Post Transitional Protein Processes Flashcards
Name 2 types of post-translational modifications and describe them.
Proteolytic Cleavage-breaking peptide bonds to remove part of the protein
Chemical modification-addition of functional groups to amino acid residues
What is protein sorting?
Proteins destined for the cytosol or importation into organelles are synthesised on free ribosomes
Proteins destined for the membrane or secretary ppathway via co-translation insertion are synthesised by ribosomes on the RER
What is required for protein sorting?
A signal, intrinsic to the protein (address)
A receptor that can recognise the signal and direst it to the correct membrane
A translocation machinery (get protein into organelle)
Energy to transfer the protein to its new place
What 3 types of proteins are targeted for secretion?
Extracellular proteins
Membrane proteins (go through secretion pathway but stop in membrane)
Vesicular proteins (lysosomes etc (through secretory ER pathway)
Name the 2 types of secretion from cells (of protien)
1) Constitutive secretion
2) Regulated secretion
What is Consitiutive secretion?
Give an example.
Constant secretion
Eg, Albumin (a plasma protein) is secreted constantly
What is Regulated secretion?
Controlled secretion (can be up or down regulated as needed)
Endocrine cells-secrete hormones
Exocrine cells-secrete digestive juices
Neuroscience cells-secrete neurotransmitters
What is a signal sequence (referring to the signal sequence on the Nterminus)
N-terminal AA sequence
5-30 amino acids in length
Central region is rich in hydrophobic residues
Able to form an a-helix
“Pre”proalbumin - the pre defines the signal sequence, this is removed during processing
What is the Signal Recognition Particle? SNP
Receptor needed to bind the signal peptide onto proteins destined for the ER.
Composed of 6 proteins and short piece of RNA
Recognise the signal peptide on the protein and the ribosome and the destination.
Explain the synthesis of proteins and their translocation across the ER membrane.
1) Ribosome in cytosol starts synthesising the protein, starting with its N-terminal signal sequence.
2) The SRP binds to the signal sequence on the protein and halts transcription. The SRP binds to the SRP receptor on a protein in the membrane called a translocon.
3) GTP is hydrolosyed to GDP, this releases the SNP and opens the translocon. Synthesis of the protein now continues but it’s synthesised through the translocon into the ER lumen.
4) A signal peptidase cleaves the signal sequence form the N-terminal of the protein.
5) The protein synth continues through the membrane into the ER lumen.
6) The ribosome detaches and the synthesised protein folds inside the ER lumen. The translocon closes again
What are the function of the ER?
Insertion of proteins into membranes Specific proteolytic cleavage Glycosylation Formation of S-S bonds Proper folding of proteins Assembly of multi-subunit proteins Hydroxylation of selected Lys and Pro residues
What is N-linked Glycosylation?
Sugars are added on an Aparagine side chain.
N-linked chain just refers to the fact the sugar is added to n amino group
Why is Glycosylation of proteins important?
Correct protein folding Protein stability (longer lived) Facilitate interactions with other molecules (io proteins on cell surface to react, immune recognition sites)
What happens when disulphide bonds form in the ER lumen?
Cytosines on AA groups can form disulphide bonds together. A PDI ensures the correct combo of bonds is formed and so the protein has the correct shape for being active. The protein is oxidised in this process
What can happen if there are folding problems in a Protein?
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 are Chaperone proteins?
They attempt to correct mis-folding of proteins in the ER.
They retain unfolded proteins in the ER until they are fixed
They act as sensors to monitor the extent of mis-folding-this mediates increased transcription of chaperones or reduces translation
What happens if mis-folding of proteins in ER lumen can be corrected?
Protein may be returned o cytosol for degradation (parts then re-used)
Protein may accumulate to toxic levels in the ER resulting in disease-may arise due to single mutation
Why is collagen a triple helix?
Non-extensible
Non-compressible
High tensile strength
Why does collagen use glycine in every 3rd position of the a-chain?
As glycine is the only amino acid with a side chain small enough to fit in the middle of the helix
What is the basic unit of collagen?
Troop collagen
What are the main steps in collagen synth and modification?
1) Syntheis and entry of chain into lumen of RER
2) Cleavage of signal peptide (remove the pre from preprochain)
3) Hydroxylation of selected proline and lysine residues
4) Addition of N-linked oligosaccharides
5) Addtiion of galactose to hydroxylysine residues (requires vitamin C, adds OH to Prolyne using an enzyme)
What is Prolyl Hydroxylase?
Associated with PDI (disulphide makes in ER) in the ER
Requires vitamin C and FE2+ ions for activity
Allows increased h bonding to stabilise triple helix
Scurvy is due to weak troop collagen triple helixes
Why don’t C-terminal amino acids join the helix?
They are in the premature protein to stop the helixes joining up inside the ER (where they’d be too big to leave). They are removed after exotosis by the transport vesicle so the Procollagen is now called Collagen and can now form fibres and cross links.
Give an example of a connective tissue disease.
Ehlers-Danlos syndrome
Mutation in collagen 5 of Lyly oxidase deficency
