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Published byWendy Lilian Atkinson Modified over 9 years ago
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Secondary structure elements helices strands/sheets/barrels turns The type of 2° structure is determined by the amino acid sequence –Chemical & physical characteristics –How? Area of research
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Amino acid ‘flexibility’ Side chain interactions
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-turn Proteins up to 1/3 turns and loops Common linker for -sheets and - helices 180 o turn involving 4 residues –H-bond between C=O and N-H Which AA?
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-turn Proline –Imino N cis conformation (6%) Glycine –Very flexible Often found on the exterior of the folded protein: solvent exposed
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3 ° and 4 ° structure 3 ° structure –Overall 3-D arrangement –Interaction of 2° structural elements 4 ° structure –Arrangement of separate chains/subunits –Non-covalently linked Possible exception: disulfide bonds 2 classes of proteins –Fibrous proteins (extended) –Globular proteins (~spherical)
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Fibrous proteins Structural roles Typically single type of 2 ° structure –Long strands of helices (eg. -keratin/collagen) –Big sheets of structure (eg. silk) Insoluble in H 2 O – conc of H-phobic on interior and surface –Buried by packing chains together Strong and flexible –(eg. hair, silk, cartilage)
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Collagen Major constituent of connective tissues (bone, tendon, ligaments, skin…) Helical 2° structure distinct from helix –3 AA/turn (tighter –Left-handed (opposite twist) collagen “triple helix” tropocollagen –Helix: 2° structure –Triple helix: 4° structure
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Collagen Gly (35%), Ala (11%) and Pro (or HyPro) (21%) Every 3 rd residue is a Gly (Gly–X-Y-Gly–X-Y) –Genetic defects when G is changed (“mutated”) eg. osteogenesis imperfecta Chains linked by H-bonds –Backbone NH of Gly and backbone C=O of X in another chain Chains also linked by uncommon covalent bonds –Side chain linkage
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Collagen Triple helix aligns and crosslinks collagen fibrils –Crosslinked via covalent bonds between Lys, HyLys and His Too many crosslinks? –↓ flexibility –aging
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Silk Fibroin Webs of insects and spiders Antiparallel -sheets –Rich in Ala and Gly –Close packing of -sheets –H-bonding between all backbone N-H and C=O Extended but flexible
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Globular proteins Variety of structures/functions –Enzymes, transport proteins, motor, regulatory, immunoglobulin Folding is compact –H philic outside –H phobic inside Human serum albumin Alcohol dehydrogenase N-acetylglucosamine acyltransferase
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Globular proteins are very compact 3° structure
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How is the 3D structure determined? X-ray crystallography –Form ‘crystals’ of the protein Regularly repeating lattice X-ray beam is diffracted by the lattice Just like a microscope –Much shorter wavelength (higher energy) light –Computer acts as a ‘lens’ Size of protein is theoretically unlimited
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How is the 3D structure determined? X-ray crystallography –Get a ‘snapshot’ of the protein in a solid-ish phase –Need highly ordered crystals –Proteins come in close contact: may influence the structure
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How is the 3D structure determined? NMR –Nuclear spins of 1 H, 13 C, 15 N, etc. Detect via energetic response to a magnetic field Response depends on chemical environment –Distance between all pairs of atoms within the molecule –Software (with plenty of help from the user) determines structures that satisfy these distances
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How is the 3D structure determined? NMR –Only fairly small (<25kDa) proteins –Need highly concentrated sample Lots of protein Very soluble NMR and crystallography are complementary techniques
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