3.1.4.1 General properties of proteins

Cards (39)

  • Proteins
    are polymers of amino acids
  • The monomers of proteins are
    amino acids
  • Dipeptide
    formed when two amino acids join together
  • Polypeptide
    formed when more than two amino acids join together
  • Amino acids have the same general structure
    ● a carboxyl group (-COOH)● an amine or amino group (-NH2)● an R group (also known as a variable side group)— attached to a carbon atom
  • R groups generally contain carbon. The only exception is

    glycine — its R group consists of just one hydrogen atom
  • All living things share a bank of
    only 20 amino acids— only difference between them is what makes up their R group
  • Amino acids are linked together by
    condensation reactions— to form dipeptides and polypeptides— A molecule of water is released during the reaction
  • Peptide bonds
    The bonds formed between amino acids
  • The reverse reaction (hydrolysis) happens when

    dipeptides and polypeptides are broken down
  • Hydrogen bond
    is a relatively weak bond formed between hydrogen atoms and other atoms (FON)— e.g. fluorine, oxygen, nitrogen
  • Primary structure
    ● is the number and sequence of amino acids in the polypeptide chain— can determine the 3D shape or tertiary structure— therefore affect the shape of the active site in enzymes
  • Proteins are much easier to explain if you describe their structure in four levels

    primary structuresecondary structuretertiary structurequaternary structure
  • The Primary Structure
    is specific for each protein — one alteration in the sequence of amino acids can affect the function of the protein
  • Secondary structure
    Hydrogen bonds form between the amino acids in the chain● makes it automatically coil into— an alpha (a) helix or— fold into a beta (b) pleated sheet
  • The many hydrogen bonds make these structures
    stable— Most channel proteins are made of alpha helices
  • Disulfide bridges
    are covalent bonds between sulfur atoms— a lot stronger than the ionic and hydrogen bonds in proteins
  • Tertiary structure
    ● The coiled or folded chain of amino acids is often coiled and folded further● More bonds form between different parts of the polypeptide chain— including hydrogen bonds and ionic bonds — attractions between negative and positive charges on different parts of the molecule
  • Tertiary Structure
    Disulfide bridges also form whenever two molecules of the amino acid cysteine come close together— the sulfur atom in one cysteine bonds to the sulfur atom in the other
  • The tertiary structure is
    the 3D shape of the polypeptide chain— All enzymes, antibodies and some hormones have this
  • The tertiary structure
    creates a specific shape due to the sequence of amino acids in the chain as ● hydrogen bonds● ionic bonds● disulfide bridges (covalent bonds) — form between R groups
  • A change to the sequence of amino acids would affect
    the secondary and tertiary structure — as these bonds would form in different places
  • Quaternary structure
    proteins made of more than one polypeptide chain joined together
  • Examples of quaternary structures
    ●Antibodies ●Haemoglobin ●Insulin ●Collagen ●Other non-protein groups may also be associated — e.g the haem group in haemoglobin
  • Quaternary Structure
    is the way these polypeptide chains are assembled together— Some proteins are made of several different polypeptide chains held together by bonds
  • The quaternary structure is the proteins

    final 3D structure
  • Not all proteins have a quaternary structure — some are made of only one polypeptide chain.
    some are made of only one polypeptide chain
  • A protiens shape determines
    its function — e.g. haemoglobin is a compact, soluble protein— makes it easy to transport— makes it great for carrying oxygen around the body
  • Proteins found in living organisms have all got different structures and shapes, which makes them

    specialised to carry out particular jobs
  • Enzymes
    ● roughly spherical in shape — due to the tight folding of the polypeptide chains● soluble ● often have roles in metabolism — e.g. some enzymes break down large food molecules (digestive enzymes)● other enzymes help to synthesise (make) large molecules
  • Antibodies
    ● are involved in the immune response ● are found in the blood● made up of two light (short) polypeptide chains two heavy (long) polypeptide chains— bonded together● have variable regions — the amino acid sequences in these regions vary greatly.
  • Transport proteins
    ● E.g. channel proteins are present in cell membranes— Channel proteins contain hydrophobic (water hating) and hydrophilic (water loving) amino acids— cause the protein to fold up and form a channel● These proteins transport molecules and ions across membranes
  • Structural proteins
    ● are physically strong● consist of long polypeptide chains lying parallel to each other with cross‑links between them● include :— keratin (found in hair and nails) — collagen (found in connective tissue)
  • Collagen has
    three polypeptide chains tightly coiled together — makes it strong— makes it a great supportive tissue in animals
  • Proteins are also used as
    chemical messengers in the body— e.g. as hormones like insulin
  • To test for a protien
    use the biuret test
  • The biuret test for proteins
    ● add a few drops of sodium hydroxide solution — test solution needs to be alkaline● add some copper(II) sulfate solution
  • If protein is present, the solution
    turns purple
  • If there's no protein the solution
    will stay blue