Download Fibrous Proteins

Fibrous Proteins
DrVivek Joshi, MD
Fibrous Proteins
 Cylindrical shape: long rod like
 Low solubility in aqueous solutions
 With a structural rather than a dynamic role in the cell
or organism
 Characteristically contain larger amounts of regular
secondary structure
 Examples: Collagen, elastin, keratin
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Collagen
 Most abundant protein in the human body
Comprises about 25 % of total mammalian protein
Present in all tissues and organs
Provides the framework that gives the tissues their form
and structural strength
Types and organization are dictated by the structural role
played in a particular organ
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Collagen
 Types and organization are dictated by the structural role
played in a particular organ
Organ/Tissue
Collagen characteristics
Tendons
Bundled in tight parallel fibers that provide great
strength
Bone
Fibers arranged at an angle to each other so as to resist
mechanical shear from any direction
Skin
Loosely woven flexible fibers
Cornea
Ordered so as to be nearly crystalline, transparent
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Collagen
 More then 20 distinct types in humans
 Made up of about 30 distinct polypeptide chains (from 30 genes)
Type
Tissue
I
COL1A1, COL1A2
Most connective tissues,
including bone
II
COL2A1
Cartilage, vitreous humor
III
COL3A1
Extensible connective tissues
such as skin, lung, and the
vascular system
IV
COL4A1, COL4A6
Basement membranes
Minor component in tissues,
containing collagen I
V
COL5A1, COL5A6
VI
COL6A1, COL6A3
Most connective tissues
VII
VIII
COL7A1
Anchoring fibrils
COL8A1, COL8A2
Endothelium, other tissues
IX
COL9A1, COL9A3
COL10A1
Tissues containing collagen II
XI
COL11A1, COL11A2
COL2A1
Tissues containing collagen II
XII
COL12A1
Tissues containing collagen I
XIII
XIV
COL13A1
Many tissues
COL14A1
Tissues containing collagen I
XV
COL15A1
Many tissues
XVI
COL16A1
Many tissues
XVII
COL17A1
Skin hemidesmosones
XVIII
XIX
COL18A1
Many tissues (e.g., liver, kidney)
COL19A1
Rhabdomyosarcoma cells
X
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Genes
Hypertrophic cartilage
Collagen
Subdivided into classes based primarily on the structures they form
C l a s s
T y p e
F i b r i l - f o r m i nI g,
I I ,
Ne t wo r k - l i k e I V,
VI I I ,
FACI T s
XI I ,
Be a d e d
I X,
Ot h e r s
XI V,
f i bVrI iI l s
T r a n s m e mdbormaani eX
nI I I ,
XV,
XVI I
XVI I I
V,
and
XI
X
f i l a m eVnI t s
An c h o r i n g
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I I I ,
XVI ,
XI X
A. Collagen
 All types have a triple helical structure
 Composed of 3 left-handed polypeptide helices twisted around
each other to form a right-handed superhelix
* Stabilized by H-bonds between
individual polypeptide chains
* Helices with approximately 3
residues per turn
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Collagen
 Amino acid sequence primarily consists of large numbers of
repeating triplets with the sequence of Gly-X-Y


