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Chapter 8
Fibrous proteins
Major fibrous protein of epithelial tissues is a keratin
Major fibrous proteins of connective tissue are:
Collagen
Elastin
Dr. Stephen C. Hardies
[email protected]
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.
a-helical coiled
coil domain
B.
coiled coil
C.
a-helix
heptad repeat: hxxhxxx, where "h" means hydrophobic
and "x" means any residue.
a-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 a-helixes.
Function: Provide insoluble structural protein for body
surfaces
Crosslinks: disulfide bonds
Multigene family: family members differ by:
tissue structure
Hard
Soft
amt. Cys
More
Less
amt. Crosslinks
More
Less
True or False?
http://biochem.uthscsa.edu/hardies-bin/survey.pl
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 Nterminus and the other the C-terminus.
b) Disulfide bonds are crosslinks between sulfur atoms of either
cysteine 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?
http://biochem.uthscsa.edu/hardies-bin/survey.pl
a) In woven bone, collagen fibers are laid down in a
disorganized array, whereas in lamellar bone they are
laid 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
Repetitive character of sequence:
-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-
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
Gap
tropocollagen
Key assembly
interaction
670 angstrom D period
Ca10(OH)2(PO4)6
gap
gap
gap
gap
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
[a1(I)]2a2(I)
[a1(II)]3
Skin, tendon, bone, dentine
[a1(III)]3
selection of 6 chains
a1(V)a2(V)a3(V)
Extensible connective tissues (skin, arteries)
a1(VI)a2(VI)a3(VI)
[a1(VII)]3
Associated with Type I
a1(VIII)a2(VIII)2
a1(IX),a2(IX),a3(IX)
Hexagonal lattice in endothelium
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
[a1(X)]3
a1(XI)a2(XI)a1(II)
Cartilage
Basement membrane
Associated with Type I
Epithelial anchors
Associated with Type II, bound to
glycosaminoglycans
Hexagonal lattice in mineralizing cartilage
Associated with Type II
[a1(XII)]3
[a1(XIII)]3, many variants
Associated with Type I
[a1(XIV)]3
[a1(XV)]3
Associated with Type I
[a1(XVI)]3
[a1(XVII)]3
Many tissues
[a1(XVIII)]3
[a1(XIX)]3
Many tissues
Membrane bound
Many tissues
Skin, membrane bound
Rhabdomyosarcoma
Prockop and Kivirikko, Ann. Rev. Biochem. 1995, 64:403-34.
fibril
Type VI
Anchoring
fiber
5 nm
50 nm
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.
Defects in Type I collagen cause:
Osteogenesis Imperfecta
Dentinogenesis Imperfecta
Blue sclera
Opalescent and cracked teeth
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?
http://biochem.uthscsa.edu/hardies-bin/survey.pl
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
Disorder
Disorders due to defects in collagen.
Collagen Defect
Clinical Manifestations
Ehlers-Danlos IV
Defect in type III
Osteogenesis
imperfecta
Dentinogenesis
imperfecta
Dystrophic
Epidermolysis Bullosa
Chondrodysplasia(vari
ous types)
Alport syndrome
Decrease in type I
Defect in type VII
Arterial, intestinal, or uterine rupture; thin,
easily bruised skin
Blue sclerae, multiple fractures, low bone
mass
Discoloration; enamel chips off the dentin;
defect is in the dentin.
Blistering of skin and mucous membranes
Defect in type II
Short-limbed dwarfism, skeletal deformity
Defects in type IV (specialized
minor forms)
Defect in type I
Defect in type II or type IX
Defect in type X
Kidney disease, hearing loss, ocular lesions
Amino terminal propeptide present
due to COL1A mutation
Hyperextensible, easily bruised skin, hip
dislocations
Scurvy
Decreased hydroxyproline
Ehlers-Danlos VI
Decreased hydroxylysine
Ehlers-Danlos V
Decreased cross-linking
Poor wound healing, deficient growth;
increased capillary fragility
Hyperextensible skin and joints, poor wound
healing, musculo- skeletal deformities
Skin and joint hyperextensibility
Osteoporosis (1-3%)
Osteoarthritis (subset)
Schmidt Metaphyseal
Chondrodysplasia
Ehlers-Danlos VII
Found for some type I defects
Short limbs, bowing of legs
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
Scurvy
Perifollicular abnormalities.
Gingival abnormalities
True or False?
http://biochem.uthscsa.edu/hardies-bin/survey.pl
What are preliminary steps to get procollagen polypeptides into
the ER?
a) Depending on the synthetic cell type, transcription factors will
select 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 it
enters the ER.
Examples of collagen nomenclature:
An individual polypeptide: a1(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:
vitamin C
HOlys
•Translation on rough ER and entry into ER.
•Hydroxylation in the ER.
+ HOpro
(or else degrade)
•Triple helix assembly.
+
+ •Glycosylation, transport to Golgi, further glycosylation.
•Secretion.
•Removal of propeptides.
•Assembly into fibrils.
+
•Crosslinking.
Elastin:
Where: elastic connective tissues.
Cellular location: extracellular matrix
Function: add elasticity to connective tissue
Structure: beta spiral
Crosslink: desmosine
Genetic defects in elastin underlie Williams Syndrome
Facial features associated with Williams Syndrome
Dental features include small
widely spaced teeth and
malocclusion.
Dynamic structure of elastin:
The structure of elastin
is called a b 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?
http://biochem.uthscsa.edu/hardies-bin/survey.pl
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?
http://biochem.uthscsa.edu/hardies-bin/survey.pl
Which of these are glycosaminoglycans?
a)
c)
b)
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
inactive
Inside out
activation
Outside in
signaling
True or False?
http://biochem.uthscsa.edu/hardies-bin/survey.pl
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’ve
seen that in:
b) bone proteins
c) matrix metalloproteases
d) keratin
Tissues of Teeth