Extracellular Matrix

Question 1

What are the types of ECM proteins?

Answer 1

Proteoglycans
Structural proteins like collagen, elastin, fibrillin
Specialiazed proteins like fibronectin and laminin

Question 2

Where are proteoglycans found?

Answer 2

Synovial joints (compressibility properties)
Vitreous humor of the eye
Arterial walls (strength)
Bone and cartilage

Question 3

What are the types of interactions proteoglycans do?

Answer 3

Interact with:
Collagen and elastin-play structural role
Fibronectin-involved in cell adhesion and migration
Laminin-found in basal laminae such as the renal glomerulus

Question 4

What is the structure of the proteoglycans?

Answer 4

Core protein
Covalently attached long, linear chains of glycosaminoglycans which are repeating disaccharide units (hexosamine+uronic acid)
Frequently sulfated
Negatively charged and repeal each other
Act as a lubricant
Occupy large space
Act as molecular sieves-Control which substances approach and leave the cell
Give flexibility to cartilage for compression and re expansion

Question 5

What are the type of proteoglycans?

Answer 5

7 which differ in the monosaccharides present in their repeating disaccharide units
Hyaluronic acid
Chondroitin sulfate
Dermatan sulfate
Heparin
Heparan sulfate
Keratan sulfate I and II

Question 6

How is the binding of proteoglycans?

Answer 6

All proteoglycans are attached to protein by covalent linkage to serine or threonine residues
Keratan sulfate I is attached via asparagine residue
Hyaluronic acid binds to proteins through non covalent linkage

Question 7

What is the process for synthesis of proteoglycans?

Answer 7

Synthesis start with the attachment of a sugars to a serine or threonine residues once a core protein enters the lumen of the ER
UDP-sugars are the substrate for synthesis
Specific UDP-sugar glycosyl transferases are responsible for sequential transfer of monosaccharides
Synthesis starts with linking sugar synthesis
Once the linking sugars are attached, 2 alternating glycosyltransferases add the sugars of the repeating disaccharide to the growing glycosaminoglycan chain

Question 8

What is the process of sulfations of proteoglycans?

Answer 8

Sulfate groups attached through N or O-sulfation
Occurs after addition of sugars
3’-phosphoadenosine 5’-phosphosulfate (PAPS) provides the sulfate groups.
Only way sulfate is actively transfer in biological system is through PAPS

Question 9

What are the function of proteoglycans?

Answer 9

Hyaluronic acid-cell migration, embryogenesis, morphogenesis, wound healing
Chondroitin sulfate-formation of bone, cartilage and cornea
Keratan sulfate-Transparency of cornea
Dermatan sulfate-Transparency of cornea, binds LDL to plasma wall
Heparin- Anticoagulant , binds to antithrombin III, releases lipoprotein lipase from capillary wall
Heparan sulfate-Component of skin fibroblast and aortic wall, commonly found on cell surface

Question 10

What is the structure of the glycoproteins?

Answer 10

Contain much shorter carbohydrates
Carbohydrate moiety is branched and is not made of repeating disaccharides
Most plasma proteins in the blood are glycoproteins

Question 11

What is the process of glycoproteins synthesis?

Answer 11

Carbohydrate chains are added in the lumen of ER and the Golgi
The initial sugar is added to the serine or threonine residues for O-linkage and asparagine residues for N-linkage
Carbohydrate chain is extended by the sequential addition of suga residues to the non reducing end

Question 12

What are the 2 types of oligosaccharide synthesis ?

Answer 12

O linked

N-linked

Question 13

What is the process of. Synthesis of O-linked glycosides?

Answer 13

Glycosylation begins with the transfer of N-acetylgalactosamine onto a specific serine or threonine
Glycoproteins destined to the plasma membrane will be integrate to the Golgi membrane with glycosyl moiety facing the Golgi lumen
Glycoproteins secreted from the cell remain freee in the lumen
Secondary vesicles bud off from the Golgi and fuse with the cell membrane releasing glycoproteins to the extracellular space

Question 14

What is the process of N-linked glycosides?

Answer 14

Initially protein does not become glycosylation with individual sugars
Instead a lipid linked oligosaccharide is first constructed
Sugars are added to the dolichol by the membrane bound glycosyltransferases
Oligosaccharide is transferred from the dolichol to an asparagine side group of the protein by a protein oligosaccharide gtransferase enzyme present in the ER

Question 15

What is the process of degradation of proteoglycans and glycoproteins?

