Lec 13 - Protein Processing Flashcards
give examples of post translational protein modification and why it is necessary
proteins are specific 3D strucutres, some need modification after translation to work - some proteins would be dangerous if activated in cell, so we wait till they leave the cell to activate them
proteolytic cleavage - breaking pepetide bonds to remove some of the protein
chemical modification - addition of functional groups
what are the two protein sorting methods?
protein that is synthesised of free ribosomes
destined for - cytoplasm, nucleus, mitochondria (posttranslational import into organelles)
Proteins synthesised by proteins on the rough endoplasmic reticulum - will end up embedded in the membrane via co trasnlation insertion, or a secretory vesicle or in lysosomes after processing in the golgi complex
what is constitutive secretion ?
slow continuous process of secretion eg saliva
what is regulated secretion ?
secretion controlled by a signal of some sort
endocrine secretion - secreting hormones
exocrine cells - secrete digestive juices
neurocrine cells - secreting neurotransmitters (neuronal synapses)
briefly explain how a secretory cell is organsied
nucleus and lots of RER at bottom of the cell
RER makes secretory proteins
then processes by the golgi apparatus
before packaged in secretory vesicles near the surface of the cell - eg pancreatic acinar duct
mitochondrion to provide then energy for protein modification and transfer
what is a signal sequence ?
a sequence of amino acids at the N terminus of a pre-protein
it allows the SRP to bind to the protein via the signal sequence
will be removed once the protein is fully processed
what is the SRP
signal recognition particle
will recognise the signal sequence and bind to it
will also recognise the ribosome, allowing it to join and translation to take place
explain the synthesis of a secretory protein
ribosome translates the signal sequence (cytoplasm)
SRP recogs and binds to signal sequence and ribosome - this stops protien synthesis
SRP binds to a SRP receptor
using GTP –> GDP as translocon is opened and SRP leaves
the protein folds into translocon space in intermembrane space
signal peptidase cleaves the signal sequence
this allows translation to continue into the ER lumen
folded completed protein is produced and ribosome leaves
explain the synthesis of type 1 membrane protein
same as secretory protein with SRP ect until
the signal peptidase cleaves signal sequence
riobosome translates protein N terminus lead way into space
until a stop transfer anchor sequence stops going inot space as its hydrophobic, acts as anchor to hold protein in membrane
translation occurs on other side of membrane until complete and C terminus is completed
ribosome leaves
what are the functions of the endoplasmic recticulum ?
Insertion of proteins into membranes
Specific proteolytic cleavage
Glycosylation - addition of carbohydrate residue
Formation of S-S bonds - rigidity , resist protein degregation
Proper folding of proteins
Assembly of multisubunit proteins
Hydroxylation of selected Lys and Pro residues
what is N linked glycosilation ?
additon of sugars to N terminus
needed for correct protein folding, stability and function
what does PDI (protein disulphide isomerase) do and why?
forms a disulphide bond between two SH’s
via a redox reaciton
this is a strong bond formation, so vital in protein folding and stability
what happens if mis folded proteins cannot be corrected ?
mis folding proteins will not function properly so may be degraded in the cytosol]
protein may accumulate to toxic levels in the ER resulting in disease - prevention of ER working properly
can be caused by mutations
diseases such as tay-sachs and alzhemiers and cystic fibrosis are examples of protein misfolding
what is the role of the golgi apparatue ?
protein comes from ER and enters the cis golgi face
protiens are sorted
proteins are phosphorylated here
GAL can be added and Manose removed
sulfation of tyrosines and carbs also occurs
sorted protiens exit from the trans golgi face
enter lysosomes, secretory vesicles, and will be tranfixed to plasma membrane
explain the structure and function of collagen fibers
Tropocollagen unit - rod shaped protein
forms a triple helix
glycine -X-Y repeating strucuture - the X and Y are mainly made of proline and hydroxyproline
glycine is used alot as it has small enough R group to fit in middle of helix strucutre
this is non extensible and non compresisble - high tensile strength
H bonds between alpha chains stabilise the structure
after some processing in the ER, the collagen s released to the golgi, more processing in the golgi
then procollagen is secreted in a transport vesicle via exocytosis
the molecule is a pro as it still has N and C terminal propetides, this prevents further cross linkage than just the triple helix - this means we dont form collagen fibers until the triple helicies have left the cell. as collagen fiber formation would destroy the cell
once out of the cell the N and C terminal propetides can be removed by proteases, -PROTEOLYTIC CEAVAGE
to give a unit of tropocollagen from procollagen
collagen molecules undergo lateral asscoation due to cross linkage between units to form a collagen fibril
fibrils aggregate to form a collagen fibers
