Chapter 8 Fibrous proteins Major fibrous protein of epithelial tissues is a keratin
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Transcript of Chapter 8 Fibrous proteins Major fibrous protein of epithelial tissues is a keratin
Chapter 8
Fibrous proteins
Major fibrous protein of epithelial tissues is keratin
Major fibrous proteins of connective tissue are:
Collagen
Elastin
Dr. Stephen C. Hardies 7-3735 437D
The fundamental building block of a keratin fiber is a coiled coil dimer of a type I and type II keratin polypeptide.
A.
coiled coil
B.
C. heptad repeat: hxxhxxx, where "h" means hydrophobic
and "x" means any residue.
-helix
-helical coiled coil domain
-Keratin
Where: Major protein of hair, skin, nails, some oral mucosa; small amounts in all epithelial cells.
Cellular location: Intracellular; not found in connective tissues
Structure: fibrous bundle of coil-coiled -helixes.
Crosslinks: disulfide bonds
Multigene family: family members differ by:
tissue structure Hard Soft
amt. Cys More Less
amt. Crosslinks More Less
Function: Provide insoluble structural protein for body surfaces
True or False?
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a) Proteins are linear chains of amino acids with polarity. Polarity means that there is directionality caused by adjacent amino acids being joined from COOH group to NH2 group. One end of a polypeptide will be termed the N-terminus and the other the C-terminus.
b) Disulfide bonds are crosslinks between sulfur atoms of eithercysteine residues or methionine residues.
c) An alpha helix can accommidate any sequence of amino acids.
Collagen
Where: 30% of total protein. Major protein of connective tissues: bones, tendons, ligaments, basement membranes, dentin, cementum, (not enamel).
Cellular location: extracellular matrix.
Structure: triple helix (tropocollagen). Subsequent to secretion, tropocollagen is assembled and crosslinked to make insoluble collagen fibers.
Function: Provides tensile strength to soft connective tissues. Tissues that must be elastic but exhibit tensile strength (e.g. ligaments) have a mixture of collagen and elastin. Collagen fibers in bone reinforce against fracture.
True or False?
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a) In woven bone, collagen fibers are laid down in a disorganized array, whereas in lamellar bone they arelaid down in a more organized parallel fashion.
b) Woven bone is stronger than lamellar bone.
c) Laminar bone is made by remodeling woven bone.
Features of collagen primary structure
-Gly-Pro-Met-Gly-Pro-Ser-Gly-Pro-Arg-
-Gly-Leu-Hyp-Gly-Pro-Hyp-Gly-Ala-Hyp-
-Gly-Pro-Gln-Gly-Phe-Gln-Gly-Pro-Hyp-
Repetitive character of sequence:
Collagen 3 stranded helix
The collagen triple helix is stabilized by an interchain hydrogen bonding network involving the hydroxyl group of hydroxyproline, the glycine carbonyl group, and water molecules.
True or False?
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a) Hydroxyproline and hydroxylysine are among the 20 amino acids inserted into polypeptides by ribosomes.
b) Glycine is NH2-CH2-COOH.
c) Glycine is the only amino acid that doesn’t have a D and an L isomer.
Type I collagen fibril
EM
670 angstrom D period
tropocollagen
Key assembly
interaction
Gap
gap
gap
gapgap
Ca10(OH)2(PO4)6
True or False?
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a) Ca10(OH)2(PO4)6 is the same mineral (hydroxyapatite) that forms enamel.
b) Whereas bone is reinforced by collagen fibers, enamel uses a different protein named amelogenin.
c) Enamel has a higher mineral content than bone.
