• Extra Cellular Matrix (ECM)
• Structure, Synthesis, Function, Disease
The Extra Cellular Matrix: ECM
• Extra Cellular: outside the cell
Matrix: structure made from a network of
interacting components
The ECM is composed of an interlocking mesh of
fibrous proteins and glycosaminoglycans (GAGs).
Components of the ECM are produced
intracellularly by resident cells, and secreted into
the ECM via exocytosis.
Functions of ECM
1-Role in establishing and maintaining cell shape,
migration, mechanical support
2-Anchorage for cells, segregating tissues from one
another,
and
regulating
intercellular
communication
3-Sequesters a wide range of cellular growth factors,
and acts as a local depot for them
4-Essential for processes like growth, wound
healing etc
What are the major proteins of the ECM?
Collagens, Proteoglycans, Elastin, Fibronectin, Laminin, Tenascin.
The collagens
• A family of fibrous proteins found in all
multicellular animals
• They are secreted by connective tissue cells, as
well as by a variety of other cell types
• They are the most abundant proteins in
mammals, constituting 25% of the total protein
mass in these animals
Structural features
• Long, stiff, triple-stranded
helical structure,
• Three collagen polypeptide
chains, called α chains, are
wound around one another in a
ropelike superhelix
• A basic unit of mature collagen
is called tropocollagen
Composition of collagens
• Collagens are extremely rich in proline and
glycine
• It is composed mainly of glycine (33%),
proline (13%), 4-hydroxyproline (9%)
• Hydroxyproline is unique for collagen and
elastin
Amino acid sequence
• Every third residue is glycine which lies in the
center of the triple helix, with the preceding
residue being proline or hydroxyproline in a
repetitive fashion
– pro-Gly-X
– hydroxypro-Gly-X
Functions of amino acids
• Proline stabilizes the helical conformation in
each a chain
• Glycine allows the three helical a chains to
pack tightly together to form the final collagen
superhelix
Hydroxylysine
• Collagen is also
composed of
hydroxylysine, which
serves as attachment
sites of polysaccharides
making collagen a
glycoprotein
Lysine
• Part of the toughness of collagen is accounted
by the cross-linking of chains via lysine
residues
How?
• Some of the lysine
side chains are
oxidized to aldehyde
derivatives, which
react with another
lysine or another
oxidized lysine via
the action of lysyl
oxidase
Types of collagens
• There are about 40 collagen genes dispersed
throughout the genome and the protein
products combine to form more than 28
different types of collagen.
• The various collagens and the structures they
form all serve the same purpose, to help tissues
resist stretching.
Classification of collagen
1. Fibril-forming collagens
 No interruptions in triple helix
 Regular arrangement results in characteristic “D” period of 67 nm
 Diameter : 50-500 nm
 Example : Types I, II, III, V, XI
Classification of collagen
2. Network-forming collagens
 Forms network in basement (Collagen IV) and Descemet’s
membrane (Collagen VIII)
 Molecular filtration
 Example : Types IV, VIII, X
Classification of collagen
3. Fibril-associated collagens with interrupted triple helices
(FACITs)
 Short collagens with interruptions
 Linked to collagen II and carries a GAG chain
 Found at the surface of fibril-forming collagens
 Example : Types IX, XII, XIV
Classification of collagen
4. Anchoring collagens
 Provides functional integrity by connecting epithelium to
stroma
 Example : Type VII
Classification of collagen
5. Beaded-filament-forming collagens
 Form structural links with cells
 Example : Type VI
 Collagen VI crosslink into tetramers that assemble into long
molecular chains (microfibrils) and have beaded repeat of 105 nm
Synthesis of collagen
•
Individual collagen polypeptide chains are
synthesized on membrane-bound ribosomes and
injected into the lumen of the endoplasmic reticulum
(ER) as larger precursors, called pro-a chains
•
In the lumen of the ER, selected prolines and lysines
are hydroxylated to form hydroxyproline and
hydroxylysine, respectively, and some of the
hydroxylysines are glycosylated
•
Each pro-a chain then combines with two others to
form a hydrogen-bonded, triple-stranded, helical
molecule known as procollagen
(Continued)
Synthesis of collagen
•
During or following exocytosis, extracellular
enzymes, the procollagen peptidases, remove
the N-terminal and C-terminal propeptides
•
The
resulting
protein,
often
called
tropocollagen (or simply collagen), consists
almost entirely of a triple-stranded helix.
