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7/28/2019 Biopolymers structure and properties.pptx
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Biopolymers structure and
properties
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Biopolymers/Natural polymers
Definition: Natural polymers are formed in natureduring the growth cycles of all organisms, hence theyare also referred to as biopolymers.
Their synthesis generally involves enzyme-catalyzed,chain growth polymerization reactions of activated
monomers, which are formed typically within cells bycomplex metallic processes
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Types
• Polysaccharide based polymers
– Starch, Cellulose in higher plants, Chitin/Chitosan
• Protein based polymers
– Protein collagen in animals, Gelatin, Silk Proteins,Albumin etc.
• Microbial polyesters
– Poly--hydroxyalkanoates
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Collagen
• Types of collagen
• Structure of collagen
• Biosynthesis of
collagen
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• Collagen is the most abundant fibrousprotein, which occur in vertebrates.
• A typical collagen molecule is a long, rigid
structure in which three polypeptides "-
chains" are wound around one another in a
rope-like triple-helix
COLLAGEN
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• Although collagen molecules are found
throughout the body, their types andorganization are dictated by the structural
role collagen plays in a particular organ.
• In some tissues, collagen may be dispersed as
a gel support to the structure, as in the
extracellular matrix or the vitreous humor of the eye.
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• In other tissues, collagen may be bundled intight, parallel fibers great strength, as in
tendons.
• In the cornea of the eye, collagen is stacked transmit light with a minimum of scattering.
• Collagen of bone occurs as fibers arranged at anangle to each other resist mechanical shear
from any direction.
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A. Types of collagen
• The collagen superfamily of proteins includes
more than 20 collagen types, and proteins
that have collagen-like domains.
• The three polypeptide -chains are held
together by hydrogen bonds between the
chains.
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• Variations in the amino acid sequence of the
-chains structural components that areabout the same size (approximately 1000
amino acids long), but with slightly different
properties.
• These -chains are combined to form the
various types of collagen found in the tissues.
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1. Fibril-forming collagens:
•
Types I, II, and Ill are the fibrillar collagens,and have the rope-like structure describedbefore for a typical collagen molecule.
• In the electron microscope, these linearpolymers of fibrils have characteristicbanding patterns reflecting the regular
staggered packing of the individual collagenmolecules in the fibril(Figure 4.3).
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• Type I collagen fibers are found in supportingelements of high tensile strength (e.g. tendonand cornea).
• Fibers formed from type II collagen molecules
are restricted to cartilaginous structures.
• Fibrils derived from type Ill collagen are
prevalent in more distensible tissues, such asblood vessels.
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2. Network-forming collagens:
•
Types IV and VII form a three-dimensional mesh,rather than distinct fibrils (Figure 4.4).
• For example, type IV molecules assemble into a sheet
or meshwork that constitutes a major part of basement membranes.
• [ Basement membranes are thin, sheet-like structures
that provide mechanical support for adjacent cells, and function as a semipermeable filtration barrier for
macromolecules in organs such as the kidney and the
lung.]
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3. Fibril-associated collagens:
• Types IX and XII bind to
the surface of collagen
fibrils, linking these
fibrils to one another
and to other
components in the
extracellular matrix(Figure 4.2).
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B. Structure of collagen
1. Amino acid sequence: • Collagen is rich in proline and glycine, both of
which are important in the formation of the
triple-stranded helix.
• Proline facilitates the formation of the
helical conformation of each -chain becauseits ring structure "kinks" in the peptide
chain.
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glycine
proline
oxygennitrogen
carbon
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• Glycine, the smallest amino acid, is found in
every third position of the polypeptide chain.
• It fits into the restricted spaces where the
three chains of the helix come together.
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• The glycine residues are part of a repeating
sequence.—
Gly—
X—
Y—
, where X isfrequently proline and Y is often hydroxyproline
or hydroxylysine (Figure 4.5).
• Most of the .- chain can be regarded as a
polytripeptide whose sequence can be
represented as (—
Gly—
X—
Y—
) 333
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2. Triple-helical structure: • Unlike most globular proteins that are folded into
compact structures, collagen, a fibrous protein, has
an elongated, triple-helical structure that places
many of its amino acid side chains on the surface of
the triple-helical molecule.
• [This allows bond formation between the exposed
R-groups of neighboring collagen monomers
aggregation into long fibers]
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3. Hydroxyproline and hydroxylysine:
• Collagen contains hydroxyproline (hyp) and
hydroxylysine (hyl), which are not present in
most other proteins.
