so what we're going to be looking at in this particular video is we're going to be covering the types of proteins uh four a levels for Cambridge a levels you have to know that there are two types of proteins this is just a big class by the way we are dividing proteins into uh two big types which are known as globular proteins and fibrous proteins now as an overview before we talk about globular proteins and fibrous proteins what I'm going to do is I'm going to draw out two polypeptide chains you can see a chain on the left you can see a chain on the right each of the circles are just amino acids and they are linked together by peptide bonds uh so the chain on the left I'm just going to call it polypeptide chain a and chain on the right is called polypeptide chain B now if you notice I'm putting some of the amino acids in wit and I'm also putting some of the amino acids in Blue uh the red amino acids signify that they are hydrophobic amino acids if you remember hydrophobic amino acids basically mean they cannot interact with water because they are non-polar and the blue ones hydrophilic amino acids can interact with water because these amino acids have polarity now compare the composition of polypeptide a and polypeptide B if you look at polypeptide a some of the amino acids innovate are hypophobic and some of the amino acids are hydrophilic we also notice that the number of hydrophilic amino acids outnumber the hydrophobic amino acids so they're more blue than red ones in polypeptide a but in polypeptide B most of the amino acids are hydrophobic so you might be thinking so okay does it matter what's the big deal why is this such Why do I have to care about this uh does it really matter if chain a has more hydrophilic amino acids and polypeptide chain B has less more hydrophobic amino acids um yes it does matter because think about it if we were to put both the chains in water what will happen we notice that polypeptide chain a starts to form a very weird folding of it it starts to curl up and form a kind of ball-like structure whereas foreign is not even going into the water it's most of the chain are just settling at the top of the water there is just one amino acid if you notice where it's kind of going inwards and the reason why it's going inwards is because it's the hydrophilic amino acid so if we were to zoom in on the polypeptide chain a we notice that the hydrophilic amino acids face outside and interact with water meanwhile the hydrophobic amino acids curl inwards because they want to avoid the water and because they want to avoid the water they interact with each other that is why those hydrophobic amino acids are near each other and this is good because this causes the chain to become a ball-like or spherical structure but for polypeptide chain B the chain generally does not interact with water and it forms a kind of thread like or fiber-like structure therefore the chain on the left that forms the ball-like shape is a globular protein and the chain on the right that does not interact with water and forms a thread-like shape is called the fibrous protein these are the fundamental differences that happens between a globular protein and a fibrous protein if we were to look at the globular protein in detail globular proteins have some very important characteristics that you must know first and foremost globular protein is a water-soluble protein the reason why it's a water-soluble protein is because hydrophilic amino acids the ones I'm highlighting are able to interact with water because they are able to interact with water they are said to be water-soluble even though even though that they do have some hydrophobic amino acids but fundamentally it uh it has mostly hypophilic amino acids that's why it's able to interact with water it's a spherical or ball shaped protein it's not circular you cannot put the word circular in the XM because the protein has a three-dimensional shape and circular it's not three-dimensional so you have to say ball shape or spherical and most of the time globular proteins are involved in chemical reactions now what do I mean by they're involved in chemical reactions some globular proteins can be enzymes which means to say they react with substrates there is a type of globular protein which is called hemoglobin which can bind to oxygen which by the way needs to bind to oxygen okay and there is also a type of globular protein we are going to see in chapter 11 okay of immunity known as antibodies and they bind to antigens so these are just some examples of globular proteins that you should know and we are going to cover all these proteins in the future so let me just basically show you three globular proteins right here I'm just blowing out these three globular proteins and let's say this globular proteins are all enzymes okay they're all enzymes and they're all on one side of the screen and you have substrates on the other side of the screen without going into the detail we just have to know that enzymes have to react with some substrates the problem is the enzyme is on one side of the screen on the left side and the substrates are on the right side of the screen there on the other side so will the enzymes be able to react with the substrate the answer is yes because the enzymes are water soluble they are able to move inside the watery environment and meet up with the substrates so because the enzymes are water soluble the globular proteins can easily move because they are water soluble the enzymes can easily move in a watery environment for example cytoplasm of the cell and carry out chemical reactions point of the matter here is just basically this globular proteins are proteins that are water soluble they can easily move in a watery environment to do whatever functions they are supposed to do we're going to look at other examples