Download Proteins of extracellular matrix

Extracellular matrix
Jana Novotná
Department of Med. Biochemistry
2nd Faculty of Medicine
Charles University
Composition of Extracellular
Matrix (ECM)
• Cells (mesenchymal origin)
- fibroblasts
- smooth muscle cells
- chondroblasts
- osteoblasts and epitelial cells
• Organic fibrillar matrix
• Organic nonfibrillar matrix
• Water
Function of ECM
• Provides support and anchorage for cells.
• Regulates and determine cells dynamic behaviour :
- polarity of cells
- cell differentiation
- adhesion
- migration
•
Provides mechanical support for tissues and organ
architecture
- growth
- regenerative and healing processes
- determination and maintenance of the structure
•
Place for active exchange of different metabolites, ions,
water.
Structure of ECM
• collagen
– the main ECM component, forms the main fibres
• elastin
• proteoglycans
- heteropolysacharides
• structural glycoproteins
- fibronectin, laminin
Collagen
• The most abundant protein in the body, making 25%-35% of all the
whole-body proteins.
• 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
and skin.
• Plays an important role in cell differentiation, polarity, movement.
• Plays an important role in tissue and organ development.
Collagen Structure
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 –A–
–A–G –X–A–G–A–A–G –Y–A–G–A–A–G –X–A–G–
–A–A–G –X–A–G –A–A–G –Y–A–G–A–A–G –X–A–
G - glycine, X - proline or hydroxyproline, Y – lysin or hydroxylysine, A – amino
acid
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.
Synthesis
1. Synthesis of a chains of preprocollagen on ribosomes.
2. Hydroxylation of lysine and
proline in rER/Golgi by lysyl-5hydroxylase and prolyl-4hydroxylase. Reaction needs aketoglutarate, O2 and ascorbic
acid is necessary to activate the
hydroxylases and Fe2+ as a
cofactor.
3. Glycosylation: addition of
galactose and glucose to some
hydroxylysine residues
(galactosyl transferase and
glycosyl transferase).
4. Assembly of a-chains to form
procollagen. This involves the
formation of disulphide bonds
between parts of the
polypeptide chains known as
registration peptides, which
occur at both ends of the preprocollagen.
5. Secretion of procollagen molecules by exocytosis into the
extracellular space.
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 crosslinks. 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.
The typical staggered array of tropocollagen molecules in
the collagen fibril. The telopeptides participate in covalent
crosslinking.
Collagen – Fiber Formation
Collagen types I, II, III, V, IX, X
Cross striated structure of collagen fiber reflect periodic comosition of
individual tropocollagen molecules.
Collagen fibrils of 1 mm diameter support the weight of 10 kg.
Collagen Interactions
Fiber forming collagen and nonfibrous collagen
Tendon
Cartilagous matrix
Collagens Are Classified Into:
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
pulposus
III
[a1(III)]3
Skin, vessel wall, reticular fibres of
most tissues (lung, liver, spleen, etc)
V
a1(V)a2(V)a3(V)
Lung, cornea, bone, fetal membranes,
together with type I collagen
XI
a1(XI)a2(XI)a3(XI)
Cartilage, vitreous body
Basement membrane collagens
IV
[a1(IV)2a2(IV)]; a1 – a6
Basement membrane
Collagen IV has a short non-helical amino-terminal domain, a long Gly-X-Y repeat
domain with numerous small interruptions and a highly conserved carboxylterminal globular NC1 domain. It polymerizes into a disulfide-bonded polygonal
network via tetramerization between amino-terminal domains and dimerization
between NC1 domains.
Elastin
•
•
•
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
proline residues.
Tropoelastin is a ~65kDa
protein that is highly crosslinked to form an insoluble
complex.
Elastin
• The most common
interchain cross-link in
elastins is the result of the
conversion of the amine
groups of lysine to reactive
aldehydes by lysyl oxidase.
This results in the
spontaneous formation of
desmosine cross-links.
Proteoglycans
Proteoglycans represent a special class of
glycoproteins that are heavily glycosylated (95%).
They consisit of core protein with one or more attached
glycosamino glycan chain(s).
Glycosaminoglycans (GAG)
Glycosaminoglycan (GAG) chains are long, linear
carbohydrate polymers
under physiological conditions they are negatively charged
(due to the occurrence of sulfate and uronic acid groups).
Disaccharide subunits are:
1. uronic acid
D-glucuronic acid or L-iduronic acid
2. aminosugar
N-acetyl glucosamin (GlcNAc) or
N-acetyl galactosamin (GalNAc)
•
Amino sugars and uronic acids are the most common building blocks
of the glycosaminoglycans.
• In the amino sugars, the hydroxyl group at C-2 of the hexose is
replaced by an amino group. This amino group is most often acetylated
and sometimes sulfated.
• In the uronic acids, C-6 of the hexose is oxidized to a carboxyl group.
Linkage of GAGs to protein core by specific
trisaccharide linker
Hyaluronic Acid & Keratan Sulfate
Chondroitin 6-sulfate & Heparan
Supfate
Dermatan Sulfate & Heparin
Synthesis
• 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.
Synthesis
• 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
glycoproteins.
• 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.
Glycosaminoglycan Occurence
• 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
•
Dermatan sulfate
•
Heparan sulfate
•
Keratan sulfate
– cartilage, bone
– skin, blood vessels
– basement membrane, component of cells surface
– cornea, bone, cartilage, often aggregated with
chondroitin sulfate
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,
hydroxyapatite)
•
cell migration and adhesion
- passageways between cells
• anchoring cells to matrix fibers
Structural Glycoproteins
• Direct linkage to collagen or proteoglycans
– anchoring collagen fibers to cell membrane
– covalent attachment to membrane lipid
• Major adhesive structural glycoproteins
– fibronectin
– laminin
Fibronectin
• Fibronectin is a high-molecular weight
(~440kDa) glycoprotein
• It binds to membrane-spanning receptor –
integrin.
• Fibronectin also binds extracellular matrix
components such as collagen, fibrin and
heparansulfate proteoglycans.
• Cellular fibronectin is assembled into an
insoluble fibrillar matrix.
Fibronectin Structure
•
•
•
•
Dimer connected at C-terminal by S-S linkage
Rigid and flexible domains
Cell binding domain RGDS
(arg, gly, asp, ser)
- binding receptor in cell membranes
Domain is binding to
- collagen type I, II and III
- heparin sulfate
- hyaluronic acid
- fibrin
Fibronectin Function
•
•
•
•
Fibronectin is involved in cell adhesion,
differentiation, growth, migration.
Anchoring basal laminae to other ECM.
Along with fibrin, plasma fibronectin is deposited
at the site of injury, forming a blood cloth that
stops bleeding and protects the underlying
tissue.
Fibronectin is necessary for embryogenesis
(guiding cell attachment and migration during
embryonic development)
Laminin structure
and function
•
•
•
•
•
•
•
cross-shaped glycoprotein
3 polypeptide chains
domain bind
- collagen type IV
- heparin
- heparin sulfate
cell surface receptor
cell adhesion
cell differentiation
anchoring the glycoprotein to
basal laminae