Answer 15

Proteoglycans and glycoproteins are brought inside the cell by endocytosis
They are degrade by lysosomal enzymes
The carbohydrate moiety is degraded by glycosidases
Lysosomes contains endoglycosidases and exoglycosidases
Endoglycosidases cleave carbohydrate chains to shorten oligosaccharides
Exoglycosidases specific for each type of linkage remove the sugar residue from the non reducing end

Question 16

What is the structure of the collagen?

Answer 16

Triple helical structure
Each polypeptide is twisted into a left handed twist into a left handed helix of 3 residues per turn
Helix structure stabilizes by hydrogen bonding
Glycine residues occur at every 3 position of the alpha chain
Repeating structure of Gly-X-Y
Prolines and hydroxyprolines increase rigidity of the collagen molecule

Question 17

How is the process of synthesis of collagen?

Answer 17

Collagen is synthesized as a preprocollagen
Contains a leader or signal sequence which is removed in the ER
Some hydroxylysines residues are further modified by the addition of galactose through an o-glycoside linkage
No further modification occurs after formation of triple helix

Question 18

Examples of nonhelical collagens

Answer 18

Several collagen do not form fibrils
Example collagen 4
Characterized by interruptions of the triple helix with stretches of protein lacking Gly-X-Y repeats

Question 19

What are some genetic diseases of collagen synthesis?

Answer 19

Ehlers-Danlos Syndrome
Alport syndrome
Scurvy

Question 20

What are the characteristics of Ehlers-Danlos Syndrome?

Answer 20

Group of genetic diseases with hyperextensibility of the skin, abnormal tissue fragility and increased joint mobility
Several types
Type 4- most serious due to the tendency for spontaneous rupture of arteries or the bowel
Type 6 due to deficiency of Lysol hydroxylase. Patients exhibits marked joint hypermobility and tendency to ocular rupture
Type 7C-deficiency of N proteinase causing formation of abnormal thin, irregular collagen fibrils manifested by marked joint hhypermobility

Question 21

What are the characteristics of the Alport syndrome?

Answer 21

X linked or autosomal genetic disease affecting type 4 collagen
Patients are presented with hematuria and may eventually develop end stage renal disease

Question 22

What are the characteristics of Scurvy?

Answer 22

Due to deficiency of ascorbique deficiency necessary for cross linking of collagen structure
Major signs are bleeding gums, subcutaneous hemorrhages and poor wound healing
Signs reflect deficiency of prolly and Lysol hydroxylase.

Question 23

What is the characteristics of the elastin?

Answer 23

CT protein
Responsible for tissues extensibility and elastic recoil
Give tissues and organs the capacity to stretch without tearing
Synthesized in a single tropoelastin molecule
Does not have hydroxylysine or glycosylated hydroxylysine
Have no extension peptides
Have no Gly-X-Y arrangement and no triple helical structure
Major cross links are desmosines(3 oxidized lysine + non modified lysine)
Cross links highly insoluble and extremely stable with very low turn over

Question 24

What is the characteristics of Williams Syndrome?

Answer 24

Elastin gene deletions found in 90% of patients
Development disorder affecting the CT and the CNS
May result in the supravalvular aortic stenosis

Question 25

What are the characteristics of the fibrillin?

Answer 25

Large glycoproteins
Structural component of microfibrils 
Found in many tissues
Secreted into ECM by fibroblasts
Incorporated into the insoluble microfibrils which appear to provide scaffold for deposition of elastin

Question 26

What are the characteristics of Marfan Syndrome?

Answer 26

Autosomal dominant disease affecting CT
Due mostly to fibrillin mutations
Most patients are tall and exhibit long digit (arachnodactyly)
Hyprextensibility of joints, cardiovascular system

Question 27

What are the characteristics of Fibronectin?

Answer 27

Soluble glycoproteins found in large amount in ECM
V shape
Consist of 2 peptide chains joined by disulfide bonds
Contains 7 domains binding to heparin, fibrin, collagen, DNA and cell surfaces
Cell fibronectin-receptor belongs to family of transmembrane integrin proteins
Fibronectin receptors contains Arg-Gly-Asp (RGD) sequence that binds to integrin

Question 28

What are the characteristics of the laminin?

Answer 28

Consists of 3 distinct elongated poly[peptide chains link together to form an elongated cruciforme shape (cross shape)
Has binding sites for type 4 collagen, heparin and integrin