Fibrous Protiens

• Fibrous proteins


  Proteins adapted for a structural function

  
• Representatives of fibrous proteins

  • Keratin
  • Collagen
  • Elastin
  • Fibrin

  
• Fibrous proteins


  • Share properties that give strength and/or flexibility to the structures in which they occur
  • The fundamental structural unit is a simple repeating element of secondary structure, mainly β structures, less α-helices
  • They do not exist as separate molecules, rather form packages of many polypeptide chains linked together by H-bonds
  • Insoluble in water, high concentration of hydrophobic amino acids; amino acids with short side chains prevail in their structure (Gly, Ala, Ser, Val)

  
• Collagen


  A family of similar, rigid, insoluble proteins: more than twenty collagen types, as well as additional proteins that have collagen-like domains

  
• Collagen comprises 25% of all body proteins

  
• Different types of collagen

  • Gel (in the extracellular matrix)
  • Packed in parallel fibers (in ligaments and tendons)
  • Packed so that provides minimum diffraction of light (eye)
  • In fibers arranged at an angle to each other to resist mechanical pressure from different directions (bones)

  
• Collagen structure


  • Built from 3 alpha polypeptide chains (in various combinations, each approximately 1000 amino acids long), which form a triple helix
  • Contains modified amino acids (hydroxyproline, hydroxylysine): (-Gly-X-Y-)333, where X is frequently proline and Y is often hydroxyproline or hydroxylysine
  • Glycosylated in hydroxylysine (glucose, galactose)
  • Aggregates in fibers
  • Covalent cross-links connect the monomers into fibers

  
• Tropocollagen


  • The basic structural unit of collagen fibrils
  • Type I contains 2 different + 1 identical polypeptide chains
  • Each polypeptide chain contains ~1050 amino acids
  • Each polypeptide forms a polyproline type II helix (left-handed, 3 amino acids per turn, a rise per amino acid of 0.30 nm, not stabilized by hydrogen bonds, rather by interactions between proline and hydroxylysine side chains)

  
• Tropocollagen triple helix


  • 3 left-handed polypeptide chains form a right-handed triple helix
  • Hydrogen bonds between the peptide groups
  • Contact at the glycine residues
  • Length 300 nm, diameter 1.5 nm
• Collagen fibrils


  • Staggered arrays of tropocollagen molecules
  • The end of each molecule extends 67 nm beyond that of its neighbor with gaps of 35 nm between the ends
  • Collagen I-III, V and IX contain hundreds to thousands tropocollagen molecules in cross section
  • A single fibril contains often more than one type of collagen

  
• Type I collagen contains: 33% glycine, 10% proline, 0.5% 3-hydroxyproline, 10% 4-hydroxyproline, 1% 5-hydroxylysine, 0.5-1% carbohydrate content (glucose-galactose glycosylation in hydroxylysine)

  
• Collagen biosynthesis


  1. Synthesis of procollagen

  2. Hydroxylation of proline and lysine residues

  3. Glycosylation

  4. Formation of the triple helix (in C to N direction)

  5. Secretion

  6. Extracellular cleavage of the propeptides of the procollagen

  7. Assembly of the collagen fibrils

  8. Cross-linking of the molecules in the collagen fibrils

  
• Procollagen synthesis


  • Pre-procollagen synthesized on the endoplasmic reticulum (contains propeptides)
  • Signal peptidase
  • Procollagen
  • Formation of interchain (N-terminal propeptides) and intrachain (C-terminal propeptides) disulfide bonds
• Hydroxylation


  • Some of the proline and lysine residues become hydroxylated to form hydroxyproline and hydroxylysine
  • The enzymes act only on non-triple helical polypeptides
• Vitamin C deficiency


  Collagen fibers cannot be cross-linked, greatly decreased tensile strength of the assembled fiber
• Scurvy: defective collagen, bleeding in the gums, capillary fragility, bruises on the limbs as a result of subcutaneous extravasation of blood
• Glycosylation


