Question 6 Keratin, Collagen ✔️ PDF

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KERATIN • • • • • • • • • • • • !

form long filaments with rod or wire like shapes! function as inert structural or storage proteins ! mostly extracellular ! generally hydrophobic ! composed of a limited subset of amino acids in repeats ! form unusual secondary/tertiary/quaternary “supersecondary” structures! e.g. collagen triple helix! often contain cross-links between chains ! e.g. disulphide bonds between keratin chains ! large structures: hair, tendons ! mammals have a - keratin ! each polypeptide chain forms a right handed a-helix !

structure of a-keratin • • • • • • ! ! • • • ! • • •

primary (1’) structure (sequence)! rich in hydrophobic amino acids ! arranged into imperfect heptachlor (7 amino acid) repeats ! every 3rd or 4th residue along the helix is hydrophobic ! facilitates bonding between 2 or more helixes ! higher than normal number of cys residues for disulphides bonding at higher levels of structure !

secondary (2’) structure ! central region of each polypeptide chain forms a right handed a-helix ! termini are globular ! two helical strands wrap together into a superhelical coiled coil! there is a parallel arrangement of the two strands (n-termini at same end)! supercoil is twisted in a left hand manner (a result of arrangement of hydrophobic amino acids in each primary sequence !

• where the two helixes touch, there are hydrophobic amino acid residues ! • side chain r groups interlock! ! the coiled coil! ! • two right handed a-helixes wound together in a left handed superhelix and connected by association and interlocking of hydrophobic side chains ! • these units combine into larger structures ! interlocking hydrophobic side chains keep dimers together in the superhelix Monomer Each polypeptide chain has an a-helical central rod domain ! Dimer! Central rod domains of two parallel polypeptide chains twist around each other ! Tetramer ! Two dimers associate a staggered anti parallel fashion ! ! ! Cross linking in filaments Disulfide bonds Keratin contains many cysteine residues ! These form disulfide bonds or bridges between keratin monomers that! • aid the formation of protofilmaents ! • Increase strength of protofilaments and intermediate filaments ! • Increase insoluability ! ! The number of inter-chain disulfide bridges determines the nature of the keratin based substance:! E.g. hair has fewer disulfide bridges and so are more flexible and elastic ! Human hair is 14% cystine and rhino horn is 18% cystine ! Keratin in fingernails hooves and claws have more bridges and are harder ! ! ! !

COLLAGEN Component of connective tissue ! Found in:! ! • dense connective tissues! • Bone, tendons, teeth, cartialage, ligaments ! ! • Loose connective tissue !

• Beneath epithelial cell layers in skin, intestines, blood vessels ! ! 1. Dense connective tissue ! Tendons and ligaments ! • connect organs to hold them together! • Join muscles to bones (tendons)! Properties ! Long fibres, various sizes ! Strong, in elastic ! Achilles’ tendon in ankles ! Must not stretch after repeated use ! ! Bones teeth and cartilage ! Provide structure and support for body ! Protection for brain ! Attachment point for muscles (bones)! ! Properties! Hard, dense, sometimes hollow or honeycombed structures ! Provides matrix for minerals ! ! Loose connective tissue ! Epithelial cell layers in skin, intestines and blood vessels ! Important component of complex extended structures but is not only component ! ! Function! Allow and response to movement including bending twisting and stretching ! Properties ! Flexible elastic deform under pressure and return to original position! ! Provide strength fibres themselves fo not stretch ! Other proteins provide stretch ! ! Properties related to function! Dense connective tissue, parallel fibres of collagen are arranged into bundle or sheets connected to bone or cartilage ! • inelastic due to additive internal strength of many molecules ! • Strength from covalent bonds connecting muscle and overlapping arrangement of fibrils ! • Gaps for mineral crystals ! Loose connective tissue ! Fibrils of collagen are arranged into networks perpendicular to the stress ! Net formation can reshape without fibres it is made of being stretched and it can readily return to original shape !

! ! ! Dense ! Stretches little! Combined strength of intramolecular and intermolecular interactions ! Overlapping structure ! ! Loose ! Stretch but doesn’t break! Return original structure! ! ! Collagen has UNIQUE structure ! ! • little variety in amino acids ! • Every third residue is glycine ! • Contains modified amino acids (hydroxyproline and hydroxylysine)! • About 1 in 3 amino acids is proline or hydroxyproline ! • Mostly repeating gly-x-pro or gly-x-hyp units ! • Pro and hyp are secondary amines that form ring structures along the backbone of the polypeptide ! • Gly and pro and hyp are incompatible with a-helix secondary structure so cant form ! • Polypeptide forms an unusual helical structure the polyproline II helix! ! Polyproline II helix ! Proline rings, many fewer of backbone bonds can rotate so fewer conformations are possible ! Hydrogen bonds cannot be formed within or along the helix ! Helix is left handed more elongated and tightly wound it repeated every three amino acids ! Most side chains are compact as amino acids with long or bulky chains are not common in the primary structure ! ! ! Collagen superhelix structure ! Tropocollagen ! ! 3 polypeptide strands are wound together in a right handed superhelical coiled coil structure ! ! Features ! Slightly staggered arrangement to allow hydrogen bonding between the helixes not

within left handed helix ! Glycine h side chains point to centre of the superhelix ! Proline and hydroxyproline are essential for the very tight helical structure ! Superhelix does not stretch because of the left hand/right hand arrangement ! ! Extra hydrogen bonds can form with the involvement of water, the OH group group of Hyp and an electron acceptor ! ! What holds the superhelix structure together?! Hydrogen bonds form between different strands within the superhelix ! Side chains are small and do not extend far from helix ! ! ! Summary ! Structure and function ! Common structural elements in collagen and a keratin make large strong inelastic insolvable extracellular structures! ! Helical polypeptides from coiled coil superhelical in which the superhelix winds in the opposite direction to the single helix ! ! Interaction between the helixes of the coiled coils to stabilise the structure ! ! To form larger structures such as fibrils or filaments the superhelix units pack together in parallel or anti parallel arrangements but units are staggered so that there aren’t any weak point in the structure ! ! Covalent cross links form between superhelices the more crosslinking the stronger the structure ! ! Multiple fibres packed and linked together form strong inelastic structures ! ! ! ! ! ! ! ! ! ! !...