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
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X = frequently proline
Y = often hydroxyproline or hydroxylysine
repeating structure is an absolute requirement for the formation of the triple
helix
glycine – the only residue with an R-group small enough to fit within the
central core of the superhelix
proline and hydroxyproline – confer rigidity because conformationally
inflexible
hydroxyproline- involved in H-bond formation that helps to stabilize the triple
helix
hydroxylysine – site of attachment of carbohydrate moieties (most commonly
glucose and galactose)
Biosynthesis of Collagen
1. Synthesis
of pro- chain
 Ribosomes start the synthesis of prepro--chain with a signal sequence
 Signal sequence directs growing polypeptide chain into the cisternae of the rough ER
 The signal sequence facilitates the binding of ribosomes to the RER and directs the
passage of pre pro chain in thr RER.
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 Signal sequence is rapidly cleaved in the ER to yield pro--chain.
Biosynthesis of Collagen
2. Modification of pro  chain
A. Post-translational hydroxylation of
proline and lysine in RER
 Catalyzed by prolyl hydroxylase and
lysyl hydroxylase
 Requires molecular oxygen and
Vitamin C
 Hydroxylated collagen can be crosslinked to triple helix collagen
 Vitamin C deficiency-Weak collagenBleeding of gums
B. Hydroxylysine residues are
modified by glycosylation with
glucose or glucosyl-galactose
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Modification of pro  chain and Ascorbic acid
• Ascorbic acid is required as a cofactor
•
•
•
•
for prolyl hydroxylase and lysyl
hydroxylase-Hydroxylation of the proline
and lysine amino acids in collagen.
Hydroxyproline and hydroxylysine are
important for stabilizing collagen by
cross-linking the propeptides in collagen.
Defective collagen impairs wound
healing.
Collagen is also an important part of
bone, so bone formation is also affected.
Defective connective tissue also leads to
fragile capillaries, resulting in abnormal
bleeding
Biosynthesis of Collagen
3. Assembly of pro  chains to form procollagen
 Carboxy-terminal globular domains fold and disulfide bonds are formed.
 Interaction of these domains initiates winding of the triple helix
 Winding of the triple helix occurs from the carboxyl towards the amino terminus
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Summary of steps 1 to 3 for modifications of the pro  chains
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Biosynthesis of Collagen
4. Secretion of procollagen
 The completed triple helix, with globular domains at each end, moves to the Golgi
apparatus.
 Completed procollagen is released from the cell via secretory vesicles
5. Extracellular cleavage of procollagen molecules
 N- and C- procollagen peptidases remove the terminal propeptidesReleases the triple helix tropocollagen molecule
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Biosynthesis of Collagen
6. Formation of collagen fibrils
Individual tropocollagen molecules spontaneously associate to form collagen fibrils.
Form an ordered, overlapping, parallel array.
Adjacent molecules arranged in a staggered pattern, each overlaps its neighbor by a length
approximately ¾ of a molecule
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Biosynthesis of Collagen
7. Assembly of collagen fibrils
into mature collagen fiber
Cu
Fibrillar array of collagen molecules serves as
substrate for lysyl oxidase.
 An extracellular enzyme
 Oxidatively deaminates some of the lysyl and
hydroxylysyl residues forming reactive
aldehydes
Stabilized by extensive cross-linking-Essential for
achieving the tensile strength necessary for the
proper functioning of connective tissue.
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Summary of collagen processing
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Collagen is highly stable
 Half-life may be as long as several months
 Any mutation that interferes with the
ability of collagen to form cross-linked
fibrils almost certainly affects the stability
of collagen.
 Degradation of collagen-Collagenase
 Cleave intact collagen fibers into smaller
fragments that can be further degraded by
lysosomal enzymes into constituent amino
acids
 Important during connective tissue
remodeling in response to growth or
injury of tissue
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Selected disorders in collagen biosynthesis and structure
Disorder
Collagen defect
Clinical Manifestations
Osteogenesis
imperfecta 1
Osteogenesis
imperfecta 2
Decreased synthesis of type I
Long bone fractures prior to puberty
Point mutations and exon
rearrangements in triple
helical regions
Poor secretion, premature
degradation of type III
Decreased hydroxylysine in
types I and III
Type I procollagen
accumulation: N-terminal
propeptide not cleaved
Decreased hydroxylysine due
to poor Cu distribution
Hydroxyproline decrease due
to ascorbic acid deficiency
Perinatal lethality; malformed and soft,
fragile bones
Ehlers-Danlos IV
Ehlers-Danlos VI
Ehlers-Danlos VII
Cutis laxa (occipital
horn syndrome)
Scurvy
Translucent skin, easy bruising, arterial
and colon rupture
Hyperextensive skin, joint hypermobility
Joint hypermobility and dislocation
Lax, soft skin; occipital horn formation
in adolescents
Poor bone growth, poor wound healing,
severe bruising
Some Ehlers-Danlos IV and some Osteogenesis imperfecta results form point mutations that
substitute another amino acid for glycine.
Collagen Diseases
1.
Ehlers Danlos Syndrome
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Heterogeneous group of generalized connective tissue disorders
Characterized by stretchy skin and loose joints
Delayed motor development
Ehlers-Danlos Syndrome (EDS)
 Inheritable defect in the metabolism of fibrillar collagen molecule
 Ehlers Danlos mostly affects Type III collagen (blood vessels)
 EDS can result from:

Deficiency of collagen processing enzyme(Lysyl hydroxylase, Procollagen
peptidase)-In most cases, the step of collagen synthesis which is disturbed in
Ehlers-Danlos is the formation of cross-links
 Mutation in the amino acid sequence of Collagen I,III or V
 The replacement of glycine with another amino acid may lead to EDS
 Glycine is used because it has the smallest side chain (-H)
 This allows for the three alpha helices to come in close contact when
they form the triple helix
 If glycine is replaced by any other AA, the side chain is bigger
 This makes it difficult for the alpha helices to come close together
 Thus, this forms a conformational change in collagen triple helix
Collagen Diseases
2. Osteogenesis imperfecta(OI)

