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COLLAGEN AND ITS DISORDERS

DR. Bhavika . Vanani.Resident Biochemistry

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• Collagen is a major structural protein of the ECM constitutes about 25% - 30% of the protein of mammals.

• The word comes from Greek language which means to produce “Glue”.

• Collagen is fibrous protein that possesses high tensile strength and cannot be stretched.

• It provides an extracellular framework for all metazoan animals and exists in virtually every animal tissue.

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• In addition, a number of proteins (eg, the C1q component of the complement system, acetylcholinesterase, pulmonary surfactant proteins SP-A and SP-D) that are not classified as collagens have collagen-like domains in their structures; these proteins are sometimes referred to as “noncollagen collagens.”

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Types of Collagen• There are many types of collagen which are

made up of around 30 distinct polypeptide chains (each encoded by a separate gene) have been identified in human tissues.

• The α chains associate in different combination to form the various types of collagen found in the tissues. For example, the most common collagen, type I, contains two chains called α1 and one chain called α2 (α12α2), whereas type II collagen contains three α1 chains (α13).

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Classification

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Basic Structure

• Basic structural unit of all collagen is a trimer of polypeptides called tropocollagen that forms a triple helix.

• Tropocollagen is a rod shaped molecule, 300nm long & 1.5 nm thick, consists of 3 helical polypeptide chains having 1050 amino acids – “α-chains”.

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• α-chains- tight, left-handed triple helix with 3.6 AA per turn.

• 3 intertwined α-chains form a superhelix with a right handed sense.

• Opposite direction of twists of individual chains and the triple helix makes tropocollagen very resistant to unwinding by tension.

• Peptide bonds are internal--resistant to digestion by proteases.

• Triple helix is stabilized by hydrogen bonds between the peptide bonds of different chains.

8• The hierarchical design of collagen

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Amino Acid composition and Sequence

• Collagen has a unique primary structure, represented as - Gly- X-Y.

• Glycine is one - third of total amino acid content of collagen followed by hydroxyproline and proline account for another one-third of amino acid content of collagen.

• X- is frequently proline, 3-hydroxyproline, glutamate, histidine, leucine or phenylalanine.

• Y- 4-hydroxyproline or hydroxylysine, threonine, lysine, arginine.

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• Proline - facilitate the formation of helical conformation of α- chain, because its ring structure causes sharp band in the peptide chain.

• Glycine fits into the restricted spaces where the three chains of the helix come together- tight binding and compaction.

• Hydroxyproline & hydroxylysine contains additional hydroxyl group which participates into additional hydrogen bond.

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BIOSYNTHESIS OF COLLAGEN

Precursors:• Collagen is one of the proteins that functions

outside the cell.

• Polypeptide Precursors of the collagen molecule are formed in Fibroblasts, osteoblasts and chondroblasts.

• These are secreted into the extracellular matrix.

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Overview of biosynthesis of collagen

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1. Polypeptide synthesis:• Pre –pro-α-chains contain a special amino

acid sequence of 15-30 hydrophobic AA at their N-terminal.

• This sequence acts as a signal that directs entry of Pre –pro-α-chains into cisternae of ER where it converted to pro-α-chains or procollagen by removal of signal sequence.

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2. Post-translational modifications:Hydroxylation: • Proline and lysine residues are hydroxylated

by enzymes prolyl hydroxylase and lysyl hydroxylase.

• This reaction requires Fe+2 and vitamin C.• In Vit. C deficiency, collagen fibers cannot

cross link- and tensile strength is decreased (scurvy).

16Hydroxylation reaction of Proline and Lysine

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Glycosylation: • Modified by glycosylation with glucose or

galactose residues linked to the hydroxyl of 5-hydroxylysine.

• Mn+2 containing UDP- glycosyltransferases.• Carbohydrate content varies from 0.5-1% in

type I and III to nearly 15% in type IV.

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3. Procollagen triple helix formation:• 3 Pro α-chains assemble together to form

triple helix.• Procollagen has a central region of triple helix

and its ends have non-helical regions of amino and carboxyl terminal extensions of 100-300 residues.

• These extensions are called Propeptides or Extension peptides – help to correctly align the 3 polypeptide chains.

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• Interchain and intrachain disulfide bonds are formed between the C- terminal extensions of the pro α-chains.

• This alignment of pro α-chains is favorable for helix formation.

• Then pro-collagen is packaged into secretory vesicles within Golgi- apparatus and then secreted out of fibroblasts .

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4. Procollagen to tropocollagen formation: • After their release the Procollagen molecules

are cleaved by procollagen peptidases.• These remove the terminal Propeptides.• Triple helical structure is now released as

mature collagen monomer, Tropocollagen.

22Formation of procollagen and tropocollagen

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5.Collagen fibril formation:• Tropocollagen spontaneously associate with

each other and form collagen fibrils (Self-assembly).

• Aggregates in a staggered head-to-tail arrangement.

