Proteins

• Amino acids

  • has an amino group (NH²) and a carboxylic acid group (COOH)
  • there are 20 types of R groups

  
• Dipeptides

  • peptide bond forms between C of -COOH and N of -NH²

  
• Primary structure

  • the sequence of amino acids bonded by covalent peptide bond

  
• Secondary structure

  • α-helix (flexible and elastic) and β-pleated sheet (stable e.g silk)

  
• Tertiary structure

  • 3D structure
  • shape is held together by H bonds between some of the R groups and ionic bonds
• Quaternary structure

  • arrangement of several polypeptide chains

  
• Functions of proteins

  • Structural
  • Contractile proteins
  • Hormones
  • Transport: haemoglobin transports oxygen around the body
  • Transport into and out of cells
  • Defence: immunoglobins protect the body against foreign invaders, fibrinogen for blood clotting
• Globular proteins

  • compact, roughly spherical
  • amino acid sequence is irregular with a wide range of R groups
  • physiological
  • generally soluble in water
  • ex: haemoglobin
• Fibrous proteins

  • long strands
  • amino acids sequence is repetitive with a limited range of R groups
  • structural
  • generally insoluble in water
  • ex: collagen, keratin