protein structure

• protein:

  • large molecules
  • monomers are amino acids
  • dipeptide: two amino acids joined together
  • polypeptide: two or more amino acids
  • proteins made of one or more polypeptides
• amino acids:

  • 20 naturally occur in proteins, basic monomer units combine form polymer polypeptide
  • 4 different chemical groups:

  1. AMINO GROUP (-NH2): basic group
  2. CARBOXYL GROUP (-COOH): acidic group
  3. hydrogen atom (-H)
  4. R (group): varies from each amino acid
• POLYPEPTIDES & DIPEPTDES:

  • amino acid linked by condensation reaction form polypeptides, molecule of H2O released
  • -OH group removed from COOH and -H removed from NH2
  • peptide bond between amino acids formed
• Proteins structure:

  • Primary structure: sequence of amino acids in polypeptide chain, determines the structure of polypeptide and its function
  • Secondary structure: hydrogen bonds form between amino acid chains, causing the polypeptide to coil into alpha helix or fold into beta pleated sheets
  • Tertiary structure: alpha helices can be twisted and folded to give a more complex 3D structure of the protein, bonds formed include disulfide bridges, ionic bonds, and hydrogen bonds
  • Quaternary structure: some larger proteins form this, containing a number of individual polypeptide chains like haemoglobin, insulin, collagen
• In secondary structure, hydrogen bonds form between amino acid chains, causing the polypeptide to coil into alpha helix or fold into beta pleated sheets
• Tertiary structure involves alpha helices being twisted and folded to give a more complex 3D structure of the protein, with bonds formed including disulfide bridges, ionic bonds, and hydrogen bonds
• Quaternary structure is found in some larger proteins and contains a number of individual polypeptide chains like haemoglobin, insulin, and collagen
• functions of proteins:

  • ENZYMES: usually spherical shape due to tight folding of polypeptide chain, soluble, roles in metabolism, e.g. break down large food molecules and some synthesise large molecules
  • ANTIBODIES: immune response, made of two short polypeptide chains & two long polypeptide chains bonded together, have variable regions
  • TRANSPORT PROTEINS: e.g. channel proteins transport molecules and ions across membrane
  • STRUCTURAL PROTEINS: physically stron, long polypeptide chain parallel to each other and cross-links between them. e.g. keratinand collagen
• Test for proteins:

  process:

  1. needs to be alkaline. place sample of solution to be tested into tube add equal volue of NaOH solution at room tep
  2. add few drops of dilute Copper (II) sulfate solution (BIURET REAGENT) and mix gently
  3. is protein present colour change from blue to lilac solution formed
• fibrous proteins: collagen

  primary structure= unbranched polypeptide chain

  secondary structure= polypeptide chain tightly wounded

  lots of AA, glycine helps close packing

  tertiary structure= chain twisted into second helix

  quaternary structure= three polypeptide chain wound together in a rope

  collagen: in tendons, join muscle to bone, when muscle contract bone pulled in direction of contraction

  globular proteins: enzymes and haemoglobin, carry out metabolic functions