•
Excision of both propeptides allows the
collagen molecules to polymerize into normal
fibrils in the extracellular space
Collagen-related diseases
• Collagen is highly cross-linked in tissues
where tensile strength is required such as
Achilles tendon
• If cross-linking is inhibited, the tensile strength
of fibers is greatly reduced, collagenous tissues
become fragile, and structures tend to tear
(skin, tendon, and blood vessels)
Diseases associated with collagen
Diseases caused by mutations
 Subtypes of osteogenesis imperfecta (collagen I)
 Ehlers-Danlos syndrome (collagen I and V)
 Alport syndrome (collagen IV)
 Certain arterial aneurysms (collagen III)
 Ullrich muscular dystrophy (collagen VI)
 Certain chondrodysplasias (collagen IX and XI)
 Kniest dysplasia (collagen II)
Scurvy
• The formation of hydroxyproline requires vitamin C
• Deficiency of vitamin C results in insufficient
hydroxylation of proto-collagen and, hence, poor
synthesis of collagen, formation of unstable triple
helices preventing formation of normal fibrils
• Non-hydroxylated procollagen
degraded within the cell
chains
are
then
• This results in weakening of the collagen resulting in
skin and gum lesions and weak blood vessels
Types of OI
• At least four types of osteogenesis imperfecta
• Designated as type I through type IV
• Type I osteogenesis imperfecta is the mildest
form of the condition
• Type II is the most severe results in death in
utero or shortly after birth
• Milder forms generate a severe crippling
disease
Mutations of OI
• Mutations in the COL1A1 and COL1A2 genes cause OI
• These mutations typically interfere with the assembly
of type I collagen molecules
• A defect in the structure of type I collagen weakens
connective tissues, particularly bone, resulting in the
characteristic features of OI
• OI types I, II, and IV have an autosomal dominant
pattern of inheritance, which means one copy of the
altered gene in each cell is sufficient to cause the
condition
Chondrodysplasias
• Mutations affecting type II collagen cause
chondrodysplasias, characterized by abnormal
cartilage, which leads to bone and joint
deformities
Ehlers-Danlos syndrome
• A heterogenous group of disorders that affect
connective tissues, which are tissues that support the
skin, bones, blood vessels, and other organs
• The signs and symptoms of Ehlers-Danlos syndrome
vary from mildly loose joints to life-threatening
complications
Mutations in Ehlers-Danlos syndrome
• Ehlers-Danlos syndrome results from defects
in synthesis of either collagen molecules type
I, III, or V or in the synthesis of collagen
processing enzymes like procollagen Npeptidase, or lysyl hydroxylase resulting in
mobile joints and skin abnormalities
Non-collagen component of Bone
Matrix
Made up of stiffening substances to resist bending and compression
(Inorganic matter ).
The bone mineral is an analogue of crystals of calcium phosphate —
hydroxyapatite Ca10(PO4)6(OH)2, a substance that can only be seen
under electron microscopy.
It is this association of hydroxyapatite with collagen fibres which is
responsible for the hardness of bone.
Elastin
• The main component of elastic fibers is elastin
• A highly hydrophobic protein, which, like
collagen, is unusually rich in proline and
glycine
• But, unlike collagen, is not glycosylated
• Contains some hydroxyproline but no
hydroxylysine
Formation of elastic network
• Soluble tropoelastin (the biosynthetic precursor of
elastin) is secreted into the extracellular space and
assembled into elastic fibers close to the plasma
membrane
• After secretion, the tropoelastin molecules become
highly cross-linked to one another, generating an
extensive network of elastin fibers and sheets
• The cross-links are formed between lysines by a
mechanism similar to that of collagen molecules
Elastin structure
• The elastin protein is composed largely of two
types of short segments that alternate along the
polypeptide chain:
– hydrophobic segments, which are responsible for the elastic
properties of the molecule; and
– alanine- and lysine-rich a-helical segments, which form
cross-links between adjacent molecules
Function of elastic fiber
• Elastin is the dominant extracellular matrix
protein in arteries
• Mutations in the elastin gene causing a
deficiency of the protein result in narrowing of
the aorta or other arteries as a result of
excessive proliferation of smooth muscle cells
in the arterial wall
• Apparently, the normal elasticity of an artery is
required to restrain the proliferation of these
cells
Diseases of Elastic Fiber
•
•
•
•
•
•
Cutis laxa
Williams syndrome
Buschke-Ollendorff syndrome
Menkes disease
Pseudoxanthoma elasticum,
Marfan's syndrome
– defects in copper metabolism (lysyl oxidase)
Glycoproteins and Proteoglycans
Glycoproteins
Proteins conjugated to
saccharides lacking a
serial repeat unit
Protein >> carbohydrate
Proteoglycans
Proteins conjugated to
polysaccharides with
serial repeat units
Carbohydrate >> protein
Glycosaminoglycans
Mucopolysaccharides
Glycoproteins
• Proteins that contain oligosaccharide chains (glycans)
covalently attached to polypeptide side-chains, in a cotranslational or posttranslational modification.
• (N- Glycosylation), the addition of sugar chains can
happen at the amide nitrogen on the side chain of
the asparagine.
• (O- Glycosylation), the addition of sugar chains can
happen on the hydroxyl oxygen on the side chain
of hydroxy-lysine, hydroxy-proline, serine, or threonine.
Functions of Glycoproteins
•
•
•
•
•
•
•
Structural
Reproduction
Hormones
Enzymes
Carriers
Inhibitors
Immunological
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