• These residues result from the hydroxylation
of some of the proline and lysine residues
after their incorporation into polypeptide
chains
(Figure 4.6).
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•
The hydroxylation is, thus, an example of posttranslational modification .
•
Hydroxyproline is important in stabilizing thetriple-helical structure of collagen because it
maximizes interchain hydrogen bond
formation.
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4. Glycosylation:
• The hydroxyl group of the hydroxylysineresidues of collagen may be enzymatically
glycosylated.
• Most commonly, glucose and galactose are
sequentially attached to the polypeptide
chain prior to triple-helix formation(Figure 4.7).
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C. Biosynthesis of collagen
•
The polypeptide precursors of the collagenmolecule are formed in fibroblasts (or in the
related osteoblasts of bone and chondroblasts
of cartilage), and are secreted into the
extracellular matrix.
• After enzymic modification, the mature collagen
monomers aggregate and become cross-linked collagen fibrils.
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1. Formation of pro- -chains:
– Collagen is one of many proteins that normally
function outside of cells.
– Like most proteins produced for export, the
newly synthesized polypeptide precursors of -
chains contain a special amino acid sequence at
their N-terminal ends.
–
This acts as a signal that the polypeptide beingsynthesized is destined to leave the cell.
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– The signal sequence:
• facilitates the binding of ribosomes to therough endoplasmic reticulum (RER)
• directs the passage of the polypeptide
chain into the cisternae of the RER.• is rapidly cleaved in the endoplasmic
reticulum precursor of collagen called a
pro--chain (Figure 4.7).
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2. Hydroxylation:
• The pro- -chains are processed by a number
of enzymic steps within the lumen of the RER
while the polypeptides are still being
synthesized (Figure 4.7).
• Proline and lysine residues found in the Y-
position of the—Gly—X—Y— sequence can
be hydroxylated hydroxyproline and
hydroxylysine residues.
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• These hydroxylationreactions require
molecular oxygen andthe reducing agentvitamin C
•
the hydroxylatingenzymes, prolyl hydroxylase and lysyl hydroxylase, are unable
to function without vit. C(Figure 4.6).
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• In the case of vit.C deficiency (therefore, a lack of
prolyl and lysyl hydroxylation), collagen fibers
cannot be cross-linked, greatly
the tensilestrength of the assembled fiber.
• Vit.C deficiency disease known as scurvy.
• Patients with vit.C deficiency often show bruises on
the limbs as a result of subcutaneous extravasation
of blood (capillary fragility) ( Figure 4.8)
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3. Glycosylation:
• Some hydroxylysine residues are modified by
glycosylation with glucose or glucosyl-
galactose (Figure 4.7).
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4. Assembly and secretion:
• After hydroxylation and glycosylation, pro--
chains form procollagen , a
precursor of collagen that has a central
region of triple helix flanked by the non-helical amino- and carboxyl-terminal
extensions called propeptides.
(Figure 4.7).
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• The formation of procollagen begins with
formation of interchain disulfide bondsbetween the C-terminal extensions of the
pro-- chains.
• This brings the three -chains into an
alignment favorable for helix formation.
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• The procollagen molecules are translocated
to the Golgi apparatus, where they arepackaged in secretory vesicles.
• The vesicles fuse with the cell membrane release of procollagen molecules into the
extracellular space.
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5. Extracellular cleavage of procollagen
molecules:
• After their release, the procollagen
molecules are cleaved by N - and C –
pro -collagen peptidases remove the terminal
propeptides releasing triple-helical
collagen molecules.
6 F i f ll fib il
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6. Formation of collagen fibrils:
• Individual collagen molecules spontaneously
associate form fibrils.
• They form an ordered, overlap ping, parallel
array, with adjacent collagen moleculesarranged in a staggered pattern.
• Each collagen molecule overlaps its neighbor
by three-quarters of its length.
(Figure 4.7).
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7. Cross-link formation:
• The fibrillar array of collagen molecules
serves as a substrate for lysyl oxidase.
• This extracellular enzyme oxidatively
deaminates some of the lysyl and
hydroxylysyl residues in collagen.
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• The reactive aldehydes that result (allysine and
hydroxyallysine) condense with lysyl or
hydroxylysyl residues in neighboring collagenmolecules form covalent cross-links (Figure
4.9).
