Post-translational Processing Of Proteins Flashcards
What are the 2 types of post-translational modification?
Proteolytic cleavage - breaking peptide bonds to remove part of protein
Chemical modification - addition of functional groups to amino acid residues
What proteins are synthesised on free ribosomes?
Protein destined for cytosol or post-translational import into organelles
What proteins are synthesised by ribosomes on RER?
Proteins destined for membrane or secretory pathway via co-translational insertion
What is required for protein sorting?
A signal, intrinsic to the protein
A receptor that recognises signal and which directs it to correct membrane
Translocation machinery
Energy to transfer protein to new place
What proteins are involved in targeting for secretion?
Extracellular proteins
Membrane proteins
Vesicular proteins (lysosomes, endosomes)
What are the 2 types of secretion from cells?
Constitutive - constantly secreted (e.g. albumin)
Regulated - endocrine, exocrine and neurocrine cells
What is a signal sequence?
N-terminal aa sequence
5-30 amino acids in length
Central region rich in hydrophobic aa residues
Able to form alpha-helix
What does the pre in preproalbumin mean?
Signal sequence which is removed during processing
What is the signal recognition particle (SRP)?
It’s the receptor needed to bind the signal peptide on proteins destined for the ER
Composed of 6 proteins and short piece of RNA
Recognises signal peptide and ribosome
Describe the synthesis of secretory proteins and their translocation across the ER membrane
Ribosome binds to mRNA, with signal sequence outside ribosome
SRP recognises signal sequence and binds to the ribosome and stops protein synthesis
SRP receptor forms part of the translocation machinery
SRP binds to receptor which is GTP dependent
Translocon opens and the SRP is released so protein synthesis can start again
Signal peptidase in the membrane cleaves the signal sequence and protein is translated into the ER
Ribosome then detaches from mRNA
What are the functions of the ER?
Glycosylation
Formation of S-S bonds
Proper folding of proteins
What is N-linked glycosylation?
Sugars are added on asparagine side chain (reaction involves amino group)
Why is glycosylation important?
Correct protein folding
Protein stability
Facilitates interactions with other molecules
Deficiencies in N-linked glycosylation lead to severe inherited human disease
What does protein disulphide isomrase do?
Role in formation of disulphide bonds in ER lumen
What happens if there are folding problems?
Protein may be trapped in mis-folded conformation
Protein contains mutation resulting in mis-folding
Protein may be incorrectly associated with other sub-units
What do ERchaperone proteins do?
Attempt to correct the problems in protein folding
It retains unfolded proteins in ER
Acts as sensors to monitor extent of protein mis-folding
What happens in mis-folding cannot be corrected?
Protein may be returned to cytosol for degradation
Proteins may accumulate to toxic levels in ER resulting in disease
What is the basic unit of collagen fibres?
Tropocollagen
- 3 polypeptides (alpha chains)
- glycine in every 3rd position
- characteristic triple helix
What are the features of the collagen triple helix?
Non-extensible
Non-compressible
High tensile strength
Why is glycine the amino acid in the collagen structure?
Glycine is the only amino acid with a side chain small enough to fit in middle of helix
How are the alpha chains held together in a collagen fibre?
H bonds between alpha chains stabilise
What are the steps of synthesis and modification of collagen in ER?
- Synthesis and entry of chain into lumen of RER
- Cleavage of signal peptide
- Hydroxylation of selected proline and lysine residues
- Addition of N-linked oligosaccharides
- Addition of galactose to hydroxylysine residues
- Chain alignment, formation of disulphide bonds between alpha chains
- Formation of triple-helical procollagen from C- to N- terminus
What is prolyl hydroxylase?
Associated with PDI (protein disulphide isomerase) in ER
Requires vit C and Fe2+ ions for activity
Allows increased H bonding to stabilise triple helix
Scurvy due to weak tropocollagen triple helices
What happens at the C and N terminals of the collagen fibre?
150 extra N-terminal amino acids do not form triple helix
250 extra C-terminal amino acids do not form triple helix
Called propeptides
What are the steps of procollagen secretion?
- Completion of O-linked oligosaccharide chains by addition of glucose
- It is then transported in a vesicle
- Exocytosis occurs
- Removal of N and C terminal propeptides
What enzyme breaks peptide bonds to remove terminal peptides?
Procollagen peptidase
Procollagen -> tropocollagen
This occurs extracellularly
Describe the steps of formation of collagen fibres
- Removal of N and C terminal propeptides
- Lateral association of collagen molecules followed by covalent cross linking
- Aggregation of fibrils
