Lecture 27 Flashcards
What is extracellular matrix (ECM)?
Spaces in connetive tissue + interstitial matrix (space between cells) + basement membrane
Components of ECM
Fibers such as collagen and elastin
Ground substance - space-filler gluethat hold cells and fibers together
Function of ECM
- Supporting framework
- Provide organized environment within which migratory cells can move and interact with one another
- Complex role in regulating the behavior of the cells it contacts
What does mechanical property of ECM depend on?
Amount of fibers
Type of fibers
Orientation of fiber
What is responsible for constant remodeling of ECM?
Matrix metalloproteinases (MMP) which need zinc and collagenases
Relationship between spread of cancer and matrix metalloproteinases
MMP is responsible for remodelling of ECM, so dysregulation of MMP can lead to spread of cancer cells.
General characteristics of fibrous protein (4)
low water solubility
contain large amounts of repetitive secondary structure
long cylindrical shape for support
structural rather than dynamic role
Characteristic of collagen (3)
- insoluble proteins secreted from cell
- found in all tissue and organs (but soft tissues have lower amount while structural organs have high)
- major ECM protein
Structure of collagen
triple-helix structure, composed of 3 long individual polypeptides (alpha-chains) organized parallel to a single axis
- All 3 alpha chains are different (same length but different aa seq)»_space; different mechanical properties
- homotrimeric : 3 identical alpha chain
- heterotrimeric : consisting of 2 or 3 distinct alpha chain
- Alpha chain not same as alpha helix
How is variation of alpha chains possible?
each alpha chain is encoded by separate gene, and different combinations of genes are expressed in different tissues
Primary structure of triple helix of collagen
1) repetitive primary structure rich in proline and lysine
- conserved sequence : Gly-X-Y-Gly-X-Y
Glycine :every 3rd position of chain which allow 3 helices to pack tightly together since it is small.
X: frequently proline which cause kinks in peptide chain, thus helps in forming helical structure (more proline, more kink, more strength)
Y: proline or lysine
- End of alpha chains do not have conserved sequences
2) Hydroxylation of proline and lysine by prolyl hydroxylase with its cofactor ‘ascorbate’ (occurs post-translationally prior to triple helix formation)
3) Glycosylation of hydroxylysine by galactose or glucose (allow mineralization in bone)
Why are proline and lysine hydroxylated in collagen?
It increases stability of triple helix by maximizing interchain hydrogen bonding (more OH, more H-bond, more structural support)
Collagen biosynthesis
- Pre-pro-alpha-chain with ER signal sequence produced from translation
- Once inside ER, ER signal is removed and proline and lysine residues are hydroxylated.
- Sugars are added to selected hydroxylysine residues by glucose or galactose, forming pro-alpha-chains
- Intra and interchain disulfide bridges form between 3 pro-alpha-chains, facilitating helix formation., forming pro-collagen.
* End of triple helix is nonhelical since it doesn’t have conserved seq, and they are called terminal extensions or propeptides - Pro-collagen is contained vesicle and bud off from ER to golgi
- Pro-collagen is packaged and secreted to cell wall where it fuses and released into extracellular space.
- terminal extensions or propeptides are cleaved by C- or N-pro-collagen peptidases, forming tropocollagen. (still soluble in water and needs more modification)
Steps of fibillogenesis
- Synthesized collagens align in a staggered fashion each over-lapping its neighbors.
- Crosslinking takes place by action of lysyl oxidase, forming collagen fibrills
- Lysyl oxidase react with amino group of lysine residue to change it into aldehyde group making it into allysine residue. Then, unmodified amino group of other lysine will react with aldehyde group of allysine to form a covalent bond = cross link (nucleophilic addition rxn)
* Lysyl oxidase is copper-dependent
Effect of copper deficiency on collagen fibril formation
lysyl oxidase which is responsible for crosslinking of collagen molecule is dependent on copper, so low copper supply leads to reduced cross-linking which leads to weaker stability and structural support of collagen
What is gap between adjacent collagen molecule along fibril involved
Bone formation; site of bone mineralization (e.g. collagen in teeth and bone is impregnated with calcium phosphate)
Sugar attached to 5’-hydroxylysyl residues in the hole region may be involved in organization of fibril assembly.
Causes of collagenopathies
- Genetic defect in collagen genes
- Genetic defect in genes encoding enzymes involved in collagen biosynthesis (eg. prolyl hydroxylase)
- dietary deficiency of vitamin C
- Deficiency in copper i.e. Menkes disease
What does vitamin C deficiency lead to in collagen fibril synthesis?
inibited hydroxylation of lysine and proline, leading to reduced interchain hydrogen bonding, leading to unstable collagen
Major characterization of scurvy
bone disease (children), haemorrhages, healing defects
Ehlers-Danlos Syndrome (EDS)
Disorder that disrupt collagen synthesis caused by defects in processing enzymes (e.g. lysyl hydroxylase or mutations in aa seq of collagen alpha chain)
- Affect connective tissue (skin, joint, blood vessel)
2 Major forms of Ehlers-Danlos Syndrome (EDS)
Type I and II (Classical EDS) caused by mutation in genes COL5A1 and COL5A1 encoding type V collagen.
Charaterized by
- fragile hyperextensible skin
- joint hypermobility (dislocations)
- delayed wound healing
- hypotonia with delayer motor development
* Also caused by mutation in gene for type III collagen which leads to mutant chain that gets degraded or accumulated causing vascular problems.
Osteogenesis imperfecta
8 different types Type 1 (mildest) Type 2 (severest) caused by mutation in COL1A1 and COL1A2
Osteogenesis imperfecta Type1
OI tarda
- caused by mutation in COL1A1 which leads to reduced –synthesis of Type 1 collagen (alpha 1 &2 chains)(quantitative alterations)
- often misdiagnosed as child abuse
menifestation of Osteogenesis imperfecta Type1
- bone fractures during childhood & adolescence due to minor trauma
- blue or grey tint to sclera
- hearing loss in adulthood
- retarded wound healing
- rotation / twisting of spine