two polypeptide chains are joined together by disulfide links or disulfide bridges and here they are here where there's r groups of amino acids containing sulfur atoms making disulfide links the shape of insulin makes it globular and it allows it to specifically bind to insulin receptors on the membranes of certain cells and these help lower blood glucose concentration so if blood glucose is going very high then we start releasing insulin from the pancreas and then insulin binds to receptor which is found in particular cell membranes and through various different interactions this causes the blood glucose to drop back down insulin as a hormone also has hydrophilic r groups on its outside making it soluble in water so this is important because it can then travel in the water and this is important because it allows the insulin to dissolve in the blood and it can transport around the body so the insulin doesn't have to stay in the same place once it's left the pancreas it can go all over the body to help make sure the blood glucose doesn't go too high a final example of a globular protein is called pepsin so globular proteins can also be enzymes such as pepsin which catalyzes digestion of proteins so there are many many enzymes in the body but pepsin is designed in the digestive system to break down polypeptides into shorter polypeptides so this could be part of protein that we've eaten for example from some meat and breaking it down into shorter polypeptides is the first step in getting those amino acids into our system so pepsin is the enzyme which does this so we find pepsin in the stomach and the stomach has a very acidic environment this is how the stomach breaks down food and it has a very very low ph at around two so there's lots of hydrogen ions in the stomach and the ph is very very low and the pepsin enzyme exists right there in that low ph environment most enzymes would actually be denatured at such a low ph because they contain some amino acids with basic r groups so a basic r group is acting as a base and bases accept hydrogen ions so this amino acid has a basic group it's an n joined to two hydrogens and this would be basic when the basic r groups accept hydrogen ions they become positively charged and they change their structure so in this case an h plus from the stomach would come in and bind onto the basic r group changing its structure and its charge because the r group is now positively charged it can affect all the ionic bonds and hydrogen bonds around the protein and therefore it will lose its shape this denatures the enzyme and therefore alters its function so if this is the optimum ph of a protein where it's all folded up and working if we had a lowered ph where there's lots of hydrogen ions so because most proteins have quite a high concentration of basic r groups they end up accepting all of these hydrogen ions changing their charge to become more positive and then the shape changes overall so the shape change causes the protein to denature and then the protein basically stops working so most proteins in this stomach environment would actually break down and lose their function the benefit with pepsin is that it has a primary structure with very few basic r groups so the tertiary structure is prevented from being affected by this low ph so these kind of basic r groups are very rare in pepsin and so it's not affected by that low ph and high h plus concentration as well as this the tertiary structure of pepsin is kept stable and in the same shape by the hydrogen bonds and the disulfide links between various amino acids so here we've got various disulfide links and we've got other interactions keeping it all in shape hey guys i hope you enjoyed the video if you're looking for an amazing a level biology resource join me today in my series of engaging bite size video tutorials just click the snap revise smiley face and together let's make a level biology a walk in the park