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

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Extra cellular matrix:

Fibrous proteins – collagen, elastin

Specialised proteins – Laminin, Fibronectin

Gel forming – Proteoglycans

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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.

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Property of fibrous proteins

Alpha helical secondary structure.

Low water solubility

A long narrow rod like structure.

Role in determining cellular structure and function.

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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)

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Collagen –many functions in many tissues !!

Dispersed as a gel – Vitreous humor

Tight parallel fibres – Tendons

Stacked for minimal scattering – Cornea

Mechanical shearing – Bone.

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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

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Outline

1. Molecular collagen (pre pro collagen and pro collagen) – soluble

2. Microfibrils – tropocollagen ( insoluble)

3. Fibrils

4. Fibres

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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.

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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

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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.

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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.

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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

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(from Kadler, 1996)

C-telopeptide

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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)

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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

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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)

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• 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.

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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

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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.

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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

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Scurvy

Malaise , Lethargy Poor wound healing Bleeding gums Weak bones Petechiae over skin Anaemia

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Vasco Di

Gama

1498

1747, James Lind and the Limeys !!

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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

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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

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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

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