Biochemistry: Collagen And Connective Tissue Flashcards
Connective tissues
Cartilage Bone Tendon Skin
§ highly specialised tissues
§ provide mechanical support and assist in movement
§ matrix around cells regulates their behaviour
§ may contain blood vessels, nerves
§ arena for fighting infection
§ diverse structure and function, but composed of same building blocks
Provide mechanical support to the body
Matrix around the cell affects behavior
Connective tissue components
§ cells – dynamically synthesise and break down the connective tissue
§ fibroblasts in skin, tendon ligament
§ chondrocytes in cartilage
§ osteocytes, osteoblasts, osteoclasts in bone
§ extracellular matrix
(a) structural components – give the tissue its mechanical properties
§ ‘ground substance’ – mostly proteoglycans (e. g. aggrecan) § elastic fibres
§ collagen fibres
(b) matricellular proteins – have regulatory, non-structural roles
Proteoglycans
Important for mechanical properties of connective tissue
consist of a protein core, decorated with sugar (called “glycans” or
“glycosaminoglycans” or “GAGs”) chains
there are various kinds of GAG chains, built from different building blocks. Most are sulfated - this is what makes it negatively charged
Sugars are negatively charged so bring water into the cell
Proteoglycans 2
§ variable in size - small (~40kDa) to large (100 kDa+)
§ hold water in tissues
§ confer viscoelastic properties
§ interact with cells, cytokines and collagen
Aggrecan
Forms huge multimeric aggregates in cartilage
§ Link protein stabilises binding to hyaluronan (HA) ‘backbone’
§ huge complexes of sulfated chondroitin & keratan sulfate are highly
hydrophilic
§ draws water into tissue and so enables cartilage to resist compression
Interaction between collagen and aggrecan gives collagen it’s properties
They’re highly hydrophilic
In osteoarthritis: degradation of aggrecan and type II collagen impairs mechanical properties of cartilage
§ in osteoarthritis, altered joint mechanics causes breakdown of aggrecan and also collagen by proteolytic enzymes
§ this impairs cartilage functions, causing pain in affected joints.
Elastic fibres (10 nm microfibrils)
§ consist of multiple components:
§ elastin
§ fibrillin-1, -2, and -3
§ fibulins (esp 4 and 5)
§ matrix-associated glycoproteins
§ structural role :
§ enable stretching of blood vessel
walls, alveoli, bladder, tendons
§ regulatory role :
§ regulate targeting and activation of
growth factors (especially TGFb)
§ form long microfibrils that sit between collagen fibre bundles
Collagens are a diverse and varied ‘family’
27 collagens, 42 genes – many roles and functions
No need to know all of the names
Collagen fibrils (collagen type I, II and III)
the main structural component of connective tissues
§ very plentiful protein - makes up 12-17% of whole-body protein content
§ collagen fibrils are made of many collagen molecules, with each molecule consisting of 3 “alpha” polypeptide chains arranged in a triple helix :
§ each chain is a single gene product
§ either 3 identical a chains (homotrimer) e.g. type 2 collagen (cartilage) is a13 (3 is subscript)
§ or two or more different a chains (heterotrimer) e.g. type I collagen (skin, bone) is a12a21
§ very stable and slowly turned over – has been extracted from dinosaur bones!