Types of Collagen
Type Composition Tissue distribution
I [1(I)]2
2(I) Skin, tendon, bone, dentine
II [1(II)]3 Cartilage
III [1(III)]3 Extensible connective tissues (skin, arteries)
IV selection of 6 chains Basement membraneV
1(V)2(V)3(V) Associated with Type I
VI 1(VI)2(VI)3(VI) Associated with Type I
VII [1(VII)]3 Epithelial anchors
VIII 1(VIII)2(VIII)2 Hexagonal lattice in endothelium
IX 1(IX),2(IX),3(IX) Associated with Type II, bound to
glycosaminoglycansX [1(X)]3 Hexagonal lattice in mineralizing cartilage
XI 1(XI)2(XI)1(II) Associated with Type II
XII [1(XII)]3 Associated with Type I
XIII [1(XIII)]3, many variants Membrane bound
XIV [1(XIV)]3 Associated with Type I
XV [1(XV)]3 Many tissues
XVI [1(XVI)]3 Many tissues
XVII [1(XVII)]3 Skin, membrane bound
XVIII [1(XVIII)]3 Many tissues
XIX [1(XIX)]3 RhabdomyosarcomaProckop and Kivirikko, Ann. Rev. Biochem. 1995, 64:403-34.
fibril
50 nm5 nm
Type VI
Anchoring fiber
Type IV collagen forms planar arrays
and makes basement membranes
Type VII collagen anchors basement membrane to underlying connective tissue (stromal) cell layer.
Type III collagen
Sometimes called elastic collagen or extensible collagen
Collagen type III micrograph visualized in polarized light, showing crimped organization of the fibrils.
Osteogenesis Imperfecta
Blue sclera
Dentinogenesis Imperfecta
Opalescent and cracked teeth
Defects in Type I collagen cause:
Type I: associated with OI.
Will probably need full crown coverage.
Abraded teeth in Dentinogenesis Imperfecta
Chronic bone fractures in Osteogenesis Imperfecta
An example of a mutation underlying Osteogenesis Imperfecta
988
Pro Gly Pro Arg Gly Arg Thr Gly Asp Ala
CCG GGT CCT CGC GGT CGC ACT GGT GAT GCT
Pro Cys Pro Arg Gly Arg Thr Gly Asp Ala
CCT TGT CCT CGC GGT CGC ACT GGT GAT GCT
True or False?
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a) DNA is a polymer of bases named A,T,G, and C.
b) The sequence of bases in a gene determines the sequence of the polypeptide that will be produced.
c) A “mutation” is a heritable change to the sequence of a gene that causes the encoded polypeptide to have altered function.
d) Some individual residue changes may have large effects on the protein function, whereas other changes may have little or no effect.
e) Sickle cell anemia is an example of an inherited disease caused by a single residue change in a protein.
Ehlers-Danlos Syndrome
Hyperplasticity of the skin
Disorders due to defects in collagen.
Disorder Collagen Defect Clinical Manifestations
Ehlers-Danlos IV Defect in type III Arterial, intestinal, or uterine rupture; thin,easily bruised skin
Osteogenesisimperfecta
Decrease in type I Blue sclerae, multiple fractures, low bonemass
Dentinogenesisimperfecta
Found for some type I defects Discoloration; enamel chips off the dentin;defect is in the dentin.
DystrophicEpidermolysis Bullosa
Defect in type VII Blistering of skin and mucous membranes
Chondrodysplasia(various types)
Defect in type II Short-limbed dwarfism, skeletal deformity
Alport syndrome Defects in type IV (specializedminor forms)
Kidney disease, hearing loss, ocular lesions
Osteoporosis (1-3%) Defect in type IOsteoarthritis (subset) Defect in type II or type IXSchmidt MetaphysealChondrodysplasia
Defect in type X Short limbs, bowing of legs
Ehlers-Danlos VII Amino terminal propeptide presentdue to COL1A mutation
Hyperextensible, easily bruised skin, hipdislocations
Scurvy Decreased hydroxyproline Poor wound healing, deficient growth;increased capillary fragility
Ehlers-Danlos VI Decreased hydroxylysine Hyperextensible skin and joints, poor woundhealing, musculo- skeletal deformities
Ehlers-Danlos V Decreased cross-linking Skin and joint hyperextensibility
Steps in collagen biosynthesis:
•Translation on rough ER and entry into ER.
•Hydroxylation in the ER.
•Triple helix assembly.