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6. Cross-linking and fiber formation: • Tropocollagen molecules extensively cross-linked

covalently both within and between molecules.• The fibrils that are formed become a substrate

for lysyl oxidase.• It contains copper and requires pyridoxal

phosphate.• It oxidatively deaminates amino group of lysyl

and hydroxlysyl residues in collagen.• Reactive aldehydes- Allysine formed.

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• Such aldehydes form aldol condensation product with other aldehydes or form Schiff bases with unoxidised lysine or hydroxylysine.

• After further chemical rearrangements, stable covalent cross links are formed.

• This cross-linking leads to the formation of mature collagen.

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Degradation of collagen

• Collagen highly stable molecule.• Half life is several months to years.• Degradation occurs during tissue repair and

normal growth and development via collagenases- zinc containing proteinases.

• These polypeptide fragments are now susceptible to intracellular proteases.

• Also seen in Osteoporosis, Tumour metastasis, Paget's dz., RA, osteoarthritis.

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Genetic defects of collagen biosynthesis and metabolism

1. Ehlers-Danlos syndrome• Heterogeneous inherited disorder affecting

musculoskeletal, skin and CV system.• C/B stretchy skin, abnormal tissue fragility,

loose joints or dislocation of joints.• Total 10 types are found.• Type IV- most serious- tendency to rupture

arteries or bowel.• Type VI- ocular rupture

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Types of Ehlers-Danlos syndrome Type Inheritan

ceCause

I Classic AD Not known

II Classic AD Not known

III Hypermobility AD Not known

IV Vascular AD Mutation in gene of type III collagen

VI Kyphoscoliotic AR Deficiency of lysyl hydroxylase

VII Arthrochalasis AD Failure to remove N-terminal propeptides in type I collagen chain(Dermatospraxis – type VII C)

IX -- X-linked recessive

Deficiency of lysyl oxidase

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Diagnosis• medical history and clinical observation• collagen gene mutation testing, collagen

typing via skin biopsy, echocardiogram, and lysyl hydroxylase or oxidase activity.

Differential diagnosis• Cutis laxa• Marfan syndrome

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Treatment• Only symptomatic treatment• Physiotherapy• Orthotic treatment• Surgery

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Ehlers-Danlos syndrome

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2. Osteogenesis Imperfecta• Incidence: 1 in 10000, Autosomal dominant.• Biosynthesis of Type I collagen defective.• Point mutation in pro- α1(I) or pro- α2(I) genes.• C/B fragile bones- K/A brittle bone syndrome or

lobstein Syndrome.• Symptoms: fractures, skeletal deformities,

dwarfism• Retarded wound healing and a rotated and

twisted spine leading to a “humped-back” appearance.

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• Extra skeletal sym: blue sclera, hearing loss, poor teeth development.

• Type I OI -Osteogenesis imperfecta tarda. This disease presents in early infancy with fractures secondary to minor trauma, and may be suspected if prenatal ultrasound detects bowing or fractures of long bones.

• Type II OI - Osteogenesis imperfecta congenita, and is more severe, and patients die of pulmonary hypoplasia in utero or during the neonatal period.

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Osteogenesis imperfecta in stillborn fetus

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3. Epidermolysis Bullosa• Dystrophic form-mutation in COL7A1 gene of Type VII collagen or Glycosylation of collagen impaired.• Skin blisters seen.

4. Dermatosparaxis• Hereditary impairment in removal of terminal

propeptide extensions.• Impaired fibril formation.• Reduced tensile strength of collagen.

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5. Alport syndrome• Both X-linked and autosomal.• Type IV collagen.• Hematuria, ESRD.

6. Schmid metaphysial chondrodysplasia• Type X alpha 1 collagen def. (in hypertrophic

cartilage)• Long bones curved & short.• Widened growth plate & metaphysis.• Normal calcium, phosphorus, urinary aminoacids.

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Other collagen disorders Scurvy• Def. of Ascorbic acid.• Impaired collagen synthesis due to def. of prolyl and lysyl

hydroxylases.• Bleeding gums, subcutaneous hemorrhage, poor wound

healing.

Menkes disease• Deficiency of copper. Enz.-lysyl oxidase.• Defective crosslinking of collagen and elastin.

Lathyrism• Inhibition of lysyl oxidase by 3-amino-propionitrile.

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References• Robert K. Murray, David A Bender, Kathleen M. Botham,

Peter J. Kennelly, Victor W. Rodwell, P. Anthony Weil. Harper's Illustrated Biochemistry,29th Edition. Newyork: Mc Graw Hill;2012. Ch 48,The extracellular Matrix; P.608-25.

• Dinesh Puri. Textbook of Medical Biochemistry, 3rd Edition.India:Elsevier,2011.Ch 5, Proteins with biological activity; P.75-81.

• Pamela C.Champe ,Richard A. Harvey, Denise R. Ferrier. Lippincott’s illustrated Reviews,4th edition. Wolters Kluwer. Ch 1,Protein structure and function; P.43-49.

• Google images

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