Week 5 Protein function

• X-ray crystallography:

  • Have highly focused beam that scatters through crystallised molecule
  • Can take image of the film
  • Not an easy experiment (can take years to crystallise protein)
  • Myoglobin
  • First protein to have 3 dimensional structure revealed
  • Binds to oxygen in muscle cells

  A) crystallised

  B) diffracted

  
• Myoglobin:

  • 153 amino acids - 8 helices
  • binds to oxygen
  • ability depends on the presence of a haem group
  • Water-soluble proteins fold into compact structures
  • Interior consists mainly of hydrophobic amino acids
  • Exterior consists of charged and polar amino acids

  A) hydrophobic

  B) charged

  C) other

  
• Polyphyrins:

  • globular molecule that consists of 4 pyrrol

  A) pyrrol

  B) porphyrin

  
• The Haem group:

  • Iron has 4 bonds to the Polyphyrins ring
  • Fe can form 2 more bonds
  • Bottom is bonded to protein
  • Top is bonded to oxygen
  • Binding of myoglobin and oxygen

  A) pyrrole

  B) methyl

  C) protein

  D) oxygen

  
• Types of haem proteins:

  • myoglobin (oxygen storage in muscle)
  • haemoglobin (oxygen transport)
  • cytochromes (generation of energy in mitochondria)
• Haemoglobin:

  • Help red blood cells carry oxygen from lungs to tissues
  • Haemoglobin gives the RBC the red colour
  • Consists of 4 chains (2 alpha & 2 beta chains)

  A) myoglobin

  B) haemoglobin

  
• Oxygen binding by myoglobin and haemoglobin:

  • binding of O to one site increases the likelihood that oxygen binds at remaining sites (cooperativity)
  • myoglobin - bind one oxygen with high affinity
  • haemoglobin - bind 4 oxygen's with lower affinity

  A) myoglobin

  B) haemoglobin

  
• Effects of oxygen binding:

  • changes the position of the iron ion
  • there are also changes to the quaternary structure
  • there are conformational changes in the haemoglobin

  
• Bohr effect:

  • Low pH = release of oxygen
  • Increase H+ conc and CO2 conc, affinity reduces and oxygen is released
  • The H+ and CO2 bind to the protein on the haemoglobin
• Effect of pH on binding of oxygen:

  • Lowering the pH results in release of oxygen from haemoglobin

  
• Myoglobin:

  • monomer
  • stores oxygen

  Haemoglobin:

  • tetramer
  • transports oxygen
  • bind oxygen cooperatively
  • binding of oxgyen can be regulated
• Sequence and structure:

  • similarities and differences can be spotted when comparing the sequence alignments
  • Differences in myoglobin and haemoglobin are due to mutations that occur after gene duplication

  A) similar

  B) identical

  C) Glycine

  D) alanine

  E) aspartate

  F) Glutamate

  G) identical

  H) similar

  
• Evolution of globin proteins:

  A) mutation

  B) gene duplication

  
• Sickle cell anemia:

  • It is a point mutation
  • There is only one amino acid different in the beta chain (Glu6 is now Val)
  • This sickles the RBC and leads to it clogging small capillaries (impair blood flow)
  • Leads to painful swelling and there is a higher risk of a stroke
  • Link to malaria
  • Untreated homozygous sickle cell individuals die in childhood
  • However, heterozygous individuals show resistance to malaria

  
• Fibrous proteins:

  • Linear
  • Give strength and flexibility
  • Alpha keratin
  • Major component in hair, nails and feathers
  • Collagen
  • Most abundant protein in mammals
  • Extracellular protein
  • Main fibrous component for skin, bone, teeth, tendon and cartilage
• Collagen properties:

  • Has a triple helix, distinct from alpha helix (secondary structure)
  • amino acid sequence in collagen is usually the repeating tripeptide unit Gly-X-Y, where X is often proline, and Y is often 4-hydroxyproline
  • helix has 3 amino acid residues per turn

  
• • 4-hydroxyproline
  • Required for collagen structure
  • Proline converts to 4hp after collagen is synthesised
  • Requires vitamin c
  • deficiency in vitamin C can lead to scurvy

  
• Structure of collagen fibrils:

  A) striations

  B) molecule

  C) heads