Biochemistry Wk 6

Question 1

what is the ECM

Answer 1

Many of the cells in tissues are embedded in an extracellular matrix

determines the shape of tissues

Question 2

Basic components of ECM:

Answer 2

structural proteins (e.g., collagens)
- proteoglycans (GAGs + protein backbone)
- adhesion proteins (link components of matrix to each other and to cells)
* Collagens, elastin and laminin are the principal structural proteins of connective tissue

Question 3

Fibrous structural proteins

Answer 3

These fibrous structural proteins are composed of repeating elements that form a linear structure.
The principal structural proteins of connective tissue are:
* Collagens
* elastin
* laminin

Question 4

Collagen

Answer 4

A typical collagen molecule is a long, rigid structure in which three polypeptides are wound around one another in a rope-like triple helix.
* The three polypeptide α chains are held together by interchain hydrogen bonds.

Question 5

types of collagen

Answer 5

type I- bone, skin, tendon, cornea, late wound repair

type II- cartilage, nucleus pulpous, vitreous body

type III- reticulin- skin, blood vessels, uterus, fetal tissue, early wound repair

type IV- basement membrane, lens

Question 6

synthesis and secretion of collagen

Answer 6

synthesis of preprocollagen, insertion of procollagen molecule into the lumen of the ER.

hydroxylation of proline and lysine residues, glycosylation of selected hydroxylysine residues requires vitamin c (cofactor)

self assembly of the tropocollagen molecule, initiated by the disulfide bond formation in the carboxyl terminal extensions , triple helix formation

procollagen is secreted from the cell by exocytosis

cleavage of the properties, removing the amino and carboxyl terminal extensions and self assembly of the collagen molecules into fibrils and then fibres Enzyme: lysyl oxidase for cross linking of collagen fibrils

Question 7

Elastin

Answer 7

Elastin is the major protein found in elastic fibers, which are located in the ECM of connective tissue of:
* smooth muscle cells,
* endothelial cells,
* microvascular cells,
* chondrocytes,
* fibroblasts.

Question 8

synthesis of elastin

Answer 8

gene- DNA- ELASTIN TRNALSATION- PROLYL HYDROXYLATION- SECRETION AND EXCISION- TROPOELASTIN ( CATALSAYED BY TROPOELASTIN PROTEASE)- TROPOELASTIN COMBINES WITH MICROFIBRILLAR PROTEIN) CROSSLINKED INTO MATURE FIBRES -> ELASTIC FIBRES

Question 9

Laminin signal transduction - Adhesion.

Answer 9

The laminin structure is a coiled α-helix, which joins the three subunits together and forms a rigid rod.
* All three chains have extensions at the amino-terminal end.
* Only the α-chain has a significant carboxyl-terminal extension that
allow laminin to bind to other components within the ECM and to
provide stability for the structure.
* Laminin 111, composed of α1β1γ1, is typical of this class of
proteins.
↑Camma

Question 10

Junctional epidermolysis
bullosa

Answer 10

Defects in the structures of laminin 5 or laminin 6 (proteins that contribute to the cohesion of the dermis and epidermis)
* severe spontaneous blistering of the skin and mucous membranes.
* A severe form of the disease is often fatal early in life.
* Death occurs as a result of epithelial blistering of the respiratory, digestive, and genitourinary
systems.

Question 11

Proteoglycans

Answer 11

• The fibrous structural proteins of the ECM are embedded in gels formed from proteoglycans.
• Proteoglycans consist of polysaccharides called glycosaminoglycans (GAGs) linked to a core protein
• The GAGs are composed of repeating units of disaccharides

Question 12

GAG- hyaluronic acid

Answer 12

cell migration in:
embryogenesis
morphogenesis and wound healing

Question 13

Synthesis of the Proteoglycans

Answer 13

The protein component of the proteoglycans is synthesized on the ER. It enters the lumen of this organelle, where the initial glycosylations occur. Uridine diphosphate (UDP)-sugars serve as the precursors that add sugar units, first to the protein and then to the nonreducing end of the growing carbohydrate chain. Glycosylation occurs initially in the lumen of the ER and subsequently in the Golgi complex.

