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Proteins > Collagen > Flashcards

Collagen Flashcards

1
Q

What type of protein is collagen?

What type of fibres does it form?

A

It is part of a family of fibrous proteins

It forms insoluble fibres and has a high tensile strength

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2
Q

What is collagen a major element of?

What organs is it found in?

A

It is present in most organs

It is a major element of skin, bone, tendon, cartilage, blood vessels and teeth

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3
Q

How does collagen hold cells together?

What is its role in developing tissues?

A

It holds cells together in discrete units called basement membranes

It has a directive role in developing tissue

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4
Q

How abundant is collagen?

A

it is the most abundant protein in mammals

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5
Q

How many different types of collagen are there?

Why are there different types of collagen?

A

There are 12 different types of collagen

The different types result from different polypeptide chains in tropocollagen

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6
Q

What are the 3 broad types of collagen?

A
  1. fibril-forming collagens
  2. network-forming collagens
  3. fibril-associated collagens
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7
Q

What are the fibril-forming collagens?

Where are they found?

A

Types I, II and III

They form long fibrils

They are involved in bone and tendon

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8
Q

What are the network-forming collagens?

What structure do they form?

A

Types IV and VII

They DO NOT form long protein ropes, they form a 2 dimensional matrix

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9
Q

What is the importance of the matrix formed by network-forming collagens?

A

It is important in the basal lamina

This ensures that tissues are properly organised

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10
Q

What are the fibril-associated collagens?

What are they involved with?

A

Types V, IX and XII

They are involved in crosslinking between other collagen molecules

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11
Q

How many protein chains does each type of collagen consist of?

A

All types of collagen consist of 3 protein chains

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12
Q

What happens to a newly synthesised polypeptide within fibroblast cells?

A

There are 3 main modifications that make the pre-pro-polypeptide into a pro-collagen molecule

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13
Q

What is the first modification that a pre-pro-polypeptide must undergo within fibroblast cells?

A

The signal peptide from the N-terminal is removed

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14
Q

What additional residues are added to the pre-pro-polypeptide in fibroblast cells?

What enzyme is involved?

A

Lysine and proline residues get additional hydroxyl groups added to them

This is catalysed by hydroxylase enzymes, which require vitamin C as a cofactor

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15
Q

What happens after the lysine and proline residues have had hydroxyl groups added to them?

A

Glycosylation of the selected hydroxyl groups on lysine with galactose and b-glucose

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16
Q

What forms the procollagen triple helical cable?

A

3 left handed helices twisting into a right-handed coil

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17
Q

What happens to the procollagen after it is formed?

A

It moves into the Golgi apparatus for final modifications

It is then packaged into secretory vesicles to enter the extracellular space

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18
Q

What enzymes act on the procollagen in the extracellular spaces of connective tissue?

What do they do?

A

Collagen peptidases perform propeptide cleavage

They remove the ends of the procollagen molecule so that it becomes tropocollagen

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19
Q

What happens to tropocollagen molecules after they have formed?

A

They aggregate to form a fibril

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20
Q

What enzyme acts on tropocollagen molecules within an aggregated fibril?

What does it do?

A

Lysyl oxidase enzyme acts on lysine and hydroxylysines

This causes covalent bonds to form between tropocollagen molecules in the fibril

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21
Q

What is the primary structure of collagen like?

A

Collagen has a repeating unit where glycine is every third residue

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22
Q

What is the typical repeating unit in collagen?

A

(Gly - X - Y)

Where X is often Pro and Y is often Hyp

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23
Q

What are the 3 different covalently modified amino acids found in collagen?

A

4-hydroxylproline (Hyp)

5-hydroxyllysine (HyL)

Allysine

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24
Q

How is allysine formed?

A

From the deamination of lysine or hydroxylysine

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25
Q

What enzymes catalyse hydroxylation of proline and lysine residues?

When does this occur?

A

Prolyl hydroxylase and lysyl hydroxylase

They BOTH require vitamin C as a cofactor

This process occurs before the polypeptide chains form a helix

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26
Q

What is hydroxyproline involved with?

A

Hydrogen bond formation

This helps to stabilise the triple helix

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27
Q

What is hydroxylysine involved with?

A

The residues are attachment sites for sugar residues

They are also involved in cross-linking between collagen chains

28
Q

What is the secondary structure of collagen?

A

Each polypeptide chain forms a left-handed helix with 3.3 residues per turn

29
Q

Why is the secondary structure of collagen different to an alpha helix?

A
  1. alpha helix has a right-handed configuration

2. the alpha helix is NOT as tightly wound

30
Q

What is the quaternary structure of collagen?

What does it form?

A

Right-handed triple helix formed from 3 polypeptide chains twisted together

This triple-helical cable is tropocollagen

31
Q

How does the arrangement of side chains in tropocollagen differ to an alpha helix?