of globular proteins in the future chapter but for now this information is sufficient and of course if we will were to look at fibrous proteins fibrous proteins are not able to be involved in chemical reactions because they cannot move around in the water so does that mean they are useless No in fact because they're not involved in chemical reactions or they cannot move around in water that makes them useful in a different way now what do I mean by they're useful in a different way for example the water cannot affect them so if you were to throw water into fibrous proteins the fibrous proteins are not affected whatsoever the fibrous proteins will not dissolve in water they will not disperse in water in fact they will hold their structure and their unaffected whatsoever which means to say fibrous proteins can have structural functions now what do I mean by structural functions structural functions are basically examples of them will be keratin keratin is a type of protein that forms nail and hair think about it when you when you just basically put your hair in water does your hair just dissolve and wither away it doesn't because hair is a fibrous protein it is unaffected by water directly nails too just because you dip your hand your fingers in water does your nail just disperse and just dissolve it doesn't because the Keratin in your nails can hold themselves together and they do not interact with water other types of structural proteins also include collagen which are known which can be found in your skin and blood vessels and you also have elastin which can be found in your Airways trachea bronchus bronchioles and alveoli and also your blood vessels too we're going to look at some of these proteins in other chapters as well now so sometimes students will be like I still don't understand why this is important okay I'm just going to attempt to give you an analogy now let's imagine you had a block of salts okay a block of salt this is just an analogy by the way this is not a protein but imagine if you had a block of salt and you had a block of wood okay I know blue wood is supposed to be brown in color I have no idea why I'm putting it in purple but yeah let's go with that all right now the salt is water soluble but the wood is water insoluble so you can imagine that the block of salt is used to represent globular proteins and the block of wood is used to represent fibrous proteins okay it's just a representation it does not mean it's the real thing now imagine then if I were to build a tower made out of a block of salt I put a person on the block of salts and I build a tower made out of a block of wood and I put a person on the block of wood now and imagine then that these two blocks are standing on water because the block of salt can interact with water it will basically start to dissolve in water it will start to disperse and break apart all right and that's not good you do not want to be standing on salt a block of salt if you are protecting yourself from the water but the wood however is unaffected the block of wood is unaffected because they are water insoluble they cannot dissolve in water same thing with the fibrous proteins as well so the globular proteins can dissolve in water but the fibrous proteins are able to maintain their shape structure and integrity in water so as you can see the person who was basically standing on the block of salt is now drowning I hope the person doesn't die and the person is standing on the block of wood is fine although why that person is smiling looking at a person who's drowning that's another matter altogether anyway so if we were to look at six globular proteins on the left and we also have about six fibrous proteins on the right and imagine then if I were to put them both in water what will actually happen same thing we've talked about it earlier those globular proteins will start to disperse in water they'll just equally disperse in water because they can move wherever they want to in a watery environment but the fibrous proteins will never be able to interact with water so they stay together and interact with each other so globular proteins can easily move around in water to do metabolic reactions or chemical reactions but fibrous proteins just stay with each other and they stick to each other so they can perform structural functions that's basically what we have to understand from this so what we're going to look at right now is we're going to be looking at an example of a globular protein we are going to be we're going to be talking about something known as hemoglobin hemoglobin is just one example of the globular protein we will be talking about hemoglobin more in chapter 8 but we do need to be introduced to what hemoglobin is one important thing I want to explain to students is because a lot of students have this confusion they say that hemoglobin and red blood cells are the same thing it is not I want you to understand that hemoglobin is a protein found in red blood cells hemoglobin is a protein a red blood cell is a cell please do not assume that hemoglobin is a red blood cell no it's not okay one is a protein one is a cell two different things all together now that that's out of the way let's look at the structure of hemoglobin now the interesting thing about the structure of hemoglobin is to make hemoglobin you need to have two polypeptides called Alpha globin polypeptides and two polypeptides called beta globin polypeptides the two alpha globin polypeptides have the same sequence of amino acids which means to say they have the same primary structure and the two beta-globin polypeptides also have the same sequence of amino acids which means to say they have the same primary structure however an alpha globin polypeptide and a beta-globin polypeptide are made up of different sequences of amino acids okay so though