S9) Protein Targeting and Collagen Biosynthesis Flashcards
What is collagen?
Collagen is a secreted protein produced by fibroblasts in the connective tissue and is the most abundant protein in the body (25-35%)
Identify some structures composed largely of collagen
- Tendons
- Ligaments
- Cartilage
- Bone
- Loose connective tissue
What is the basic unit of collagen?
Basic unit is tropocollagen
Describe the structure of the collagen molecule
- Consists of 3 polypeptides
- Rod-shaped protein
- (Gly-X-Y)n repeat (glycine in every 3rd position in chain)
- Characteristic triple helix (right-handed)
Which type of bonding in predominant in a collagen molecule?
H-bonds between α chains stabilise structure
State three structural benefits of the triple helix structure of collagen
- Non-extensible
- Non-compressible
- High tensile strength
Why is glycine a central part of the collagen molecule?
Glycine is the only amino acid with a side chain small enough to fit in the middle of the helix
Which other monomers form an integral part of the collagen structure (Gly-X-Y)n ?
- Proline
- Hydroxyproline
What is prolyl hydroxylase and what does it do?
Prolyl hydroxylase is an enzyme which catalyses the formation of hydroxyproline and allows increased H-bonding to stabilise the collagen triple helix
What is scurvy?
- Scurvy is a condition which arises due to insufficient Vitamin C (ascorbic acid) intake
- It presents with weakness, fatigue, and sore arms and legs
Why does scurvy occur?
Prolyl hydroxylase requires ascorbate as a cofactor to function so its absence compromises the enzyme’s function and and weak tropocollagen triple helices form
Describe the tissue distribution of Type I collagen
Type I collagen is found in the skin, tendons, ligaments and bone
Describe the tissue distribution of Type II collagen
Type II collagen is found in cartilage and intervertebral discs
Describe the tissue distribution of Type III collagen
Type III collagen is found in the foetal skin and cardiovascular system
Describe the tissue distribution of Type IV collagen
Type IV collagen is found in the basement membrane
Describe the tissue distribution of Type V collagen
Type V collagen is found in the placenta and skin
The synthesis and modification of collagen involves three stages; synthesis in the ER, procollagen secretion and the formation of the collagen fibre.
In 7 steps, describe the synthesis and modification of collagen in the ER
⇒ Synthesis and entry of chain into lumen of rER
⇒ Cleavage of signal peptide (prepro α chains → pro α chains)
⇒ Hydroxylation of proline and lysine residues
⇒ N-linked glycosylation
⇒ Addition of galactose to hydroxylysine residues
⇒ Formation of disulphide bonds
⇒ Formation of triple-helical procollagen from C- to N-terminus
The synthesis and modification of collagen involves three stages; synthesis in the ER, procollagen secretion and the formation of the collagen fibre.
In 4 steps, describe the secretion of procollagen from the cell
⇒ O-linked glycosylation (addition of glucose)
⇒ Transport vesicle
⇒ Exocytosis
⇒ Conversion of procollagen to tropocollagen (removal of N- and C-terminal propeptides)
The synthesis and modification of collagen involves three stages; synthesis in the ER, procollagen secretion and the formation of the collagen fibre.
In 3 steps, describe the formation of the collagen fibre
⇒ Lateral association of collagen molecules
⇒ Covalent cross-linking
⇒ Aggregation of fibrils
Summarise protein target signals to the ER in terms of:
- Nature of signal
- Signal location in the primary sequence
- Folding during transfer
- Specialist proteins involved
- Signal retention/cleavage
- Energy requirement
- Nature of signal: Hydrophobic signal sequence
- Signal location in the primary sequence: N-terminus
- Folding during transfer: unfolded
- Specialist proteins involved: SRP, SRP receptor & protein translocator
- Signal retention/cleavage: cleaved by signal peptidase
- Energy requirement: yes for hydrolysis of GTP
Summarise protein target signals to the nucleus in terms of:
- Nature of signal
- Signal location in the primary sequence
- Folding during transfer
- Specialist proteins involved
- Signal retention/cleavage
- Energy requirement
- Nature of signal: NLS (basic)
- Signal location in the primary sequence: various positions on surface
- Folding during transfer: folded
- Specialist proteins involved: importin & RanGTP for transport and displacement of NLS
- Signal retention/cleavage: retained
- Energy requirement: yes for hydrolysis of GTP
Summarise protein target signals to the mitochondria in terms of:
- Nature of signal
- Signal location in the primary sequence
- Folding during transfer
- Specialist proteins involved
- Signal retention/cleavage
- Energy requirement
- Nature of signal: amphipathic signal
- Signal location in the primary sequence: N-terminus
- Folding during transfer: held partially unfolded by chaperones
- Specialist proteins involved: Tom & Tim channel complex, chaperones
- Signal retention/cleavage: cleaved
- Energy requirement: yes, ATP hydrolysis by mHsp70
Summarise protein target signals to the lysosomes in terms of:
- Nature of signal
- Signal location in the primary sequence
- Folding during transfer
- Specialist proteins involved
- Signal retention/cleavage
- Energy requirement
- Nature of signal: Mannose-6-phosphate
- Signal location in the primary sequence: signal patch for distinguishment
- Folding during transfer: folded
- Specialist proteins involved: M-6-P receptor in trans-Golgi
- Signal retention/cleavage: phosphate cleaved by phosphatase
- Energy requirement: Yes
Summarise protein target signals for ER retention in terms of:
- Nature of signal
- Signal location in the primary sequence
- Folding during transfer
- Specialist proteins involved
- Signal retention/cleavage
- Energy requirement
- Nature of signal: KDEL (Lys-Asp-Glu-Leu)
- Signal location in the primary sequence: C-terminus
- Folding during transfer: folded
- Specialist proteins involved: KDEL Receptor
- Signal retention/cleavage: retained
- Energy requirement: only for formation of budding vesicle