•
This cross-linking is essential for achieving the tensilestrength necessary for the proper functioning of
connective tissue.
• Any mutation that interferes with the ability of
collagen to form cross-linked fibrils affects thestability of the collagen].
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Contribution of Collagen
The major fraction of connective tissue iscollagen
• This component is important because it
contributes significantly to toughness inmammalian muscle
• Gelatin serves as the functional ingredient in
temperature—Dependent Gel-type desserts
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Solubility
• Some of the collagen is soluble in neutral saltsolution
• Some in soluble in acid
• Some is insoluble
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The collagen triple helix
A case of structure following composition
• The unusual amino acid composition of collagen isunsuited for alpha helices or beta sheets
• But it is ideally suited for the collagen triple helix; threeintertwined helical strands
• Much more extended than alpha helix, with a rise perresidue of 2.9 Angstroms
•
3.3residues per turn• Long stretches of Gly-Pro-Pro/Hyp
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The Fabric of Collagen
• The collagen monomer is a long cylindricalprotein about 2800 Å long and 14-15 Å indiameter
• It consists of three poly peptide chains woundaround each other in a suprahetical fashion
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Collagen degradation
• Collagen can be degraded by the enzymecalled collagenase.
• Activity of collagenase will be reduced if
collagen is crosslinked with metal ions whichact as a enzyme poisons.
• Gelatin is the byproduct of collagen
degradation by acid or alkaline digestion
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Stability of collagen
• Affected by dehydration – Contact with reagents which reduce hydrophobic
interaction
– Heat – Mucopolysaccharides
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Contents:
•
Elastin – Structure of elastin
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ELASTIN
• In contrast to collagen, which forms fibers
that are tough and have high tensile
strength, Elastin is a connective tissueprotein with rubber-like properties.
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• Elastic fibers composed of elastin and
glycoprotein microfibrils are found in the
lungs, the walls of large arteries, and elastic
ligaments.
• They can be stretched to several times their
normal length, but recoil to their original
shape when the stretching force is relaxed.
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A. Structure of elastin
• Elastin is
– an insoluble protein polymer
– synthesized from a precursor,
tropoelastin , ( a linear polypeptide composed of about 700 amino acids that are primarily
small and nonpolar) (e.g. glycine,
alanine, and valine).
• Elastin is also,
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Elastin is also,
– rich in proline and lysine,
– contains a little hydroxyproline
– contains no hydroxylysine.
• Tropoelastin is secreted by the cell into theextracellular space.
• There, it interacts with specific glycoproteinmicrofibrils, such as fibrillin, which function as ascaffold onto which tropoelastin is deposited.
• Some of the lysyl side chains of the tropoelastin
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Some of the lysyl side chains of the tropoelastin
polypeptides are oxidatively deaminated by lysyl
oxidase forming allysine residues.
• 3 of the allysyl side chains + one unaltered lysyl
side chain from the same or neighboring
polypeptides form a desmosine cross-link.
(Figure 4.12).
This produces Elastin - an extensively
interconnected, rubbery network that can stretch
and bend in any direction when stressed
connective tissue elasticity
(Figure 4.13).
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Elastin is stable to relatively high temperaturesand chemical reagents due to low content of aminoacids with polar side chains. The enzyme elastase,hydrolyses elastin at peptide bonds after small
hydrophobic residues, particluarly alanine.
Mechanical properties of elastin and
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Mechanical properties of elastin andcollagen fibers
Fibers Modulus of
elasticity
Mpa
Tensile
strength
Mpa
Ultimate
elongation %
Elastic fibers 0.6 1 100
Collagenfibers
1000 50-1000 10
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61
mechanics of BIO materials
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Extracellular Macromolecules
1. GlycosaminoglycansProteoglycans
GlycoproteinsMucins
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Extracellular Macromoleculesmacromolecule % carb.
