Collagen Flashcards
What are general characteristics of collagen?
Composed of 3 polypeptide a-chains (NOT alpha-helices) → form collagen triple helix
- 46 collagen genes → 28 collagen types/proteins
- 2 large groups: Firbil-forming (long fibrils) and sheet-forming (in basement membrane) collagens
- Homotrimers or heterotrimers
- Form aggregates by association of mutliple triple helices
- 25% of the protein content in the body is collagen
- Found in all mutlicellular animals
What is the structure of collagen
Structure of 1 a-chain:
- G-X-Y-G-X-Y…
- Left-handed / counter-clockwise
- X ~ proline (often) → stabilize helical conformation
- Y ~ hydroxyproline (often)
- Glycine → smallest aa, placed at the center of triple helix (mutation in G → destabilization → disease)
Structure of triple-helix:
- Right-handed / Clockwise
- 1.5 nm large
What interaction is important for triple helix stability?
X — Hydrogen with H2O — OH-Y (hydroxyproline)
What are the steps of synthesis of collagen (1st part - triple helix)?
- Synthesis of pro-a-chain (from ribosome + translocon complex → ER)
- Hydroxylation of selected prolines and lysines
- Glycosylation of selected hydroxylysines (addition of galactose / glucose mono-, di-saccharides)
- Self-assembly of 3 pro-a-chains
- Procollagen triple helix formation
- Secretion (out of the ER by vesicle → extracellular space)
What is the role of the propeptide? (pro-a-chain)
- Guides intracellular formation or the triple-helical collagen molecules
- Prevent intracellular formation of large collagen fibrils
What are the steps of the formation of collagen fibrils?
- Secretion of procollagen triple-helix into extracellular space
- Cleavage of propeptides (in deep invagination of the cell surface)
- Self-assembly into fibrils
- Aggregation of collagen firbils to form collagen fibers (entropy driven, no enzymes)
*Collagen molecules are much less soluble than procollagen
How does extracellular procollagen processing occur (to collagen triple-helix)?
What is it dependent on?
Zinc (Zn2+)-dependent (metalloprotease)
- Procollagen N-proteinase:
ADAMTS-2, -3, -14 = A disintegrin and metalloproteinase with thrombospondin motifs
-2, -14 → skin and others
-3 → cartilage - Procollagen C-proteinase:
Tolloid family
- BMP-1 = Bone Morphogenetic Protein-1, actually a protease
- mTLD = mammalian tolloid
- TLL-1 = tolloid-like-1
*Removal of the C-propeptide decreases critical concentration for self-assembly (of larger fibrils)
What is the structure of generic collagen microfibrils?
Triple helix → 300 nm in length → 1000 AA
D-period = length of stagger between adjacent molecules → 64nm periodicity in human dermis (67 in rat tail tendon)
- Collagen type I and III can co-polymerize
- 5 triple helices → 1 microfibril (4nm in diameter) → longitudinal growth
*Entropy driven self-assembly
+ and - charges from each triple helix align (attracted) + gap of 64 nm between each triple helix in length → Hole zone (dark, has the charges, dye was able to go in ~40 nm) + overlap zone (just proteins, dye couldn’t go in, ~ 24 nm) → striated collagen fibrils
What are the dimensions of collagen molecules?
When assembled into high order polymers → 10-300 nm in diameter, many hundred micrometers long
- Clearly visible in EM
- Larger can sometimes be seen in light microscopy
Which enzymes are responsible for hydroxylation of specific prolines and lysines of a-chains in the ER?
What is the effect of these hydroxylations?
Lysyl hydroxylase (3 isoenzymes)
At delta position
→ Glycosylation (galactose (mono) or glycosylgalactose (di)) + crosslink sites
Prolyl hydroxylase (3 isoenzymes)
- At gamma position
→ Intermolecular hydrogen bonds (specifically 4-hydroxyproline)
*Vitamine C dependent reaction (reduced iron in the enzyme)
What is the mechanism of the hydroxylation reaction of proline?
Membrane bound dioxygenase → Takes O2 and distributes the 2 oxygens to different locations → 1 forming the new hydroxyl group, other one on a-ketoglutarate after CO2 was removed
- Iron needed for electron flow
- VitC needed in active center of the enzyme (for Fe2+)
Prolyl residue + O2 + a-ketoglutarate → 4-Hydroxyprolyl + CO2 + Succinate
Required for the reaction (at the arrow): Prolyl hydroxylase + ascorbate (Vit C) + Fe2+
*Same for Lysyl hydroxylase
What is the importance of VitC in the prolyl hydroxylase reaction?
In absence of substrate (of collagen), prolyl hydroxylase oxidizes Fe2+ to Fe3+ → inactivates the enzyme
*VitC = antioxidant → reduced Fe3+ → Fe2+ the reaction can occur
What are the consequences of a defect in collagen hydroxylation?
Scurvy (lack of vitamin C)
- Rupturing of blood vessels
- Defective wound healing
- Loss of teeth
- Lethal if no treatment
Solution: Sauerkraut → keeps long + rich in VitC
What is the importance of crosslinks in Collagens?
What ion is the reaction dependent on?
Strenghtening of collagen fibers by formation of covalent cross-links between hydroxy-lysine residues
- Extracellular reactions
- Lysyl Oxidase (LOX) and 4-LOX-like enzymes → copper dependent
Reaction: Hydroxylysines on procollagen → oxydative deamination by LOX → reactive aldehyde (allysine) → Aldol condensation reaction (non enzymatic) → Aldol crosslinks (→ further cross-link products → tri-, tetra- and pentafunctional cross-links)
1st reaction needs Lysyl oxidase + O2 + H2O + Cu2+ → byproducts = NH3 + H2O2
2nd reaction = non enzymatic → byproduct = H2O
What 5 enzymes are involved in collagen processing?
What ion are the reactions dependent on?
- Prolyl hydroxylase → adds hydroxyl group to some prolines (VtiC + Fe2+, ER/Golgi)
- Lysyl hydroxylase → adds hydroxyl groups to some lysines (VitC + Fe2+, ER/Golgi)
- N- and C-Propeptidases → cleave collagen peptides outside the cells (Zn+, Extracellular)
- Lysyl oxidase → converts some lysines and hydroxylysines to allysines and hydroxy-allysines → reactive aldehydes react with free hydroxy-lysines to form covalent cross-links (Cu2+, Extracellular)