Collagen structures (triple helix)
Each molecule of fibrillar collagen is made up of 3 intertwined (triple helix) polypeptides
§ unique amino acid composition of Gly-X-Y repeats
(X= often proline, Y = often hydroxyproline)
§ tight packing of triple helix depends on glycine being every 3rd residue - § glycine is the smallest amino acid
§ proline & hydroxyproline provide rigidity and stability -
§ hydroxyproline is an unusual amino acid, mostly found in collagen
§ stabilised by hydrogen bonds between chains
Collagen fibril synthesis
(Only need to know first steps)
processing is essential for stability
post-translational modification
§ Hydroxylation (NB for chain stability!) § glycosylation
assembly of three alfa chains
disulfide bond formation
assembly of triple helix
What vitamin is hydroxylation dependent on
(& scurvy)
Vitamin c
Vitamin C deficiency causes reduced collagen hydroxylation, and causes scurvy
reduced hydroxylation of proline weakens collagen triple helix
weaker collagen fibrils leads to scurvy:
§ bleeding gums
§ loss of teeth
§ skin lesions, bruises § poor wound healing
§ joint pain & weakness
Connective tissues have different collagen composition and organisation
Skin
§ 60% type I collagen
§ 30% type III collagen
§ meshwork of fibres
Tendon/Ligament
§ 90+% type I collagen
§ 5% type III collagen
§ parallel fibres
Bone
§ 90% type I collagen
§ 3% type V collagen
§ sheets (lamella)
Cartilage
§ 95% type II collagen
§ meshwork
No need to learn percentages
In bone: type I collagen fibrils form ‘lamellae’
§ type I collagen is principal protein component
§ type I collagen fibers are arranged into ‘lamellae’ (concentric circles)
§ hydroxyapatite crystal deposits harden bone
Collagen In tendon:
tensile strength is provided by type I collagen fibrils arranged in parallel bundles
In tendon:
the insertion (‘enthesis’) has a different matrix composition & organisation
Uncalcified fibrocartilage:
As for tendon plus:
type II collagen: 2-5%
proteoglycan (aggrecan): 3%
Tendon:
type I collagen: 90+%
type III collagen: ~5%
proteoglycan: 0.5%
No need to learn %
enthesis is a common site of tendon pathology
§ is a region where mechanical stress is concentrated
§ common site of microdamage and ‘overuse’ injury
§ e.g. tennis elbow, golfer’s elbow
Changes associated with tendinopathy:
§ loss of collagen fibril organisation and reduced fibril diameter
§ changes in collagen content
§ increased type III collagen
§ increased turnover
§ vascular and nerve infiltration (painful tendons only)
§ failure to repair
Collagen in Skin and blood vessels:
are rich in type III collagen
§ type III collagen forms mixed fibrils with type I collagen
§ deficiencies in type III collagen (COL3A1 gene) that affect fibril formation lead to fragility of skin and blood vessels (e. g. Ehlers Danlos Syndrome Type IV)
examples of acquired connective tissue diseases are:
Scurvy – a disease of impaired collagen synthesis
§ Vitamin C deficiency causes poor proline hydroxylation
§ structure of fibril-forming collagens (types I, II and III) is weakened
§ skin/bone/gums affected
Diseases of altered mechanics
§ abnormal mechanics lead to altered connective tissue structure, which
impairs the tissue function
§ e. g. tendinopathy
§ often affects the enthesis
§ altered collagen content and organisation
§ e. g. osteoarthritis
§ increased breakdown of collagen (type II) and aggrecan in cartilage
Genetic conditions of connective tissue
§ the conditions reflect the function of the protein and where it is expressed
§ phenotypic effects of mutations are often highly variable depending on the specific
mutation, and so therapy is often guided by genotype
§ rarer forms of these diseases can be caused by mutation of other genes
Mafan syndrome
Protein affected: fibrillin 1
Connective tissue component: elastic fibers
Genes: FBN1
Tissue affected: CV system, eyes, lungs, tendons, ligament
§ affected individuals are tall, with long hands/feet and cardiovascular effects
§ mutation in fibrillin-1 (>1800 different mutations have been reported, often missense)
Osteogenesis imperfecta
Protein affected: type 1 collagen
Connective tissue component: collagen fibers
Genes: COL1A1 , COL1A2
Tissue affected: Bone
§ type I collagen is abundant in bone, so this mutation causes ‘brittle bone disease”
§ position of mutation determines severity of phenotype
§ most common mutations cause glycine to be replaced with larger amino acid (e. g. cysteine) that distorts packing of the collagen helix
TYPE I Osteogenesis imperfecta
§ mildest and most common form
§ mutations cause a quantitative defect, with ~half the amount of type I collagen produced (called haploinsufficiency)
§ bones fracture easily
§ sclerae can appear blue – thin sclerae allow the pigmented coat of the choroid to be visible
TYPE II Osteogenesis imperfecta
§ most severe form of OI – perinatal lethal
§ mutations cause a qualitative defect, with
abnormal type I collagen produced
§ very fragile bones – intracranial fractures, severe breathing difficulties
e.g. lack of skull mineralisation, severe osteopenia, multiple fractures, long bone and rib deformities
Vascular Ehlers Danlos Syndrome (EDS)
Protein affected: type III collagen
Connective tissue component: collagen fibers
Genes: COL3A1
Tissue affected: blood vessels, skin
Alport syndrome:
Protein affected: type IV collagen
Connective tissue component: collagen fibers
Genes: COL4A3, COL4A4, COL4A5
Tissue affected: Basement membrane of many tissues (kidneys, ears, eyes)
Classical EDS
Protein affected: type V collagen
Connective tissue component: collagen fibers
Genes: COL5A1, COL5A2
Tissue affected: skin, joints