glycosaminoglycans* (GAGs) 100
proteoglycans* 90-95
glycoproteins 2-30
fibrous proteins 1-2
Examples of functions:
mechanical support lubrication
cushioning adhesivescell spacers selective filters
* aka mucopolysaccharides, mucoproteins, respectively 1
Extracellular matrix in tissues
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Extracellular matrix in tissues
• ground substance + fibers
• macromolecules between cells – ground substance molecules
GAGs/proteoglycans (mostly carbohydrate)
– fibersfibrous proteins:
structuraladhesive
• especially abundantin connective tissue
adhesionmolecules
Adapted from Hypercell
extra-cellularmatrix
basallamina
underlying cells
epithelial cells
2
GAG structure OO – A sugar
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GAG structure
• exist as: –
independent moleculese.g., hyaluronate & heparin –parts of larger structures
e.g., in proteoglycans
•
heteropolysaccharidesrepeating structure:disaccharide ( AB)n ABABAB…
– where A is usually 1 uronic acid (hexose with C6 as COO – )
– & B is 1 glycosamine (amino sugar) derivative
• unbranched –glycosidic linkage –anomeric C of 1 unit linked to hydroxyl of adjacent unit
OHO
OHOH
C
OH
OHOH
O
OH
CH2
NH2
g
B sugar
3
GAG structure: repeating units 4
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GAG structure: repeating unitsGAG A sugar B sugar
hyaluronate glucuronate N-acetyl glucosamine
OO
OH OH
COO
OOH
O
O
CH2 OH
CH3 O
NH1,3
1,4
–
25*
GAG structure: repeating units
4
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GAG structure: repeating unitsGAG A sugar B sugar
hyaluronate glucuronate N-acetyl glucosamine
chondroitin sulfate glucuronate N-Ac galactosamine 4-SO4
dermatan sulfate iduronate "
heparan sulfate glucuronate glucosamine N-SO3, 6-SO4
heparin iduronate 2-SO4 "
keratan sulfate galactose N-Ac glucosamine 6-SO4
*opposite configuration in iduronateglucuronate/iduronate: epimers at C5 glucose/galactose: epimers at C4
OO
OH OH
COO
OOH
O
O
CH2 OH
CH3 O
NH1,3
1,4
–
25*
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9.5 Proteoglycans
• Example: syndecan - transmembrane protein -inside domain interacts with cytoskeleton,outside domain interacts with fibronectin
Hyaluronate (aka hyaluronan)
5
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Hyaluronate (aka hyaluronan)• mol wt: 106 – 107 (5000 – 50,000 monosaccharide
units)
• very polar: 2 hydroxyls/unit 6 heteroatoms/unitCOO – every other unit
binds cations: Na+, Ca++
2 1 3 4 5 6
–
– –
(glucuronate –N-acetyl glucosamine)3 (glcUA –glcNAc)3
A B A B A B
Display of HAin motion
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Proteoglycan Functions
• Modulation of cell growth processes
– Binding of growth factor proteins by proteoglycansin the glycocalyx provides a reservoir of growthfactors at the cell surface
• Cushioning in joints
– Cartilage matrix proteoglycans absorb large
amounts of water. During joint movement, cartilageis compressed, expelling water!
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Proteoglycans (PGs)
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g y ( )• composed of as many as 200 GAG chains covalently bonded to a core protein via
serine side chains• molecular weight range: 105 – 107 • GAG chains: chondroitin sulfate, heparan sulfate,
dermatan sulfate, keratan sulfateExamples• decorin
– many connective tissues – binds type I collagen, TGF-
• perlecan – basal laminae – structural & filtering function
• aggrecan• syndecan (slide 13)
from Alberts et al.
Fig. 19-57
GAG chains
coreprotein
AlbertsT 19-3:Dcrn GAGchndSO4
/drmSO4
9
Proteoglycans: aggrecan
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oteog yca s: agg eca
• ~100 GAG chains/molecule
• ~100 monosaccharides/GAG chain• each "bristle" = 1 GAG chain
• each GAG chain is either chondroitin sulfate
or keratan sulfate• GAG chains linked to ser side chains of core protein
coreprotein
GAG chains11
An aggregate of aggrecans & hyaluronanj GAG PG 1m
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gg g gg y• major GAG –PG
in cartilage
• link proteins bindnoncovalently
• with bound H2O,disperses shocks,compressive force
• ~ cell size• adhesion proteins
link to collagen &cells
• degraded bychondroitinsulfatase, etc
1m
hyalur-
onan
link proteins
keratansulfate chondroitin
sulfate
Alberts et al. Fig. 19-41
core protein
12
Proteoglycans:d
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g ysyndecan
GAG chains
core
• cell-surface PG• core protein domains
–intracellular –transmembrane –
extracellular5 GAGs attached
• functions –interactions
• cell-cell• cell-matrix
–growth factor receptor
outside
inside