L13- Post Translational Modification Flashcards
Which type of proteins are synthesised by:
A) free ribosomes
B) ribosomes on the rough ER
A) Proteins destined for cytoplasm or import into organelles (nucleus, mitochondria etc)
B) proteins destined for membrane or secretory pathway by co translational insertion
What is required for protein sorting?
- a signal (address) that is intrinsic to the protein
- a receptor that recognised the signal and directs it to the correct membrane
- a translocation machinery
- energy to transfer protein to new place
What is the difference between constitutive secretion and regulated secretion?
- Constitutive secretion: happens all the time, not particularly regulated e.g. albumin (plasma protein) is secreted constantly
- Regulated secretion: controlled and is up or down regulated
Eg. Endocrine cells secreting hormones (insulin), exocrine cells secreting digestive juices and neurocrine cells secreting neurotransmitters
What is a signal sequence?
- N-terminal AA sequence
- it is 5-30 AA in length
- the central region is rich in hydrophobic AA’s
- able to form an alpha helix
- the “pre” name is the signal sequence
Outline the steps of synthesis of secretory proteins and their translocation across the ER membrane.
- Signal sequence synthesised by ribosome at n terminus
- Signal recognition particle (SRP) binds to signal sequence ribosomes- temporally stops protein synthesis
- SRP binds to SRP receptor on the ER membrane
- hydrolysis of GTP to GDP + pi, releases their SRP and the translocon opens and Newly synthesised protein gets through pore into lumen of ER
- Signal peptidases cleave signal sequence (breaks peptide bond)
- The rest of the protein is translated and ribosome leaves
- Protein is folded
What are some of the functions of the endoplasmic reticulum (name the 3 important ones first) ?
- Glycosylation
- Formation of S-S (disulphide) bonds
- Proper folding of proteins
- Insertion of proteins into membranes
A) What is N-linked glycosylation?
B) Why is it important?
A) The addition of sugars to an asparagine side chain (involving an amino acid group)
B) - correct protein folding
- protein stability
- facilitates interactions with other molecules
- deficiencies in N-linked glycosylation lead to severe inherited human diseases: congenital disorders of glycosylation (CDG)
What is the role of protein disulphide isomerise (PDI) ?
- forms disulphide bonds
What is an AA residue with lots of sulfide bonds?
Cysteine
What happens if there are protein 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 mechanisms help attempt to correct folding problems?
ER chaperone proteins such as BiP (binding immunoglobulin protein) and calnexin and calreticulin
- retain unfolded proteins in the er
- act as sensors to monitor extent of protein misfolding: mediate increased transcription of chaperones, mediate reduction in translation
What happens if misfolding cannot be corrected?
- protein may be returned to cytosol for degradation
- protein may accumulate to toxic levels in the ER resulting in disease
What’s direction do proteins move through the Golgi apparatus?
Cis to trans movement
Outline the basic structure of collagen?
- Basic unit is: Tropocollagen:
- 300nm rod shaped protein
- 3 polypeptide (alpha chains)
- glycine in every 3rd position along each alpha chain: GLY-X-Y repeat (glycine small enough to fit in middle of helix)
- characteristic triple helix (right handed)
- LOTS of hydrogen bonds between alpha chains stabilise the structure
A) What are the 3 characteristics of collagen?
B) What makes collagen have these cxtics?
A) non-extensible, non-compressible and high tensile strength
B) Glycine in every 3rd position (lots of H bonds) and mostly proline or hydroxyproline in x AND y positions
- h BONDS between alpha chains stabilise structure
