1. Collagen

2. Objectives
1- Structure of collagen 2- Distribution of collagen 2- Functions of collagen 3- Biosynthesis of collagen 4- Degeneration of collagen 5- Diseases of collagen (Clinical Applications)

3. What is collagen ?
Collagen is the most abundant protein in the human body.
Collagen is a fibrous protein
Collagen serves as having structural functions in the body .
Collagen is found in components of
- Skin
- Connective tissue
- Blood vessel walls
- Sclera & cornea of the eye

4. General structure of collagen
Collagen protein has primary & secondary levels
of structure.
A typical collagen molecule
is a long, rigid structure in which three polypeptides
(a chains) are wound around one another
in a rope-like triple helix.
Types and organization of collagen molecules
are governed by the structural role collagen plays in a particular organ.
in some tissues, collagen is dispersed as a gel as in the vitreous humor of the eye.
in other tissues, collagen may be arranged in parallel fibres that provide great strength as in tendons
in cornea of eye, collagen is stacked so as to transmit light.

6. Types of collagen
Collagen super family of proteins includes more than 20 collagen types
The three polypeptide a chains are held together by hydrogen bonds between chains
a chains are ~1000 amino acids long but with slightly different properties
These a chains are combined to form the various types of collagen found in tissues.
Examples:
Type 1 collagen: Most common collagen
contains two a1 chains and one a2 chain
a12 a2
Type II collagen: contains three a1
a13

8. most abundant types of collagen

9. Detailed structure of collagen
1- Amino acid sequence
Collagen is rich in proline & glycine
Proline :
by its ring structure facilitates the formation of the helical conformation of each a chain.
Glycine (smallest amino acid):
present in every third position of the chain.
It fits into the spaces where the three chains of the helix come together.
Glycines are parts of a repeated sequence [-Gly- X – Y – ] where:
X is frequently Proline Pro & Y is often Hydroxyproline Hyp (can be Hydroxylsine Hyl)
So, most of the a chain can be POLYTIPEPTIDE with sequence of:
(- Gly- Pro – Hyp -) 333

11. Detailed structure of collagen (cont.) 2-Triple-helical structure
Collagen as a fibrous protein has an elongated , triple-shaped helical
structure in which side chains (R- groups) of its amino acid are on the
surface of the triple-helical molecule.
This allows bond formation between the exposed R-groups of
neighbouring collagen monomers resulting in their aggregation into long
fibres.

12. Detailed structure of collagen (cont.)
3- hydroxyproline & hydroxylysine Hydroxyproline & hydroxylysine are not present in most proteins, however, collagen contains them. These two amino acids result from the hydroxylation of some proline & lysine amino acids after their incorporation into poly peptide chains during protein biosynthesis. (i.e. post-translational modification) Hydroxyproline is important in stabilizing the triple-helical structure of collagen because it increases (maximises) interchain hydrogen bond formation.

13. prolyl hydroxylase enzyme ascorbic acid (vitamin C)
Detailed structure of collagen (cont.)
Proline is hydroxylated by
prolyl hydroxylase enzyme
which requires
ascorbic acid (vitamin C)
as a coenzyme

14. Detailed structure of collagen (cont.)
4- glycosylation
The hydoxyl group of hydroxlysine amino acids of collagen may be glycosylated by
enzymatic reaction.
Glucose & galactose molecules are attached to the polypeptide chain prior
to triple helix formation.

15. Biosynthesis of collagen
The polypeptide precursors of collagen molecules are formed intracellularly in fibroblasts (or in osteoblasts of bone & chondroblasts of cartilage) & then secreted into the extracellular matrix.
In the extracellular matrix, mature collagen monomers are formed.
Collagen monomers aggregate & become cross-linked to form
collagen fibrils.

