Protein

• Monomer = repeating structurally similar subunits
• Structure of amino acid:

  NH2CHRCOOH

  R - variable region

  COOH - carboxyl acid

  NH2 - amino group
• Bonding between R groups:

  • folding of polypeptide chain -> tertiary structure
  • joining polpeptide chains -> quarternary structure
• Amino acids joined by peptide bonds to form a polypeptide chain, formed by condensation reactions
• Condensation reactions = forming of a chemical bond by the elimination of water
• Hydrolysis reaction = breaking of a bond by the addition of water
• Structure of protein:

  • primary structure - sequence of amino acid in a polypeptide chain
  • secondary structure - determined by the arrangement of hydrogen bonds between carboxylic acid and amino group -> folding of the primary structure to give alpha helix and beta-pleated sheets
  • tertiary structure - specific 3D shape due to folding of secondary structure -> held in place by disulphide bridges (s), ionic bonds (charged R group), hydrogen bonds (between R groups)
  • quarternary structure - one or more polypeptide chains bonded together -> bonds form between R bonds, same bonding as tertiary
• Disulphide bridges = strong covalent bonds form between R-groups containing sulphur atoms

  • only tertiary and quarternary
• Ionic bonds = form between R groups with charges

  • only tertiary and quarternary
• Hydrophobic interactions = when polypeptide chains fold, hydrophobic R-group cluster close to each other in the interior of the folded chains

  • doesn’t interact with water
• Hydrophilic interactions = R-groups are on the outside, attracted to water

  • Interact with water
• Peptide bonds = join amino acids to form primary structures
• Fibrous:

  • long strands formed from long parallel polypeptide chains joined together
  • have a structural use
  • insoluble in water
  • maximum is secondary structure
  • e.g. Keratin, collagen
• Globular:

  • often have spherical or globular structure
  • often carry out metabolic functions
  • can be soluble in water
  • can be tertiary or quarternary
  • catalyse reactions , enable transport across membranes
  • e.g. Hormones
• Test for Proteins:

  • Mix Solution with biuret reagent in a boiling tube
  • Cover the top of the boiling tube and invert it once
  • if solution turns purple, protein is present