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SPECIALIZED TISSUE PROTEINS:COLLAGEN AND
ELASTIN
Dr.S.Chakravarty MBBS, MD
Learning objectives:
Describe the structure and formation of collagen and elastin
List the various steps in Post translational modification of collagen
Mention the role of Vitamin C and copper in stabilizing the collagen structure
List the types of collagen and its distribution in the body
Describe the defects of collagen and elastin and its associated clinical conditions
2
Extra cellular matrix:
Fibrous proteins – collagen, elastin
Specialised proteins – Laminin, Fibronectin
Gel forming – Proteoglycans
3
Functions of extracellular matrix
Regulation of proliferation, differentiation, migration and cell-cell recognition
Prevents or limits the movement of bacteria and cancer cells
Damage leads to various diseases like osteoarthritis, Glomerulonephritis etc.
4
Property of fibrous proteins
Alpha helical secondary structure.
Low water solubility
A long narrow rod like structure.
Role in determining cellular structure and function.
5
Types of fibrous proteins:
Col lagen - most abundant protein in body; rigid, insoluble.
Elasti n - stretchy, rubber-like, lungs, walls of large blood vessels, ligaments
Kerati n - tough fibers (hair, nails, outer epidermis)
6
Collagen –many functions in many tissues !!
Dispersed as a gel – Vitreous humor
Tight parallel fibres – Tendons
Stacked for minimal scattering – Cornea
Mechanical shearing – Bone.
7
Types of collagen:So far, 28 types of collagen have been identified and described. The five most common types are:
Fibril forming Tissue Function
Type 1 (90%) Tendon, bone, ligaments and skin Resistance to tension
Type 2 Hyaline and elastic cartilage Resistance to pressure
Type 3 Skin, muscle, blood vessels Structural framework for expanding tissues
Network forming
Tissue Function
Type 4 Basement membrane Filtration and support
Anchoring fibrils Tissue Function
Type 7 Epithelium Anchors basal cells to underlying stroma
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Type I collagen is stronger than steel !!
Structure of Collagen
9
Outline
1. Molecular collagen (pre pro collagen and pro collagen) – soluble
2. Microfibrils – tropocollagen ( insoluble)
3. Fibrils
4. Fibres
10
Collagen triple helix
Left handed helix
3 such strands wound together
Usmle!
About 25-30% of the total weight of body is collagen.
Major fibrous element of tissues like bone , teeth , tendons , cartilage and blood vessels.
Each polypeptide has about 1000 amino acid residues.
1/3 of the a.a are Gly residues i.e every 3rd residue is glycine.
The repetitive a.a sequences can be denoted by Gly-X-Y , where X and Y are commonly Proline and Hydroxyproline .
The collagen is a rod like structure .
The three polypeptide chains are held in a helical conformation by winding around each other.This results in formation of a superhelical cable with 3.3 amino acids per turn and each turn separated by 2.9 A.
The strands are H-bonded to each other ( H-donated by NH grp and H-accepted by C=O )
Further stabilization by H –bonds between OH- groups and the bridging water molecules.
15
Quarter staggered Arrangement The trophocollagen molecules are arranged in in such a
way that each row moves ¼ length over last row and the 5th row repeats the same position of the first row.
Molecules in each row separated by 400 A and adjacent and adjacent rows by 680 A.
The collagen fibres are further strengthened by covalent cross links b/w lysine and hydroxy-lysine
An electron micrograph of collagen from skin
Formation of pro alpha chains:
Cytoplasm of fibroblasts
Formation of pro alpha chains: with signal sequence at N-terminal ends.
Rough-endoplasmic reticulum: signal sequence directs proteins to RER.
Removal of signal sequence
18
Events in the RER :Formation of pro-collagen
Hydroxylation of proline and lysine Requires a dioxygenase with Fe .
(Vit C keeps the iron reduced ) Glycosylation – hydroxylysine with glucose. Spontaneous disulfide bond formation at C terminal
peptides
formation of triple helix.
3. Assembly in the Golgi and release of pro-collagen to extra cellular matrix.
19
USMLE concept!
4. Extracellular matrix
Stabilizing force H-bonding between Gly of one chain and Pro of another ~1 H-bond per triplet
Extra cellular cleavage of N and C-terminal propeptides – pro collagen peptidases.
20
Tropo collagen molecules:
Terminals (ends) of the triple-helix are different C-telopeptides N-telopeptides
Terminals are non-helical
Helps in triple helix formation N-TERMINAL INTRACHAIN
DISULPHIDE BONDS C-TERMINAL- INTERCHAIN +
INTRACHAIN DISULPHIDE BONDS
21
(from Kadler, 1996)
C-telopeptide
22
Elevated levels can be used in the confirmation of increased bone turnover.
Elevated levels can identify persons with osteoporosis who have elevated bone turnover and who, as a result, are at increased risk for rapid disease progression.
The patient's response to antiresorptive osteoporosis treatment can be monitored through this test.
This test can be used to monitor and assess how effective antiresorptive therapy has been in patients treated for disorders such as osteopenia, osteoporosis, and Paget disease.