  Some of the hydroxylysine residues become glycosylated with galactose or glucosyl-galactose, or N-acetylglucosamine-mannose
• Triple helix assembly


  1. Formation of C-interchain disulfide bonds is a prerequisite for the helix formation

  2. Delay in formation or imperfection of triple helical structure may cause overhydroxylation or overglycosylation
• Secretion


  1. Translocation to Golgi apparatus, where the fibrils are packaged in secretory vesicles

  2. Fusion with the membrane and secretion – only triple-helical procollagen

  3. Improperly coiled molecules are degraded
• Extracellular release of the propeptides

  1. and C-procollagen extracellular peptidases
• Assembly of collagen fibrils


  Each one is overlapping its neighbor by a length approximately three-quarters of a molecule

  
• Extracellular lysyl oxidase


  • Oxidatively deaminates some of the lysyl and hydroxylysyl residues in collagen
  • Reactive aldehydes that result (allysine and hydroxyallysine) can condense with lysyl or hydroxylysyl residues in neighboring collagen molecules to form a variety of covalent cross-links

  
• Collagen degradation


  1. Extracellular collagenase – cleaves a single peptide bond about three-quarters down the length of the triple helix

  2. Resulting fragments unravel and are further degraded by other proteases

  3. Intact triple helical collagen is resistant to common proteases (such as pepsin, trypsin)

  
• Ehlers-Danlos syndrome (EDS)


  • Deficiency of collagen-processing enzymes (e.g., a procollagen peptidase deficiency)
  • Or mutations in the amino acid sequences of collagen types I, III or V
  • Heterogeneous group of generalized connective tissue disorders that result from inheritable defects in the metabolism of collagen
  • Stretchy skin and loose joints

  
• Osteogenesis imperfecta (brittle bone syndrome)

  • A heterogeneous group of inherited disorders
  • Bones easily bend and fracture, retarded wound healing, rotated and twisted spine
  • Type I osteogenesis imperfecta tarda – fractures in early infancy
  • Type II osteogenesis imperfecta congenita: glycine replaced by amino acid with bulky side chain, which prevents folding of the triple helix, more severe, patients die in utero or in the neonatal period of pulmonary hypoplasia
• Elastin


  Fibrous insoluble protein with rubber-like properties; forms elastic fibers composed of elastin and glycoprotein microfibrils in the extracellular matrix
• Elastin structure


  • Protein polymer, extensively interconnected, rubbery network
  • Precursor - tropoelastin, a linear polypeptide composed of about 700 amino acids that are primarily small and nonpolar (glycine, alanine, valine; contains proline, lysine, and not much hydroxyproline)
  • Secreted by the cell into the extracellular space, where it interacts with specific glycoprotein microfibrils, such as fibrillin, which function as a scaffold onto which tropoelastin is deposited

  
• Elastin properties


  • Hydrophobic
  • Unusual conformation
  • Free folding
  • Reversibly stretchy
• Desmosine cross-links


  • Oxidative deamination of lysyl residues by lysyl oxidase → allysine residues
  • Three of the allysyl side chains plus one unaltered lysyl side chain form a desmosine cross-link
• α1-antitrypsin (α1-AT)


  • In blood and other body liquids, synthesized by the liver
  • Inhibits a number of proteolytic enzymes (serine protease inhibitor)
  • α1-AT inhibits the elastase, released from activated and degenerating neutrophils (the elastase degrades the connective tissue of alveolar wall, resulting with emphysema because lung tissue cannot regenerate)
  • Deficiency of α1-AT: results in emphysema
• Smoking


  • Causes methionine oxidation, (methionine is necessary for the connecting of α1-AT to elastin)
  • The inhibitory effect of α1-AT is reduced
  • Smokers with α1-AT deficiency suffer from lung destructions and have less chances for survival