Function of ECM
• Provides support and anchorage for cells.
• Regulates and determine cells dynamic behaviour :
- polarity of cells
- cell differentiation
• Provides mechanical support for tissues and organ
- regenerative and healing processes
- determination and maintenance of the structure
• Place for active exchange of different metabolites, ions,
Structure of ECM
– the main ECM component, forms the main fibres
• structural glycoproteins
- fibronectin, laminin
• The most abundant protein in the body, making 25%-35% of all the
• Collagen contributes to the stability of tissues and organs.
• It maintains their structural integrity.
• It has great tensile strenght.
• The main component of fascia, cartilage, ligaments, tendons, bone
• Plays an important role in cell differentiation, polarity, movement.
• Plays an important role in tissue and organ development.
Collagen is insoluble glycoprotein (protein + carbohydrate)
Collagen polypeptide primary structure:
- G – X – A – G – A – A – G – Y – A – G – A – A – G – X – A – G −
G - glycine, X - proline or hydroxyproline, Y – lysin or hydroxylysine, A – amino
Proline and hydroxyproline constitute about 1/6 of the total
sequence, provide the stifness of the polypeptide chain.
Carbohydrates : glucose, galactose
• Three helical polypeptide units twist to form a triple-helical collagen
molecule: a molecular "rope" which has some bending stiffness and does
not undergo rotation.
• The tropocollagen molecule has a length of approximately 300 nm and a
diameter close to 1.5 nm.
• In the typical fibrillar collagens, only short terminal portions of the
polypeptides (the telopeptides) are not triple helical.
1. Synthesis of a chains of pre-procollagen
2. Hydroxylation of lysine and
proline in rER/Golgi by lysyl-5-
hydroxylase and prolyl-4-
3. Glycosylation: addition of
galactose and glucose to some
(galactosyl transferase and
4. Assembly of a-chains to form
procollagen. Reaction needs the
formation of disulphide bonds
between registration peptides,
at both ends of the prepro-collagen.
5. Secretion of procollagen molecules by exocytosis into the
6. Cleavage of registration peptides is catalysed by procollagen
peptidases. The resulting molecule is called tropocollagen.
7. Oxidation – deamination of the hydroxylysine, the removal of (NH2)
group has a net oxidative effect and the formation of covalent cross-links.
Reaction is catalyzed by lysine oxidase (or catalase).
8. Self-assembly or polymerization of tropocollagen molecules form
collagen fibrils. Cross-linkage between adjacent tropocollagen
molecules stabilizes the fibrils.
Posttranslation Modification of Collagen
Hydroxylation of some prolyl and lysyl residues
O2 (or superoxid)
vitamin C (ascorbic acid)
Proline + a-ketoglutarate + O2 + Fe2+ → 4-hydroxy-proline
+ Fe3+ + CO2 + succinate
Hydroxyproline stabilizes molecule of tropocollagen.
The typical staggered array of tropocollagen molecules in
the collagen fibril. The telopeptides participate in covalent
Collagen – Fiber Formation
Collagen types I, II, III, V, IX, X
Cross striated structure of collagen fiber reflect periodic composition of
individual tropocollagen molecules.
Collagen fibrils of 1 mm diameter support the weight of 10 kg.
1. Fibril-forming collagens (I, II, III, V, X)
2. Fibril-associated collagens (FACIT)
3. Network-forming collagens
4. Anchoring fibrils collagens
5. Transmembrane collagens
6. Basement membrane collagens
7. Other collagens with unique function
Major Collagen Types
Fibril forming collagens
(Most abundant collagen family - 90 % of the total collagens)
Type Molecular composition Tissue distribution
I [a1(I)2 2a(I)] Bone, dermis, tendon, ligaments cornea
II [a1(II)]3 Cartilage, vitreous body, nucleus
III [a1(III)]3 Skin, vessel wall, reticular fibres of most
tissues (lung, liver, spleen, etc)
Lung, cornea, bone, fetal membranes,
together with type I collagen
Cartilage, vitreous body
Basement membrane collagens
IV [a1(IV)2a2(IV)]; a1 – a6 Basement membrane
Short non-helical amino-terminal domain, a long Gly-X-Y repeat domain with
numerous small interruptions, and a highly conserved carboxyl-terminal globular
It polymerizes into a disulfide-bonded polygonal network via tetramerization
between amino-terminal domains and dimerization between NC1 domains.
• Elastin is a major protein component
of tissues that require elasticity such
as arteries, lungs, bladder, skin and
elastic ligaments and cartilage.
• It is composed of soluble tropoelastin
protein containing primarily glycine
and valine and modified alanine and
• Tropoelastin is a ~65kDa protein that
is highly cross-linked to form an
• Polypeptide chains are cross-linked
together to form rubberlike, elastic
fibers. Each elastin molecule uncoils
into a more extended conformation
when the fiber is stretched and will
recoil spontaneously as soon as the
stretching force is relaxed.
