Chapter4_BCH461-part2

Chapter4_BCH461-part2 - The Four Levels of Protein...

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The Four Levels of Protein Structure Tertiary structure is the overall three-dimensional arrangement of all atoms in a protein; some proteins contain two or more separate polypeptide chains, or subunits, which maybe identical or different. The arrangement of these protein subunits in three-dimensional complexes constitutes quaternary structure. Secondary structure Primary structure Tertiary structure Quaternary structure
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Two major groups of tertiary/quanterary protein structures Fibrous Proteins: having polypeptide chains arranged in long strands or sheets Globular Proteins: having polypeptide chains folded into a spherical or globular shape
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Fibrous Proteins • Much or most of the polypeptide chain is organized approximately parallel to a single axis (filamentous or elongated form) • Fibrous proteins are often mechanically strong. • Fibrous proteins are usually insoluble • Usually play a structural role in nature - they hold things together
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Fibrous Proteins • Include the major proteins of skin and connective tissue and of animal fibers like hair and silk • The amino acid sequence of each of these proteins favors a particular kind of secondary structure • Include α - and β -keratins, and collagen
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Protein Structure Characteristics Examples of Occurrence Keratin -Helix, crosslinked by disulfide bonds Tough, insoluble protective structures of varying hardness and flexibility -keratin of hair, wool, feathers, and nail Keratin or Fibroin -Conformation Soft, flexible filaments -keratin or silk fibroin Collagen Collagen triple helix High tensile strength, without stretch Collagen of tendons, bone matrix Secondary Structures and Properties of Fibrous Proteins
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Alpha Keratin • Found in hair, fingernails, claws, horns and beaks • Individual molecules contain long sequences - over 300 residues in length - that are wholly α -helical
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Alpha Keratin Pairs of these helices twine about one another in a left-hand coiled-coil structure Primary structure of helical rods consists of 7- residue repeats: (a-b-c-d-e-f-g) n , where a and d are nonpolar . Promotes association of helices! (Dimerization) In different tissues, α -keratin is hardened to differing degrees by the introduction of disulfide cross links
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Alpha Keratin
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Permanent Waving Reduction and Reoxidaxion of Disulfide Bonds
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Fibroin and Beta Keratin Proteins that form extensive beta sheets • Found in silk fibers • Alternating sequence: (Gly-Ala/Ser) n • Since residues of a beta sheet extend alternately above and below the plane of the sheet, this places all Glys on one side and all Alas and Sers on other side!
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Fibroin and Beta Keratin • This allows Glys on one sheet to mesh with Glys on an adjacent sheet (same for Ala/Sers) • Structure stabilized by extensive hydrogen bonding between peptide linkages and by optimized van der Waals interactions between sheets
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Silk Fibroin
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Silk Fibroin
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• Principal component of connective tissue (tendons, cartilage, bones, teeth) • Collagen helix is
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This note was uploaded on 10/10/2009 for the course BCH 461 taught by Professor Lefler during the Spring '08 term at ASU.

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Chapter4_BCH461-part2 - The Four Levels of Protein...

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