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Monday, May 30, 2011

Globular and Fibrous Proteins


Fibrous Proteins
  • Little or no tertiary structure.
  • Long parallel polypeptide chains.
  • Cross linkages at intervals forming long fibres or sheets.
  • Usually insoluble.
  • Many have structural roles.
  • E.g. keratin in hair and the outer layer of skin, collagen (a connective tissue).
Globular Proteins
  • Have complex tertiary and sometimes quaternary structures.
  • Folded into spherical (globular) shapes.
  • Usually soluble as hydrophobic side chains in centre of structure.
  • Roles in metabolic reactions.
  • E.g. enzymes, haemoglobin in blood.
Haemoglobin
An example of a globular protein
  • Reddish-purple oxygen carrying pigment found in red blood cells.
  • Made up of 4 polypeptide chains.
  • 2 identical alpha-chains and 2 identical beta-chains.
  • Nearly spherical - hydrophobic side chains point inwards.
  • Outward pointing hydrophilic side chains maintain solubility.
  • Each polypeptide chain contains a haem group.
  • Haem group is a prosthetic group (i.e. an important permanent part of a protein molecule which is not made from amino acids) - when combined with 4 polypeptide chains it forms a conjugated protein.
  • Haem group has an iron ion (Fe2+) at its centre.
  • The iron combines with oxygen at high oxygen concentrations and releases oxygen at low oxygen concentrations.
  • One haemoglobin molecule can carry 4 oxygen molecules.
  • Colour is bright red when combined with oxygen / purplish if not.
Collagen
An example of a fibrous protein
  • Found in skin, tendons, cartilage, bones, teeth, walls of blood vessels.
  • Structural protein in most animals.
  • Collagen molecule made of 3 polypeptide chains, each in the shape of a helix (not as tightly wound as an alpha-helix).
  • Each chain contains 1000 amino acids - every third amino acid is glycine (the smallest amino acid).
  • 3 helical chains wind around each other to form a three-stranded 'rope'.
  • Glycine allows the 3 polypeptides to lie close together to form a tight coil (any other amino acid would be too big).
  • 3 strands held together by H bonds.
  • Each 3 stranded molecule interacts with others next to it. Bonding causes fibres to form.
  • The ends of parallel molecules are staggered, preventing weak areas from running across the collagen fibre.
  • Very strong - one quarter the tensile strength of mild steel.