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31 Cards in this Set

  • Front
  • Back
Polypeptides are synthesized by _____?
Where are polypeptides bound for secretion synthesized?
ribosomes ass. w/ RER
What is transcription?
What is a codon?
A sequence of 3 consecutive DNA bases: one amino acid
What is translation?
mRNA -> protein
What does translation require?
adaptor molecules (tRNA, ribosomes [ribosomal protein, rRNA])
What are the subunits of the ribosome?
Large (60s) and small (40s)
What are they composed of
rRNA and protein
How many steps are involved in translation?
To which end of the previous A.A. is the new A.A. attached to?
carboxyl end
During the elongation phase, an __________ molecule binds with the codon at the ___ site of the ribosome?
amino-acyl tRNA; A
What does this pairing result in?
GTP hydolysis of the elongation factor
What ensures that the correct A.A. is being added to the chain (a translational proofreading)?
A delay in the time from when the amino-acyl tRNA binds and the amino acid is added to the chain allows for assurance of the correct anticodon.
What inactivates elongation factors?
phosphorylation by kinases
Therefore, protein synthesis is indirectly regulated by ______
the kinases that inactivate elongation factors
3.angiotensin II
6.intracellular free A.A.'s
What type of cells commonly have many free polysomes?
erythroblasts; synth. lots of protein for internal use
What type of cells have many ER-bound polysomes?
cells that synthesize proteins for export
What is the Signal Hypotesis?
mRNA's of proteins destined for secretion have a sequence (approx. 20 A.A.'s) that directs the mRNA/ribosome complex to be translocated to the ER.
What enzyme cleaves the signal sequence?
signal peptidase
What molecule recognizes the signal ER sequence?
Signal-recognition particle (SRP)
Why does translation pause after SRP binding to the signal sequence?
To allow time for travel to ER membrane
What is the complex that allows the polypeptide chain to be injected to the lumen of the ER?
Once in the ER, what protein modifications can take place?
1. Signal Sequence cleaved by signal peptidase
2. Disulfide bridge formation - cysteines bond. Maintains s(x) integrity of folded proteins
3.N-Linked Glycosylation
How does N-linked glycosylation occur?
a precursor oligosaccharide is attached to Asn on the polypeptide chain.
What catalyzes this reaction?
Oligosaccharyl transferase
What is the purpose of this transfer?
Marks the state of protein folding. Chaperones will bind to incompletely folded proteins, prevents bad folding.
What happens if the protein in the ER folds improperly?
Translocated back into cytosol (via Sec61) for proteosomal proteolysis (death!)
What post-translational modifications happen at the Golgi?
1.Removal of precursor oligosaccharide
2.Addition of sugars
3.Proteolytic processing of pro-proteins
4.Addition of glycosaminoglycan (GAG) to proteoglycans
Where does protein sorting take place?
Trans golgi network
What are the two modes of secretion for proteins intended to be exported?
Constitutive - not a response to signal; move straight to membrane

Regulated - restrained until a signal arrives (hormone, etc.)
What do lysosomal proteins bind to before leaving the golgi?
mannose-6-phosphate receptors in trans-golgi network