16. Biosynthesis of collagen (cont.)
intracellular stage (in fibroblasts)
1- Formation of pro-a chain (protein biosynthesis on ribosomes of RER)
2- Hydroxylation of some proline & lysine amino acids on Y-position
of Gly – X – Y sequence of pro-a chain by enzymes prolyl & lysyl hydroxylases (requires vitamin C)
In cases of vitamin C deficiency (Scurvy):
Hydoxylation of proline & lysine will not occur, with no cross linking resulting in decrease in the tensile
strength of collagen fibers.
Clinical manifestations: bruises in the limbs due to subcutaneous extravasation of blood (capillary fragility)
3- Glycosylation: some hydroxylysine amino acids are glycosylated by glucose or glucose-galactose
4- Assembly & Secretion;
Pro-a collagen form procollagen which is a precursor of collagen that has a central region of triple helix, flanked
by non-helical amino – and caboxy- terminal extensions of the pro-a chain.
Procollagen molecules are translocated to the Golgi apparatus where they are packaged in the secretory
vesicles which fuse with cell membrane causing the release of procollagen into extracellular space.

17. Intracellular Stage of
Collagen Synthesis
(in fibroblasts)

18. Bruises in lower limb in a case of scurvy

19. Biosynthesis of collagen (cont.)
Extracellular Stage (in matrix)
5- Extracellular cleavage of procollagen molecules:
Procollagen molecules are cleaved by N- & C- procollagen peptidases
which remove the terminal nonhelical peptides resulting in the formation of triple helical tropocollagen
molecules.
6- Formation of collagen fibrils:
Individual tropocollagen molecules associate to form a collagen fibril.
Collagen fibril form an ordered, overlapping, parallel array with adjacent collagen molecules in a
staggered pattern.
7- Cross-link formation of collagen fibrils
by the enzyme lysyl oxidase which oxidatively demainates some lysine and hydroxylysine amino acids
in collagen forming aldhydes.
An aldehyde from one collagen polypeptide link with lysine or hydroxylysine amino acids of
a neighboring collagen polypeptide to form covalent cross-links resulting in mature collagen
formation.

20. Extracellular Stage of Collagen Synthesis (in matrix)
Procollagen
Tropocollagen
(Collagen molecule)
Collagen Fibril
Extracellular Stage of
Collagen Synthesis
(in matrix)

21. Collagen Molecule
Collagen Fibril

22. on one collagen polypeptide
Cross-links formation in
collagen
Aldhyde group formed
on one collagen polypeptide
NH2 of lysine on
neighbor collagen
polypeptide
Cross-link between adjacent collagen polypeptides

23. Degradation of collagen
Half-lives of normal collagen are several years.
Collagen fibrils is degraded by collagenases (matrix metalloproteinases) that cleave a collagen molecule into two fragments 3/4 & ¼ length.
Then by other matrix proteases, these fragments are degraded to individual amino acids.

24. Collagen diseases
Defects in any one of the steps of collagen fibre synthesis can result in a genetic disease involving inability of collagen to form fibres properly (& thus collagen will not provide tissues with the needed tensile strength).
More than 1000 mutations have be identified in 22 genes coding for 12 of the collagen types.

25. Collagen Diseases (cont.)
Ethlers-Danlos syndrome (EDS)
Results from inherited defects in the metabolism of fibrillar collagen molecules.
EDS can result from:
Deficiency in collagen-processing enzymes
as lysyl hydroxylase deficiency or procollagen peptidase deficiency
Mutations in the amino acid sequences of collagen type I, II, III, or type V.
(most important mutations are in of genes for type III)
Collagen containing mutant chains is not secreted. So, it is degraded or accumulated in cells.
Clinical Manifestations:
Lethal vascular problems of arteries (Collagen III is an important component of arteries)
Fragile & stretchy skin
Loose joints

26. Collagen diseases (cont.)
Osteogenesis imperfecta (OI)
(Brittle bone syndrome)
Bones easily bend & fracture
Retarded wound healing
Rotated & twisted spine leading to humped-back appearance