A heterogeneous group of inherited
disorders-Also k/a brittle bone syndrome

Distinguished by bones that easily bend and
fracture

Other common features:

Retarded wound healing

Rotated and distorted spine leading to a
hump-back appearance
 Type I OI

Osteogenesis Imperfecta Tarda

Decresed production of α1 and 2 chains
 Type II OI

More severe clinical presentation

Death in utero or neonatal period

Mutation in gene for pro- α1 and pro -2
chains of Type I collagen

Most common mutation-Glycine replaced
by bulky side chain amino acid
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Ehlers-Danlos v/s Osteogenesis Imperfecta
 Ehlers-Danlos
 Defect in processing of collagen
 Leads to hyperelasticity and hypermobility
 Mostly affects collagen type III
 Incidence: 1:5,000
 Osteogenesis Imperfecta
 Defect in amount of collagen processed
 Symptoms of brittle bones and bruising
 Hearing loss and dental imperfections
 Mostly affects collagen type I
 Incidence: 1:10,000
 The 12-year-old son of a circus contortionist says that his
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skin scars and bruises easily. Physical examination reveals
distensible skin and hypermobile joints.You suspect an
inherited defect in which of the following molecules?
A. Fibrillin
B. Laminin
C. Fibronectin
D. Collagen
E. Keratin
F. Elastin
 A 64-year-old woman presents to your office complaining of bleeding gums
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while brushing her teeth. Physical examination reveals gingival bleeding and
perifollicular hemorrhages. On careful history, the patient states that she lives
alone and that her diet primarily consists of tea and toast. This patient’s
symptoms are most likely caused by enzyme hypoactivity in which of the
following compartments?
Nucleus
Cytoplasm
Golgo apparatus
Rough endoplasmic reticulum
Lysosomes
Extracellular space
Mitochondria
Elastin
 Synthesized as a soluble monomer called “tropoelastin”
 Composed primarily of small, non-polar amino acids
 Examples: glycine, alanine, valine
 Also rich in proline and lysine
 Some proline are hydroxylated to hydroxyproline
 A few lysine residues are oxidatively deaminated to allysine by lysyl




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oxidase
 Occurs after secretion of tropoelastin from the cell
Connective tissue protein with rubber-like properties
Gives tissues and organs the capacity to stretch without tearing
Elastin fibers can be stretched to several times their normal length, but
recoil to their original shape when the stretching force is relaxed
Found in lungs and walls of large blood vessels, elastic ligaments, and skin
Elastin
 Desmosine
 A tetrafunctional cross-link unique to elastin
 Formed from 3 allysines and 1 lysine
 Formation results in an extensively interconnected, rubbery network
• Can stretch and bend in any direction when stressed
• Gives elasticity to connective tissue
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Elastin
 Characteristics of mature, extracellular form:
 Highly insoluble
 Extremely stable
 Has a very low turnover rate
 Exhibits a variety of random coil conformations -
permit the protein to stretch and subsequently
recoil during the performance of its physiologic
functions
 Elastin degradation
 Catalyzed by Elastase- Neutrophil elastase
 A powerful protease
 Released into the extracellular space
 Degrades elastin of alveolar walls as well as
other structural proteins in a variety of tissues
 Can destroy alveolar wall if unopposed by 1antitrypsin
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1- antitrypsin /1-AT/AAT/1-antiproteinases
 Inhibits proteolytic enzymes that hydrolyze and destroy proteins
 Present in blood and other body fluids
 Most of the 1-antitrypsin in plasma is synthesized and secreted by the liver
 The rest is synthesized by several tissues including monocytes and alveolar
macrophages.
 1- antitrypsin deficiency

Due to different gene mutations

Inheritance of 2 abnormal alleles leads to an increased risk for
emphysema

1-AT methionine-Required for binding AAT to target proteases.
Smoking causes oxidation and inactivation of methionine residuesmokers with AAT-Poor survival

Treatment: weekly intravenous administration of 1- antitrypsin

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Collagen v/s Elastin
Collagen
1.- Many differentgentic types
2.- Triple helix
Elastin
One genetic type
No triple helix; random coil
conform ations allowing stretching
3.- (Gly-X-Y)n repeating structure No (Gly-X-Y)n repeating structure
4.- Presence ofhydroxylysine
No hydroxylysine
5.- Carbohydrate containing
No carbohydrate
6.- Intram olecular aldol crosslinks Intram olecular desm osine crosslinks
7.- Presense of extension peptides No extension peptidesduring biosynthesis
during biosynthesis
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Concept map for fibrous proteins, collagen, and elastin
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Thank You