Protein

• Protein


  The most complex and diverse group
• Functions of proteins


  • Structure (e.g. collagen, keratin)
  • Enzymes (e.g. amylase, pepsin, catalase)
  • Transport (e.g. haemoglobin)
  • Pumps (e.g. Na+-K+ pump in cell membranes)
  • Motors (e.g. myosin, kinesin)
  • Hormones (e.g. insulin, glucagon)
  • Receptors (e.g. rhodopsin)
  • Antibodies (e.g. immunoglobulins)
  • Storage (e.g. albumins, casein)
  • Blood clotting (e.g. thrombin, fibrin)
  • Lubrication (e.g. glycoproteins in synovial fluid)
  • Toxins (e.g. diphtheria toxin)
• Amino acid


  • Central carbon atom with four different chemical groups attached: hydrogen atom (-H), basic amine group (-NH2), acidic carboxylic group (-COOH), variable "R" group/side-chain (-R)
• Zwitterion


  Amino acid at neutral pH
• Formation of a peptide bond

  1. Head-to-tail (N' to C') reaction

  2. Condensation reaction

  3. Involves formation of water
• Breaking of a peptide bond

  1. Hydrolysis reaction

  2. Involves addition of water
• Humans can produce 10 of the 20 amino acids, the rest we obtain from our diet
• Peptide


  Short chain of amino acids covalently linked to each other with peptide bonds
• Polypeptide


  Long chain of amino acids having more than 40 amino acids
• Primary structure of proteins

  • Linear sequence of amino acids in a polypeptide chain
• Secondary structure of proteins

  • Most basic level of folding or coiling in a protein, includes alpha-helix and beta-sheets held together by hydrogen bonds
• Tertiary structure of proteins


  • Compact 3D structure of a protein molecule resulting from the folding of the already-folded chain of amino acids, held by intermolecular forces like hydrogen bonds, disulfide bonds, ionic bonds, and hydrophobic interactions
• Quaternary structure of proteins


  • Three-dimensional arrangement of two or more polypeptides or a polypeptide and a non-protein component in a protein molecule, held by bonds similar to tertiary structure
• Proteins with quaternary structure


  • Haemoglobin
• Amino acids


  • Alanine (Ala, A)
  • Serine (Ser, S)
  • Valine (Val, V)
  • Glycine (Gly, G)
• Peptide bond


  Covalent bond formed between the carboxyl group of one amino acid and the amine group of the next amino acid
• Peptide bonds are formed by condensation reaction and broken by hydrolysis reaction
• Tertiary structure


  The compact structure of a protein molecule resulting from the three-dimensional coiling of the already-folded chain of amino acids
• Quaternary Structure of protein

  The three-dimensional arrangement of two or more polypeptides, or of a polypeptide and a non-protein component such as haem, in a protein molecule
• Quaternary Structure of protein


  • Haemoglobin molecule
• Haemoglobin

  Made up of 2 α chains, 2 β chains. Each chain has its own haem group. Each haem group contains an iron atom, bonds reversibly with oxygen molecule
• Amino acids


  • 3-letter abbreviation
  • 1-letter abbreviation
• Four types of bonds which fold protein into its 3-D shape

  • Hydrogen bonds
  • Disulphide bonds
  • Ionic bonds
  • Hydrophobic interactions
• Hydrogen bonds


  Occur between H atoms and O/N nitrogen atoms found in strongly polar R-groups of amino acid residues. They can be broken down by higher temperature and pH changes
• Hydrogen bonds


  • Between –OH and –NH/-C=O, between –OH of Ser and C=O of Glu
• Disulphide bonds


  Strong covalent bonds between sulphur atom of R-groups
• Ionic bonds


  Formed between positive and negative charged R groups of amino acid residues
• Hydrophobic interactions


  Occur between R groups which are non-polar or hydrophobic
• Hydrophobic interactions


  • Glycine and Alanine
• Haemoglobin (Hb)


  Made up of four polypeptide chains. It consist of two alpha chains and two beta chains. It curls up into a globular protein where non-polar hydrophobic R groups point into the centre of the molecule while the polar hydrophilic R groups remain on the outside. The hydrophic interactions help to maintain its 3D shape. The outward pointing hydrophilic R group helps to maintain its solubility. Each chain contains a prosthetic group called haem and each contains an iron ion. One oxygen molecule can bind with each iron ion.
• Collagen molecule


  Consist of 3 polypeptide chains wound around each other to form a triple helix. Each chain contains 1000 amino acid - every third is a glycine. Glycine allows the 3 polypeptides to lie close together to form a tight coil
• Collagen - Staggered arrangement

  1. bonds and cross-links form between these polypeptides which gives the structure strength. Ends: Cross-links between side chains
• Globular Protein


  • These tend to form ball-like structures where hydrophobic parts are towards the centre and hydrophilic are towards the edges, which makes them water soluble. They usually have metabolic roles, for example: enzymes in all organisms, plasma proteins and antibodies in mammals
• Globular Protein


  • Haemoglobin
• Fibrous Protein


  • These proteins form long fibres and mostly consist of repeated sequences of amino acids which are insoluble in water. They usually have structural roles, such as: Collagen in bone and cartilage, Keratin in fingernails and hair
• Fibrous Protein


  • Collagen