post translational processing of proteins and protein targeting Flashcards
post translational modification
proteolytic cleavage- breaking of peptide bonds or chemical modification by addition of a phosphate group. These may activate proteins.
where are proteins destined for cytosol or into other organelles made?
free ribosome
where are proteins destined for secretory pathways made?
on ribosomes attached to ER they are then secreted by constitutive or regulatory secretion
what are constitutive and regulatory secretion?
C= secreted all the time eg saliva R= release stimulated by hormones eg insulin
how do proteins enter organelles? specifically the ER
1) mRNA binds to ribosome and translation begins
2) signal recognition molecule binds to N terminus with the recognition sequence on it. translation stops.
3) SRP binds to SPR signal protein receptor in ER membrane
4) SRP is released and translation begins, going through membrane
5) on the other side of the membrane the signal sequence is chopped off as its not needed.
6) once translation is finished protein falls through into ER lumen.
functions of the ER
glycolysation s-s bond formation folding of proteins proteolytic cleavage insertion of proteins into membrane assembly of multiple subunit proteins hydroxylation (+OH group)
what is glycolysation and its purpose
addition of carbohydrate chain =
protein stability
correct folding
helps interactions with other molecules
what are disulphide bonds and their functions?
between sulphur R groups , much stronger bonds and this is important outside the cell. Enzyme called disulphide isomerase ensure correct type of disulphide bond is formed
modifications that occur in the Golgi?
phosphorylation
formation of disulphide bonds
removal/addition of certain proteins. Enter through cis faced leave packaged into vesicles at trans face post modification.
what happens if there is a problem with protein folding?
an ER chaperone will correct it by retaining it in the ER or unfolding and refolding the protein occurs. problem is if it can’t be corrected then proteins are held in the RER and this prevents it from doing its job = toxic
structure of collagen
basic unit = tropocollagen x3 wrapped around each other.
glycine every 3rd amino acid and h bonds between chains keep the structure stable. Gives It high tensile strength, non compressible and can’t be extended.
synthesis and modification of collagen
1) synthesis occurs and lumen of ER
2) 1 peptide is cleaved
3) hydroxylation of specific residues occurs
4) N-linked oligosacherides are added
5) galactose is added to hydroxolysine
6) propyl hydroxylase is added to allow hydrogen bonding to stabilise triple helix. (requires vitamin C for its synthesis)
7) chains alined and disulphide bonds formed
8) formation of triple helical pro-collagen
9) chains of glucose added
10) packaged into transport vesicle and exits via exocytosis into extracellular space
11) removal of N and C terminus = tropocollagen
12) covalent cross links form between tropocollagen molecules
13) fibrils aggreagate= collagen
what is Ehlers-danlos syndrome (EDS)
mutation in collagen type V which makes skin stretchy