After synthesis, the proteoglycan is secreted from the cell.
Its structure resembles a bottle brush, with many GAG chains extending from the core protein .

The proteoglycans may form large aggregates, noncovalently attached by a “link” protein to hyaluronic acid

Question 14

Interactions between the cell membrane and the components of ECM

Answer 14

• The proteoglycans interact with adhesion protein, fibronectin, which is attached to the cell membrane protein integrin
• cross-linked fibers of collagen also associate with this complex - ECM

Question 15

Degradation of Proteoglycans

Answer 15

Lysosomal enzymes degrade proteoglycans, glycoproteins, and glycolipids, which are brought into the cell by the process of endocytosis.
* Lysosomes fuse with the endocytic vesicles, and lysosomal proteases digest the protein component.

Question 16

Degradation of Proteoglycans

Answer 16

Degradation of Proteoglycans
The carbohydrate component is degraded by lysosomal glycosidases.
Lysosomes contain both endoglycosidases and exoglycosidases.
The endoglycosidases cleave the chains into shorter oligosaccharides.
exoglycosidases remove the sugar residues from the nonreducing ends.

Question 17

Mucopolysaccharidoses

Answer 17

Deficiencies of lysosomal glycosidases cause partially degraded carbohydrates from proteoglycans, glycoproteins, and glycolipids to accumulate within membrane-enclosed vesicles inside cells.
* These “residual bodies” can cause marked enlargement of the organ, with impairment of its function.
* This condition known as the mucopolysaccharidoses

Question 18

hurler syndrome

Answer 18

developmental delay, corneal clouding, hepatosplenomegaly

alpha L iduronidase

Heparan sulfate
demartan sulfate

Question 19

hunter disease

Answer 19

mild hurler- aggressive behaviour, no corneal clouding

iduronate 2 sulfatase

heparan sulfate
dermatan sulfate

Question 20

Integrins

Answer 20

Integrins are transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion. Integrins have two main functions, attachment of the cells to the ECM and signal transduction from the ECM to the cells.
They are also involved in a wide range of other biological activities, including extravasation, cell-to-cell adhesion, cell migration, and as receptors for certain viruses, such as adenovirus, echovirus, hantavirus
The presence of integrins allows rapid and flexible responses to events at the cell surface

Question 21

Adhesion Proteins

Answer 21

fibronectin is a prime example
* Fibronectin is a high-molecular weight glycoprotein of the extracellular matrix that binds to integrins.
* Fibronectin also binds to other extracellular matrix proteins such as collagen, fibrin, and heparan sulfate proteoglycans
* It is involved in cell adhesion, growth, migration, and differentiation.
* Loss of adhesion protein capability can lead to either physiologic or abnormal cell movement

Question 22

Matrix Metalloproteinases

Answer 22

Matrix metalloproteinases (MMPs) are calcium-dependent zinc- containing endopeptidases

• These enzymes are capable of degrading all kinds of extracellular matrix proteins, but also can process a number of bioactive molecules.
• They are known to be involved in the cleavage of cell surface receptors, the release of apoptotic ligands , and chemokine/cytokine inactivation.
• MMPs are also thought to play a major role in cell behaviors such as cell proliferation, migration (adhesion/dispersion), differentiation, angiogenesis, apoptosis, and host defense.

Question 23

Regulation of MMP activity

Answer 23

Regulatory processes include transcriptional regulation, proteolytic activation, inhibition by the circulating protein α2-macroglobulin, and regulation by a class of inhibitors known as tissue inhibitors of metalloproteinases, or TIMPs.
It is important that the synthesis of TIMPs and MMPs be coordinately regulated because dissociation of their expression can facilitate various clinical disorders, such as certain forms of cancer and atherosclerosis.