Jaime Moore , Susan Thibeault
Journal of Voice Volume 26, Issue 3 2012 269 - 275
• Desmosine (isodesmosine)
- the most common interchain
• conversion of NH3 groups of
lysine (hydroxylisine) to
reactive aldehydes by lysyl
• desmosine cross-link is
Special class of glycoproteins heavily glycosylated (95%).
Core protein with one or more
attached glycosamino glycan
• Long chain, linear carbohydrate polymers
• Negatively charged under physiological
conditions (due to the occurrence of sulfate and
uronic acid groups).
Disaccharide subunits are:
1. uronic acid
D-glucuronic acid or L-iduronic acid
N-acetyl glucosamin (GlcNAc) or
N-acetyl galactosamin (GalNAc)
Amino sugars and uronic acids are the most common building blocks of
• amino sugars -OH at C-2 is replaced by an amino group. This amino
group is most often acetylated and sometimes sulfated.
• uronic acids C-6 of the hexose is oxidized to a carboxyl group.
Linkage of GAGs to protein core by specific
• The protein component is synthesized by ribosomes and
transocated into the lumen of the RER.
• Glycosylation of the proteoglycan occurs in the Golgi
apparatus in multiple enzymatic steps.
• First a special link tetrasaccharide is attached to a
serine side chain on the core protein to serve as a
primer for polysaccharide growth.
• Then sugars are added by glycosyltransferase.
• Some glycosyltransferases catalyse sugar transfer to
tyrosine, serine and threonine to give O-linked
glycoproteins, or to asparagine to give N-linked
• Mannosyl groups may be transferred to tryptophan to
generate C-manosyl tryptophan
• The completed proteoglycan is then exported in
secretory vesicles to the extracellular matrix of the cell.
Proteoglycans can be categorised depending upon the
nature of their glycosaminoglycan chains.
• Hyaluronic acid (does not contain any sulfate)
– non-covalent link complex with proteoglycans
• Chondroitin sulfate
– cartilage, bone
• Dermatan sulfate
– skin, blood vessels
• Heparan sulfate
– basement membrane, component of cells surface
• Keratan sulfate
– cornea, bone, cartilage, often aggregated with chondroitin
Function of Proteoglycans
• organize water molecules
- resistent to compression
- return to original shape
- repel negative molecules
• occupy space between cells and collagen
• high viscosity
- lubricating fluid in the joints
• specific binding to other macromolecules
• link to collagen fibers
- form network
- in bone combine with calcium salts (calcium carbonate,
• cell migration and adhesion
- passageways between cells
• anchoring cells to matrix fibers
• Direct linkage to collagen or proteoglycans
– anchoring collagen fibers to cell membrane
– covalent attachment to membrane lipid
• Major adhesive structural glycoproteins
• High-molecular weight (~440kDa) glycoprotein
• Attached to cell membrane by membrane-spanning
receptor – integrin.
• Crosslinks and stabilizes other components of ECM
• Enhances cell addhesion to extracellular matrix
components (collagen, fibrin and heparansulfate
• Related to blood clotting - soluble FN crosslinks
platelets together using membrane bound heparin
• protein dimer connected at C-terminal by S-S linkage
• rigid and flexible domains
• cell binding domain RGD
(Arg, Gly, Asp)
- binding receptor in cell membranes
• RGD domain binds to
- collagen type I, II and III
- heparin sulfate
- hyaluronic acid
• related to cell adhesion, differentiation, growth,
• anchoring basal laminae to other ECM;
• plasma fibronectin forms a blood cloth, along with fibrin;
• related to cell movement - groups of embryonic cells
follow a FN pathway - FN guides macrophages into
• cross-shaped glycoprotein
• 3 polypeptide chains
• domain bind
- collagen type IV
- heparin sulfate
• cell surface receptor RGD
• cell adhesion
• role in cell differentiation
• anchoring the glycoprotein to
• Glycoprotein, essential for
formation of elastic fibers (a
sheath surrounding the amorphous
• Produced by fibroblasts.
• A group of three proteins, fibrillin-
1, -2 and -3.
• The main role - maintaining the
structural integrity of tissues, the
regulation of cytokines – TGF-b
• In humans, defects in the fibrillin-1
and fibrillin-2 genes have been
linked to diseases that affect the
cardiovascular, skeletal and ocular
systems, including Marfan
• Groups of
• mediate the attachment
between a cell and its
• integrins perform
outside-in signaling and
they also operate an
• Link cytoskeleton to ECM
• Fibronectin receptor is
• Abundant in the extracellular matrix of developing
• Tenascin-C contains an RGD motif and is recognized by
diverse integrins. Mainly synthesized by the nervous
system and connective tissues.
• Tenascin-R is found in the nervous system
• Tenascin-X and tenascin-Y are found primarily in muscle
• Tenascin-W is found in kidney and developing bone.