A

In an alpha helix, all the side chains sit outside the helix

In tropocollagen, the glycine residues are packed into the centre of the molecule

32
Q

What are extension peptides?

A

Collagen chains are synthesised with additional amino acids at each end

These are extension peptides

33
Q

What is the first stage in assembly of collagen?

A

Disulphide bonds form between the C-terminal extensions

This pins the 3 strands together and holds them in place during triple helix formation

34
Q

How come disulphide bonds can form between C-terminal extensions?

A

Cysteine residues have free -SH groups that can form disulphide bonds

35
Q

What happens after disulphide bonds have formed in collagen assembly?

A

The triple helix is secreted into the extracellular space

Peptidase enzymes cleave off the extension peptides

36
Q

What happens once the extension peptides have been cleaved off?

A

Tropocollagen spontaneously forms fibrils

They are stabilised by covalent cross-links between the strands

37
Q

What is the purpose of covalent cross-links between and within tropocollagen molecules?

A

They give tropocollagen strength and rigidity

38
Q

Between which residues can cross-links form?

A

Between lysine and allysine

Or between 2 allysine residues

39
Q

What makes up a collagen microfibril?

A

A collagen microfibril consists of a staggered array of tropocollagen molecules

The tropocollagen molecules are held together by cross-links

40
Q

What are nucleation sites?

What happens there?

A

Holes between tropocollagen fibrils

Calcium deposition occurs at the nucleation sites to form bone

41
Q

How are nucleation sites positioned and why?

A

They are evenly positioned along the fibre to give an even deposition of collagen

This produces strong bones

42
Q

What is formed from large groups of collagen fibrils?

A

A larger fibre

The fibres are then incorporated into a primary fibre bundle

43
Q

What is the endotendon?

A

The membrane that surrounds each primary fibre bundle

This holds separate units together and increases the strength of the tendon

44
Q

What is the entire tendon structure surrounded by?

A

A membrane - the epitendon membrane

45
Q

What is the turnover rate of collagen like?

A

It is very stable and does not have a high turnover rate under normal conditions

46
Q

When may collagen breakdown be necessary?

A

Tissue repair

Growth

Tissue remodelling (e.g. pregnancy)

47
Q

What enzymes catalyse the break down of collagen?

A

Collagenases

48
Q

What family of enzymes do collagenases come from?

A

Metalloproteinases

These are enzymes which break down protein and have a metal ion in their active site

49
Q

Why are collagenases important in cancerous tumour formation and metastasis?

A

In order for a tumour to spread, it must penetrate the basement membrane

Tumours secrete collagenases to achieve this

50
Q

What causes Dupuytren’s contracture?

A

excess collagen production

51
Q

What does Dupuytren’s contracture affect?

A

It affects the connective tissue in the hand, causing the fingers to bend and become stiff

52
Q

How is Dupuytren’s contracture treated?

A

Injection of collagenases

53
Q

What causes Ehlers-Danlos syndrome?

A

A range of different mutations in the collagen chain

It is often a missing or ineffective lysyl oxidase enzyme

It is an inherited disorder

54
Q

What is the consequence of having a missing or ineffective lysyl oxidase enzyme?

A

No covalent crosslinks can form to stabilise the final collagen molecule

55
Q

What are the consequences of Ehlers-Danlos syndrome?

A

It leads to fragile stretchy skin and hyperextendable joints

The specific symptoms vary depending on the mutation that causes the disease

56
Q

What are the consequences of having hyperextendable joints?

A
  1. frequent dislocations
  2. joints coming out of place without prior injury
  3. problems with blood vessels
  4. problems with eyesight
57
Q

What causes osteogenesis imperfecta (brittle bone disease)?

What is it?

A

Mutations in type I collagen

It is a range of inherited disorders leading to an increase in the risk of bone fractures

58
Q

Why is osteogenesis imperfecta often difficult to diagnose?

A

It often arises as a spontaneous mutation

This means there is no family history to suggest a genetic disorder

59
Q

What commonly causes osteogenesis imperfecta?

How does this impact collagen?

A

Mutation of a glycine residue

Replacing a glycine residue with an AA with a large side chain affects formation of the triple helix

60
Q

What is characteristic of Type I osteogenesis imperfecta?

A

It is the least severe

It is shown by curvature of the bones

61
Q

Why is there no such thing as type II osteogenesis imperfecta?

A

Babies with this type do not survive birth

62
Q

What is characteristic of type III osteogenesis imperfecta?

A

Severe bone damage and curvature of the spine and long bones

Bones are weak and any weight put on them causes them to break

Height is restricted

63
Q

What is characteristic of type IV osteogenesis imperfecta?

A

It often results in amputations

This is because the bones are so damaged that they cause too much pain

64
Q

What is significant about type III and IV osteogenesis imperfecta?

A

Sufferers are almost always in wheelchairs

65
Q

What treatment may be used for milder cases of brittle bone disease?

A

Steel rods may be inserted into the bones of sufferers to aid them in walking

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