they are made up of amino acids the sequences and types of amino acids are different therefore the alpha globin and beta globin have different primary structures so that being said let's move on to the next part we notice that the alpha globins will start to form a secondary structure you can see those Alpha Helix right there and the beta-globins will also start to form their own secondary structures now this secondary structure is just a representation this is not exactly what happens in reality but it helps you understand that you can see the alpha Helix forming in those chains after they form the secondary structure each of the chain will fold further to form a tertiary structure if you if you remember the previous videos the reason why a polypeptide chain forms a tertiary structure is due to the interaction of the r groups and they form the r groups can form hydrogen bonds ionic bonds disulfide Bridges and weak hydrophobic interactions now comes one of the more important parts each of those tertiary structures that you can see over there starts to interact with each other so the four polypeptide chains are now interacting with each other okay you can see it intersecting each other like that and if I were to zoom in the middle may have something known as hydrophobic bonds or hydrophobic interactions they may have disulfide Bridges they may have ionic bonds they may also have like the blue color ones over the as hydrogen bonds that and this is basically known as a quaternary structure okay but that's not done yet to make hemoglobin however each hemoglobin if you notice I'm putting some green color bright green color uh circles in that this bright green color circles are representing something known as a hem group so each polypeptide chain will have something called A Hymn Group which is a non-protein okay and this none this heme group is important because number one it contains iron and number two it binds to oxygen that's what allows hemoglobin to function so when we have the polypeptide chain and we add the heme groups to the chain that is how you make one hemoglobin molecule or one hemoglobin protein so what we have to understand for this is hemoglobin is actually a quaternary structure why do we call it a quaternary structure because to make one hemoglobin protein it is actually consisting of four polypeptide chains interacting with each other the unique thing about hemoglobin is for hemoglobin to function you need four polypeptide chains and you also need four hem groups without the Hem group The hemoglobin is useless so the heme group also needs to be there as well so just going back to the left over here I'm just going to cover this so we have the polypeptide chain on the left they are all just a sequence of um amino acids the other primary structure and they form the secondary structure and from the secondary structure they form tertiary structures they form the prime quaternary structure you add the heme groups and lo and behold hemoglobin is made hemoglobin is an example of a globular protein because it's water soluble and it also has a spherical shape and they do they are involved in chemical reactions because the chemical reaction is they have to bind to oxygen and bring the oxygen to our body cells and we're going to look at that in chapter eight so we are we have looked at one example of a globular protein now let's look at one example of fibrous proteins now for fibrous protein over here the most famous example of fibrous protein you have to say is collagen collagen is in an interesting one now the structure of collagen is made as below we have a polypeptide chain I'm just following that one in red another polypeptide chain in blue in the middle and another polypeptide chain in green which is at the bottom okay and each of these polypeptide chain have different primary structures the reason why they have different primary structure is because yeah the reason why it's different is because their sequence of amino acids are slightly different from each other you don't have to know the names of the chains by the way but what's interesting over here is the chains will start coiling within themselves to form something known as a collagen molecule or a collagen protein that is one collagen protein right there and that one collagen molecule is a quaternary protein why is it a quaternary protein because it is made up of three polypeptide chains never ever lose the definition of a loose side of the definition by the way quaternary proteins basically means proteins made up of two or more polypeptide chain so just like hemoglobin proteins collagen proteins are also quaternary as well and the shape of collagen is referred to as triple helix why is it triple helix because it's made up of three polypeptide chains and Helix because they are spiraling within each other or twisting now I'll close the inspection of each of the chain I'm just throwing out each of the chain here in detail I want you to see that each of the amino acids have different sizes because if you remember uh in our video on amino acids I did say that there are 20 different types of amino acids required to build proteins and each amino acid have different sizes and what makes the amino acid different is because of their R groups so I'm putting I will if you see the top right corner there will be a link to the video on amino acids but I'll also put it in the description if you want to look at the video on amino acids as well what I also want you to notice here is each of the chain have different types of amino acids and they have varying sizes now but a very interesting thing is there is a pattern the pattern here is as follows every third amino acid in the chain is glycine which is the smallest amino acid in out of the 20 amino acids glycine is referred to as the smallest amino acid you have