[Music] hi and welcome back to precise lessons by the end of this video you should be able to describe the general structure of fibrous proteins you should then be able to describe the properties and functions of three fibrous proteins these are collagen keratin and elastin now we've already seen that many proteins are globular proteins remember that globular proteins tend to be roughly spherical in shape and globular proteins are water-soluble globular proteins have a huge range of functions including enzymes hormones and oxygen carrying proteins now another category of proteins are fibrous proteins and as you'll see fibrous proteins are very different to globular proteins unlike globular proteins fibrous proteins often play a structural role for example in bones or tendons or in the walls of blood vessels such as arteries and unlike globular proteins fibrous proteins tend to form long rope-like molecules and i'm showing you that here for the fibrous protein collagen fibrous proteins also have a large proportion of amino acids with hydrophobic r groups and this means that unlike globular proteins fibrous proteins are insoluble in water so in this video we're going to take a closer look at three fibrous proteins these are collagen keratin and elastin now we find collagen in tendons which connect muscles to bones and in ligaments which connect bones to each other we also find collagen in the skin the key feature of collagen is that it's a strong molecule and this is due to its structure the polypeptide chains in collagen rub tightly together to form a triple helix and i'm showing you that here now in the collagen polypeptides every third amino acid is glycine and the r group of glycine is a hydrogen atom this means that glycine has the smallest r group of any amino acid and this allows the collagen polypeptides to wrap very tightly around each other as the polypeptides up around each other a large number of hydrogen bonds form between the polypeptide chains and these hydrogen bonds help to stabilize the quaternary structure of the protein the polypeptide chains are also joined to each other by strong crosslinks now a large number of these triple helical molecules join together to form larger structures called microfibers and fibrils as you can see the molecules are staggered and this means that there are no weak spots now if the molecules were arranged like this instead then weak spots would run across the structure and this would reduce the strength of the collagen so by staggering the molecules like this these weak spots are avoided collagen is further strengthened by cross-linking between different triple helical molecules coming up we're going to look at the fibrous proteins keratin and elastin [Music] okay now keratin is a fibrous protein which is found in hair fingernails and the outer surface of skin keratin is extremely strong and is insoluble in water like a lot of fibrous proteins keratin consists of long stranded molecules however keratin contains a very high proportion of the amino acid cysteine and we saw cysteine in the video on bonding in proteins cysteine is used to form disulfide bonds and remember that disulfide bonds are strong covalent bonds so because it contains a high proportion of cysteine keratin molecules contain a large number of disulfide bonds and this contributes to the strength of keratin molecules okay a third example of a fibrous protein is called elastin the skin contains a large amount of elastin fibers and these help to make the skin supple and elastic we also find elastin in the walls of arteries elastin fibers stretch when blood pulses through the artery and then recoil in between pulses helping the artery to return to its normal shape elastin molecules are long strands containing hydrophobic regions the strands are also cross linked to each other normally the hydrophobic regions on different strands associate causing the elastin molecules to group together however when they're stretched the strands move apart but remain attached at the cross links after stretching the elastin molecules reassociate springing back together and this makes elastin a very elastic molecule okay so hopefully now you can describe the structure and functions of fibrous [Music] proteins