•Glycosylation, transport to Golgi, further glycosylation.
•Secretion.
•Removal of propeptides.
•Assembly into fibrils.
•Crosslinking.
Collagen hydroxylation
Perifollicular abnormalities. Gingival abnormalities
Scurvy
True or False?
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What are preliminary steps to get procollagen polypeptides intothe ER?
a) Depending on the synthetic cell type, transcription factors willselect specific preprocollagen genes from a family of such genes be transcribed and translated.
b) The preprocollagen polypeptide is released into the cytoplasm.c) A special sequence on the N-terminus called the ‘signal peptide’
directs the preprocollagen to a pore in the ER through which itenters the ER.
Examples of collagen nomenclature:
An individual polypeptide: 1(I) procollagen
Assembled triple helix with propeptides still on: Type I procollagen
Triple helix after propeptide removal:
Type I tropocollagen
Assembled into 50 nm fiber:
Collagen microfibril
A third lysine (from a third tropocollagen) can add to make pyridinoline.
Crosslinks involving hydroxylysine are more stable than those involving lysine.
There is also a crosslink involving histidine.
Steps in collagen biosynthesis:
•Translation on rough ER and entry into ER.
•Hydroxylation in the ER.
•Triple helix assembly.
•Glycosylation, transport to Golgi, further glycosylation.
•Secretion.
•Removal of propeptides.
•Assembly into fibrils.
•Crosslinking.
+ (or else degrade)
vitamin C
+
HOlys HOpro
++
Elastin:
Where: elastic connective tissues.
Cellular location: extracellular matrix
Function: add elasticity to connective tissue
Structure: beta spiral
Crosslink: desmosine
Facial features associated with Williams Syndrome
Dental features include small widely spaced teeth and malocclusion.
Genetic defects in elastin underlie Williams Syndrome
Dynamic structure of elastin:
The structure of elastin is called a spiral, and is loosely held together by the hydrophobic force.
It is easily deformed to an extended configuration, but will relax back to a compact conformation.
Repeating unit: PGVGV
Fibrillin
Where: connective tissue; extracellular matrix
Structure: forms outer envelope of elastin microfibrils
Genetic defects in fibrillin result in Marfan’s Syndrome, characterized by a tall gaunt appearance, joint problems, an often leading to death by aortic aneurysm.
Abraham Lincoln is thought to have had Marfan’s syndrome.
Aortic Aneurysm
Enzymes that turn over connective tissue
Common names: collagenase, gelatinase, elastase
Formal names: Matrix Metalloproteases (MMPs)
Function: degrade connective tissue in support of tissue remodeling, wound healing, and cell migration (including during metastasis).
Family of Zn++ -requiring zymogens embedded in connective tissue. They can be activated in a cascade starting from a cell surface MMP.
True or False?
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Zymogens are inactive forms of enzymes that will become activated by cleavage to remove a propeptide. Zymogenes are also called “proenzymes”. Examples of zymogens are:
a) trypsinogen
b) collagen
c) coagulation factors
d) DNA polymerase
e) cathepsin
Proteoglycan aggregate
Where: major component of ground substance, the material within which collagen and other fibers are assembled to form connective tissue.
Function: make space, absorb water and allow compressibility through water flow, act as reservoir of Ca++ prior to mineralization.
Proteoglycan aggregate
True or False?
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Which of these are glycosaminoglycans?
a) b)
c) d)
Proteoglycan breakdown associated with mineralization
Adhesion proteins
Where: cell surface
Functions: adhere cells to extracellular proteins (or to other cells), sense presence of extracellular proteins, sense mechanical stress in tissues.
Integrins
cell membrane
inactiveOutside in signaling
Inside out activation
True or False?
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a) Integrins would be a good choice of adhesion protein to use for a cell like a platelet that has to very rapidly switch from a nonbinding to a tight binding mode.
Integrins bind the sequence RGD in ligand proteins. We’veseen that in:
b) bone proteinsc) matrix metalloproteasesd) keratin
Tissues of Teeth