Day 10, Lecture 3: Proteins III: Fibrous Proteins

Question 1

Relevance of Fibrous Proteins

Answer 1

Question 2

Overview of Collagen

Answer 2

• Most abundant protein (skin, cartilage, tendons, bone). About 25% of protein in adult and 15-20% of protein in child
• Family of at least 28 different genes for use in various tissues
• Primary structure
  
  • mostly GPX sequence repeats
  • about 1000 amino acids
• Forms triple helix of three interwined, open left-handed polypeptide helices
• Special features:
  
  • Small glycine residue > helices wrap together closely
  • Peptide bonds are perpendicular to the chain axis; allows the carbonyl to point to other adjacent chains
  • Triple helices self-assemble into fibrils that reinforce tissues
• Post-translational modifications:
  
  • Proteolytic processing at several steps
  • hydroxyproline (HP or Hyp) allows an extra H-bond to form with a peptide carbonyl from another chain to stabilize triple helix
  • Hydroxylysine (HK)
    
    • site of glycosylation
  • Crosslinking of lysines
    
    • Adds extra strength to fibrils

Question 3

Biosynthesis of Collagen

Answer 3

• mRNA→Preprocollagen, signal peptide removed during translation
• Procollagen form has extra amino and carboxyl termini peptides
• Proline and lysine (K) are hydroxylated by prolyl hydroxylase (note: requires cofactors Iron and vitaminC/ascorbate)
• HydroxyK is glycosylated
• Prochains associate and carboxyl-termini peptides are disulfide-linked
• Triple helix formation is initiated at C-terminus and zips up towards N-terminus
• Molecule is secreted out of cell
• Amino and carboxyl peptides cleaved to form tropocollagen
• Self-assembly of many tropocollagens into a 1/4 staggered array
• Cross-linking of lysines initiated by action of lysyl oxidase (note: copper is cofactor) - makes allylysine=all. Then Schiff base formation with another K.

Question 4

Collagen roles in aging, cell adhesion, wound healing

Answer 4

• Aging
  
  • More cross-linking of the lysines of collagen molecules with time, therefore fibers stiffen as one ages
• Cell adhesion
  
  • In addition to structural roles, collagen fibrils interact with other cell surface and matrix molecules (e.g. Integrins, fibronectin, laminin and proteoglycans) which in turn bind to other cell surfaces
• wound healing
  
  • Scar tissue
    
    • consists mostly of collagen (Ex. Liver Cirrhosis; after damage from virus,alcohol or drugs, dead cells are replaced with collagen fibers
  • Tissue remodeling and repair requires controlled collagen degradation
  • In infection, the abscess is normally walled off with collagen to contain the microbes, but some bacteria secrete collagenases to digrest the protective barrier and escape into the surrounding tissue

Question 5

Scurvy

Answer 5

• Disease of Defective Collagen
• Vitamin C (ascorbate) is required for Hydroxyproline formation
• Deficiency leads to a triple helix that is not stabilized by extra H-bond from this special AA
• Therefore triple helix “melts” at body temperature

Question 6

Osteogenesis Imperfeta

Answer 6

• “brittle bone disease”
• Disease of defective Collagens
• Typically a point mutaiton (usually glycine to X) impairs triple helix formation
• Particulary bad for the structure when mutation at the C-terminus (collagen zips up C→N direction) and the folding process stalls out early

Question 7

Ehlers-Danlos Syndrome

Answer 7

• “rubber or elastic man”
• Disease of Defective Collagens
• Heterogeneous group with: stretchy skin, loose joints, poor wound healing, failure of vessel/organ structures
• Typically caused by no cross-linking (no strengthening) or Failure to remove N-peptides (collagen not converted to most insoluble form)
• remember crosslinking requires Lysyl oxidase and copper

Question 8

Effect of Mutations in a Multimeric Protein

Answer 8

• Collagen is a triple helix that can be made of more than 1 species of polypeptide (depending on the fiber and the tissue)
• There are 2 alleles of each collagen gene species (remember about 28 families) per diploid cell
• Two Scenarios:
  
  • Bad gene is a null or nonsense mutaiton (and the defective polypeptide is degraded quickly):
    
    • 50% of the normal collagen fibers are made
      
      • patient outcome: mild to moderate disease
  • Bad gene is a missense mutation in which the defective polypeptide does not fold correctly but still incorporates into collagen fibers:
    
    • 3/4 of all of the fibers are bad since poisoned with the faulty subunit
      
      • Patient outcome:
        
        • Severe to lethal disease. Dominant

Question 9

Elastin

Answer 9

• The “rubber” protein forms a 3-dimensional network of cross-linked polypeptides
  
  • Primary structure:
    
    • mostly small nonpolar AAs, plus alot of Pro (causes kinks) and Lys (K) (sites for cross-linking)
  • Lysine crosslinks
    
    • 2 chains together with 4 cross-links called desmosine (initiated by lysyl oxidase)
  • Reversible stretching
    
    • Elastin helps large arteries act as a secondary pump in circulatory system because heart is a pulsatile pump > stores energy and smooths out blood flow

Question 10

Alpha-1 antitrypsin deficiency

Answer 10

• Alpha-1 antityspin (alphaAT) :
  
  • protease inhibitor produced in the liver
• Disease causing gene alterations result in abnormal folding of the protein
  
  • Decreased activity
  • failure to be normally secreted from the cell
  • accumulaiton within the cell

Question 11

Keratin

Answer 11

• Primary structure
  
  • Tandem repeats of 7 AAs (the equivalent of two turns of an alpha-helix) that form a nonpolar and a polar surface
  • Form micro to macroscale→ alpha helix, protofibril, microfibril, macrofibril of hair
  • Many cysteines that are cross-linked so that keratin fibrils are insoluble and resistant to stretching (permanent waves undo Cys cross-links)

Question 12

Effect of smoking on elastin in the lungs

Answer 12

• Emphysema
  
  • inflammatin leads to loss of elastin