proteins

• amino acids are the monomers for proteins
• general structure

  variable group R

  
• NH2 AMINE GROUP

  COOH CARBOXYL GROUP

  R GROUP SIDE GROUP
• a condensation reaction between two amino acids forms a peptide bond
• dipeptides formed by condensation of 2 amino acids
• polypeptides are formed through the condensation of many amino acids
• a functional protein may contain one or more polypeptides (these are proteins with a job)
• the r group is different for every amino acid
• the r group is different for every amino acid
• Dna coding determines the sequence of amino acids
• a peptide bond is formed using the OH part of the carboxyl group of one amino acids and the H part of the amine group in the second amino acid
• protein is a polypeptide that folds into a 3D shape

  polypeptides don’t have a function UNTIL they are folded
• four levels of structure in a protein

  • primary
  • secondary
  • tertiary
  • quaternary
• a polypeptide chain does not become a protein until it folds up into a distinct 3D shape and becomes functional
• PRIMARY STRUCTURE:

  • this is the unique sequence of amino acids
• primary structure

  this is the unique sequence of amino acids

  peptide bonds are found between the amino acids,

  there is a carboxyl group at one end and an amine group at the other
• Secondary structure

  • the way in which the polypeptide bond is either coiled into an ALPHA HELIX or folded into a BETA PLEATED SHEET
  • hydrogen bonds form between the amino acids and hold the structure in place
• TERTIARY

  • determines the function of the protein, eg active sites in enzymes
  • bonds occur between the R groups on the individual amino acids and cause the alpha helix/ beta pleated sheet to FOLD
  • the folds are held together by ionic bonds, hydrogen bonds and disulphide bridges (covalent bonds between sulphur atoms found on R groups of amino acids)
• QUATERNARY

  • proteins that contain more than one polypeptide chain
  • eg collagen, haemoglobin
• a peptide bonds forms between two amino acids during a condensation reaction
• to test for proteins

  • add sample to biurets solution
  • turns purple for positive
  • blue for negative
• if bonds between the R groups on individual amino acids are broken, the protein will stop working properly and is denatured
• if a protein is denatured:

  shape+function is lost

  it become non functional
• conditions causing proteins to denature:

  pH unstable

  temp too high
• at cold temps the enzymes don’t work as fast but are still working
• fibrous denatured enzymes:

  • lose structural strength
• Globular denatured enzymes

  • insoluble + inactive
  • they become non functional which is permanent
• chemical test for protein:

  • add biuret reagent (sodium hydroxide + copper sulfate)
  • the sodium hydroxide provides alkaline environment so the test can work
  • blue to lilac, protein is present
  • negative test is blue
• in a test for proteins, de-ionised water would be tested as well for a control experiment, as there is no protein just H and O
• During a protein chemical test, the pH of the tests may be measured after because the test can only work in alkaline conditions
• During a protein chemical test, the pH of the tests may be measured after because the test can only work in alkaline conditions
• During a protein test, have to make sure there is enough sodium hydroxide for ALKALINE conditions !!!!
• Tertiary

  • additional bonds forming between the R groups (side chains)
  • The additional bonds are:
  • Hydrogen (these are between R groups)
  • Disulphide (only occurs between cysteine amino acids)
  • Ionic (occurs between charged R groups)
• • A polypeptide chain will fold differently due to the interactions (and hence the bonds that form) between R groups. The three-dimensional configuration that forms is called the tertiary structure of a protein