14 - Post Translation Modifications + Collagen Flashcards
What are the two ways that proteins are post-translationally modified?
- Proteolytic cleavage
- Chemical modifications
What is needed for protein sorting?
- Intrinsic signal sequence
- Receptor for sequence
- Translocation machinery
- Energy for translocation
What will you see histologically if a cell secretes a lot of stuff?
- Extensive RER
- Lots of secretory vesicles
What are the characteristics of a signal sequence?
- N-terminus
- Hydrophobic residues
- 5-30 aa in length
- Forms alpha helix
- Gives precursors “pre” prefix, e.g preproalbumin
What is the SRP?
- Signal recognition particle
- Made of 6 proteins and short piece of RNA
- Recognises ribosome and signal sequence
How is a secretory protein synthesised?
What is the function of the RER?
- Insertion of proteins into membrane
- Glycosylation
- Formation of S-S bonds
- Proteolytic cleavage
- Hydroxylation (Pro, Lys)
- Folding of proteins
- Assembly of multisubunit proteins
What is N-linked glycosylation and why is it done?
- Adding sugar to amino group (e.g asparagine)
- Protein going into uncontrolled environment, increases stability and prevents from degradation
- Allows protein to interact with other molecules, e.g signalling
What is CDG?
Congenital disorders of glycosylation
Lack enzymes to glycosylate
Autosomal recessive
Leads to mental retardation
How do disulphide bonds form?
- in er, not in cytoplasm as that is a reducing environment
- Protein disulphide isomerase ensures disulphide bonds occur between correct sulphurs so folds correctly
What happens to misfolded proteins in ER?
- Chaperones can unfold proteins and refold them
e.g Calnexin, calreticulin, BiP
When lots of misfolded proteins there is an increase in transcription of chaperone proteins or a reduction in translation
What happens if misfolding cannot be corrected?
- Protein returned to cytosol for degradation
- May accumulate in toxic levels in the ER and cause disease as ER cannot function
What happens to a protein between it leaving the ER and leaving the cell?
What is the basic unit of collagen and what is it’s structure?
TROPOCOLLAGEN
- Gly every 3rd position
- Mostly Proline and Hydroxyproline
- H bonds between a chains stabilise (HIGH TENSILE STRENGTH)
Why is there so much glycine in collagen?
- Amino acid with smallest side chain so can fit in middle of the helix
- May have a mutation where you don’t have glycine every third, collagen can’t form properly as triple helix can’t form as amino acid too big to fit in middle of helix
What are the names of collagen as it goes through synthesis?
- Preprocollagen
- Procollagen
- Tropocollagen
- Collagen
What is the process of collagen synthesis?
What is Ehlers-Danlos syndrome?
- Lysyl oxidase or collagen type V deficiency
- Weak connective tissue
What is scurvy and why is it caused?
- Weak connective tissue due to a Vitamin C deficiency
- Weak tropocollagen triple helices as lack of prolyl hydroxylase activity so weak tropocollagen (say why)
What is the role of prolyl hydroxylase?
- Hydroxylates proline
- Associated with PDI
- Requires Vitamin C and Iron (II) for activity
- Allow increase h-bonding to stabilise triple helix
What is type I collagen made up of?
Two alpha 1 and one alpha 2
Why would introduction of cysteine to alpha collagen chains be a problem?
Would cause S-S bonds to form between separate alpha chains so they would agregate and cannot separate
Why might a patient have some normal alpha 1 bands but abnormal bands too?
- May be heterozygous
Why do people with osteogenesis imperfecta have blue sclera and irregular ossification?
- Thin collagen making up dense CT of scelera which shows the colour of the underlying choroidal tissue
- Ossification based on hyaline cartilage, which is made up of collagen. Also most of matrix of bone is made up of collagen so poor bone structure
Why may the mature mRNA of collagen be longer than the actual polypeptide?
Post-translational modification, e.g preprocollagen, tropocollagen etc