to know this this is very important and glycine is found in every third position of each chain whether it's the chain at the top chain of the middle or chain at the bottom glycine is in position number three position number six position number nine and position number 12. I'm also drawing out the glycine molecule here just to show you you have the carboxylic acid group you have the amine group N in purple I'm drawing out the hydrogen which is the other group that's why glycine is the smallest amino acid because it's our group just consists of a hydrogen now you might be thinking so what if it's glycine at every third position or at every regular intervals uh the reason is because where there is glycine it allows the chain to become closer to each other and when the chain comes closer to each other that's what allows the polypeptide chain to coil within each other so because glycine is quite small the chains can come closer and where I'm highlighting is where the twisting happens that's how it allows the three polypeptide chain to tightly coil around each other to form something known as the triple helix okay and it does not easily unravel and lo and behold this is how you get a collagen molecule because a collagen molecule is made up of three polypeptide chains tightly coiling around each other and the tight coils are only possible because of the size of glycine because glycine is small it allows the chain to uh to be closer to each other and twist around each other that's the significance of glycine In This Moment so we know that one collagen molecule is just basically made up of a triple helix of tightly coiled together polypeptide chains that's good what happens if I just basically start taking many collagen molecules as you can see here I'm just basically taking many collagen molecules and I'm starting to draw this orange color lines between the collagen molecules those orange color highlighted areas are known as crosslinks of covalent bonds holding each collagen molecule in a staggered manner okay in a staggered manner now students will get very confused with this what does it mean by a staggered manner a staggered manner just basically means each collagen is attached to another collagen and they form a kind of stair-like fashion I'm just drawing it on the right hand side you can see how it just forms a kind of like it looks like stairs it looks like many stairs just bundled up with each other that's what it's called standard all right now again is this really important to know it is because allowing collagen one collagen molecule to be linked to another collagen molecule so these covalent bonds allow us to form something known as a collagen fibril so I'm just going to do I'm just going to show you a comparison between staggered crosslinks and also straight crosslings in this case as you can see the staggered crosslinks they are forming a kind of like a stair-like pattern they look like stairs but the straight cross links are just basically well it's straight okay so you have the collagen fibril which have straight clost links and you have collagen fibrils that are stack at crosslinks now imagine if I were to exit a force onto both fibrils if I were to put a force between both factors look at the collagen fibril on the right what happens is the highlighted areas there is a weak spot in the middle and that's not good to have because what happens if you have a weak spot is the collagen molecule of the collagen fibril will just basically the collagen fibril will just break apart because of those weak spots but because you have staggered because when it's staggered there are no weak spots the weak spots do not run along this spiral so there is no proper weak spot so when you exit a force in the left cellulose fibril it bends but it does not break that is why staggered crosslings are very important in a cellulose fiber that's the most important thing we have to know over here so in this situation over here we have one collagen molecule and if we have many collagen molecules forming a Crosslink in a staggered way that's when we get something called a collagen fibril I'm just going to color it in purple okay just to show it over here um that is one collagen fiber collagen fibril is made up of many collagen molecules linked together by covalent bonds and if I have many collagen fibrils joining together that's when we get something called collagen fiber and you may have thought hey wait a second this looks a bit like cellulose in carbohydrates and you are not going to be wrong you are not wrong by the way collagen fibers just like cellulose have a very high tensile strength okay the high tensile strength means thick they will not be easily broken down when you try to put Push Pull or stretch them this is why it's important to have collagen in places where it is subjected to a lot of pressure for example the wall of arteries your tendons which are connecting muscles to Bone and also your skin okay you can pull your skin in many directions and it just doesn't give way because of the collagen in that situation and the main difference is collagen is actually quite flexible all right they're quite flexible they are unlike cellulose cellulose are quite rigid but the difference is collagen which is a protein is quite flexible so in this diagram over here you can see I I know it's zoomed out a little bit so I hope you can actually see this you can actually see on the left hand side we have one collagen molecule which is just a protein and when we have many collagen molecules linked together in a staggered Way by forming crosslinks they form something called collagen fibril but when you have many collagen fibril grouped up together you get something known as a collagen fiber so with that we are actually done with the comparison between globular proteins and also fibrous proteins