Post-translational Modification of Proteins Flashcards
(20 cards)
What is the destination of proteins synthesised on:
Free ribosomes?
ER ribosomes?
Free: cytoplasm or organelles
ER: membrane or secretion
What do proteins being translated on free ribosomes all require for translocation?
Signal sequence
Energy
Specialist proteins (receptors, peptidases)
Translocation channels
What is the path followed by proteins intended for secretion?
Cotranslational translocation (translation of the protein is completed while translocating):
Protein is surrounded by trans-ER and buds off in vesicle
Vesicle fuses with cis-Golgi network
Protein is moved through the Golgi (cisternal transport)
Sorting and modifying within the Golgi
Protein is surrounded by grams-Golgi and buds off in vesicle
Protein is secreted (constitutive/continuous or regulated/controlled flow)
(Retrograde transport of protein occurs throughout the process)
How does the ER modify proteins?
N-linked glycosylation:
Sugars are added to the amine group of Aspargine
(Mistakes cause Congenital Disorders of Glycosylation)
Disulphide bond formation:
Two free SH groups (cysteine) are reduced to form a S-S bond (cystine)
Protein Disulphide Isomerase catalyses reaction
How does the Golgi modify proteins?
O-linked glycosylation:
Sugars are added to the hydroxy groups of Serine and Threonine
(Mistakes cause abnormal ECM - used in proteoglycans)
What happens to mis-folded proteins?
Proteins mis-folded are retained in the ER by chaperonins
Chaperonins also:
- Increase transcription of chaperonins
- Decrease translation of proteins
If mis-folding cannot b corrected proteins:
- Return to the cytoplasm for degradation
- Accumulate in the ER (Toxic)
- Can be incorporated into other proteins as subunits
How is insulin formed?
Preproinsulin —> Proinsulin (removal of signal sequence on entry to the ER)
Disulphide bonds are formed in the ER
Proinsulin —> Insulin (removal of C-chain in Golgi)
Why is there a varied and complex yield of proteins?
Different amounts of proteolytic enzymes in different locations
What are the stages of collagen formation?
Preprocollagen Procollagen Tropocollagen Collagen fibrils Collagen fibre
What is the structure of collagen?
Triple helix of alpha chains with a high tensile strength (non-extensible/compressible)
H-bonds between NH of Glycine and C=O of amino acid 4 down the chain (stable alpha chains)
Gly-X-Y repeat units (small side-chain on Glycine fits inside triple helix)
X/Y: Proline/Hydroxyproline (form strong H-bonds)
How is procollagen formed?
Preprocollagen is synthesised during entry to the ER
Cleavage of the signal peptide forms procollagen alpha chains
How are procollagen alpha chains modified in the ER?
Hydroxylation of proline and lysine (prolyl hydroxylase)
N-linked glycosylation of hydroxylysine with galactose
(These modifications allow stronger H-bonds to form)
How do procollagen alpha chains form a triple helix?
Chains align and Disulphide bonds form between them (C -> N)
How is the triple helix of procollagen modified in the Golgi?
O-linked glycosylation
Packaging into a vesicle and exocytosis into the cytoplasm
How does procollagen form tropocollagen?
Procollagen peptidases remove N + C terminal propeptides
How does tropocollagen form collagen fibrils and fibres?
Collagen fibrils are formed when tropocollagen molecules laterally associate and form covalent cross links
Aggregation of collagen fibrils form collagen fibres
How do proteins targeted for the ER get there?
SRP pauses translocation and translocates protein in the ER
Signal sequence in cleaved by signal peptidase
After modification in the Golgi the KDEL sequence on C-terminus binds with a KDEL receptor
KDELR transports protein back to ER and dissociates at neutral pH
How do proteins targeted for the nucleus get there?
Protein is synthesised on a free ribosome
Nuclear Translocation Sequence (NTS) contains positively charged amino acids which bind to importin
Importin translocates protein into the nucleus
RanGTP releases protein and returns importin to cytoplasm (hydrolysing GTP)
How do proteins targeted for lysosomes get there?
Synthesised on the RER
Contain Mannose-6-Phosphate and patch signal
Binds to M6P receptor and is transported to lysosome
Change in pH releases protein
How do proteins targeted for the mitochondria get there?
Protein synthesised on free ribosomes but prevented from folding
Amphipathic signal found on the N-terminal
Protein is translocated through TIM and TOM and folds once signal sequence is cleaved