This test can also serve as an adjunct means of monitoring patient response to other treatments for diseases with increased bone turnover, such as rickets & osteomalacia.
Cross links formed by lysyl/prolyl oxidase
- copper co-enzyme
Oxidative deamination of lysines and hydroxylysines forms Allysine (aldehyde) This reacts with amino group of nearby lysine or hydroxylysine to form interchain cross-link.
Very important for tensile strength of collagen.
Cu2+/vitamin B6
USMLE concept !
Both extremes are bad !! Excessive cross links
problem in OLD AGE Hardening of ligaments (STIFF)Prone to tear
Less cross links Weak collagen Menke’s disease
due to decreased Cu (discussed later)
24
Covalent X-links between Allysine and hydroxylysine
Tropocollagen molecule
triple helix of a-chains.
28Kaplan USMLE step 1 lecture notes
Elastin
Helps in retaining the shape after stretching.
Connective tissue protein.
lungs, large blood vessels, elastic ligaments
29
Elastic fibres
Outer cover - Microfibrils containing fibrillin and microfibril associated glycoproteins (15%)
Core of amorphous elastin –single polypeptide chain of 800 amino acids-85%
Non-polar amino acids – gly, ala, val. Also rich in pro, lysine. ( no OH-proline or OH-lysine)
30
• 3D network of cross-linked polypeptides – (tropo elastin)
• cross links involve Lys and alLys –lysyl oxidase
• 4 Lys can be cross-linked into desmosine
• Desmosines account for elastic properties
Elastin
Desmosine
Elastin Structure and Function Elastin interconverts between a number of conformations, both
disordered (upper two on left) and b-spiral (bottom left). After cross-linking, when elastin is stretched (or compressed) it is
less stable and it returns to the disordered conformations.
6
Elastin Cross-linking
Some lysine residues in elastin are deaminated and oxidized to the aldehyde level.
They combine with each other and with other lysines to form lysinonorleucine and desmosine cross-links
NH
CH
O
CH2 CH2 CH2 CH2NH CH2
CH2CH2
NH
CH
O
CH2
lysinonorleucine
+CHNH
CH
CH2
CH2
CH2 CH
CH2
CH2
CH2
CH2
CH
CH2CH2CH
NH
NH
NH
OO
O
CH2
CHNH
O
CH2
desmosine
7
USMLE concept !
Major Differences Between Collagen and Elastin
Collagen Elastin 1. Many different genetic types One genetic type
2. Triple helix No triple helix; random coil conformations permitting stretching
3. (Gly-X-Y)n repeating structure No (Gly-X-Y)n repeating structure
4. Presence of hydroxylysine No hydroxylysine
5. Carbohydrate-containing No carbohydrate
6. Intramolecular aldol cross-links Intramolecular desmosine cross-links
7. Presence of extension peptides No extension peptides present during biosynthesis during biosynthesis
Degradation of elastin:
Serine type elastase: neutrophils, macrophages, fibrblasts.
Matrix metalloproteinases – mmp-12 and 7, gelatinases.
35
Keratin Keratin is rich in cysteines.
Its secondary structure is mostly a-helical.
The helices form coiled coils (on right).
The coiled coils pack into higher order elongated structures.
Keratin properties depend strongly on the degree of disulfide cross-linking. With low levels of cross-linking, it is flexible (hair, skin). It can be made very hard with additional cross-linking
(claws, horns).8
2 nm
Keratin Cross-linking
The structure of keratin is strengthened by disulfide cross-links from one helix to another.
CH CH2 S
NH
O
CHCH2
NH
S
O
CH CH2 SH
NH
O
disulfidecross-link
CHCH2
NH
SH
O
two cysteines
10
Scurvy
Malaise , Lethargy Poor wound healing Bleeding gums Weak bones Petechiae over skin Anaemia
39
40
Vasco Di
Gama
1498
1747, James Lind and the Limeys !!
41
MCQ 1
A culture of fibroblast cells is provided with equal all the 20 amino acids. After 10 days , the concentration of the amino acids is assessed .Which amino acid will have the lowest concentration?
A. Lysine B.methionine C.Glycine D.proline E.Cysteine
42
1. Elastin fibres in the alveolar walls of the lungs can be stretched easily during inspiration and recoil to their original shape once the force is released. This process facilitates expiration. The property described can be best explained by:
a) Heavy posttranslational hydroxylationb) High content of polar amino acidsc) Chain assembly to form a triple helixd) Interchain crosslinks involving lysinee) Abundant interchain disulfide bridges
43
3. A 14-year old male presents to your office complaining of easy bruising. Physical examination reveals soft and loose skin as well as multiple ecchymoses in the forearm and pretibial regions. Histologic evaluation with electron microscopy shows collagen fibrils that are abnormally thin and irregular. Which of the following stages of collagen synthesis is most likely impaired in this patient?
a) RNA signal sequence recognitionb) Amino acid incorporation into polypeptide chainc) Triple helix formationd) Lysine residue hydroxylatione) cleavage of propeptides
44