Steps In protein Synthesis Flashcards
What direction is mRNA translated
-5’ -3’
What is translation
-assembly of amino acids from tRNA into a peptide chain sequence directed by mRNA template and done by ribosomes
List the 3 steps of translation
1 initiation
2 Elongation
3 termination
Which organism has translation and transcription linked, how and why
- prokaryotes
- translation starts before transcription is done
- genetic material not separated from ribosomes via nucleus
What does initiation need before starting peptide formation and list them
- assembly of components of translation
- 2 ribosomal subunits
- 1st aminoacyl-tRNA
- mRNA bound to ribosomes
- GTP
- initiation factors
List the initiation factors in eukaryotes and prokaryotes and what they do
1 prokaryotes
- IF 1
- IF 2 GTP : beings charged met-tRNA to P site
- IF 3 : prevents premature association of subunits
2 eukaryotes
- eIF 3 : prevents immature association of subunits
- eIF 2 GTP : beings charged met-tRNA to P site
- eIF 4 : locates AUG along with small subunit
Mechanism by which ribosomes recognizes nucleotide sequence to initiate translation ( AUG )
1 Shine-Dalgarno sequence - E. coli has purine rich region 6-10 bases upstream of initiation codon
-16S rRNA component of 30S subunit has sequence complimentary to SD and binds to it
2 5’ cap - no SD sequence In eukaryotes. Small subunit ( 40S with eIF-4 proteins ) binds close to cap at 5’ and scans for AUG
Use of eIF-4 proteins near cap and poly A tail cap
-mediate circularization of the mRNA and prevent use of incompletely processed mRNA
How is met-tRNA bound to initiator codon and energy source and factors responsible
- AUG recognized by special tRNA and IF-2-GTP ( pro ) and eIF-2-GTP ( eu ) factors binds it to AUG
- GTP hydrolyzed to GDP
Special features of initiation tRNA
- only one to go directly to P site
- only one recognized by eIF-2
What type of initiate tRNA and its aas is carried in prokaryotes
-N-formulated methionine
When and how is methionine N-formulated
- after it has been added to tRNA
- uses transformylase ( enzyme ) and N10-formyl-THF as carbon donor
-N10-formyl-THF is cleaved by enzyme and N-formly added to N link ( COH ) of methionine forming fMet-tRNA
What usually happens to methionine
-removed before translation is complete
What happens after net-tRNA has been bound to mRNA
/large subunits then joins complex and functional Ribosome formed with charged initiator tRNA
Function of guanine nucleotide exchange factor
-reactivates eIF-2-GTP and IF-2-GTP by replacing GDP with GTP
How does elongation occur in translation
-amino acids are added to carboxyl of present chain via amide ( peptide bond ) of incoming aas
What is decoding and mediator
-bringing of aminos up-tRNA next on template mRNA to A site
- facilitated by EF-Tu-GTP and EF-Ts ( prokaryotes )
- EF-1alpha-GTP and EF-l beta gamma ( eukaryotes )
What catalyses peptide bond formation and where does it occur
- peptidytransferase
- intrinsic to 23S rRNA in large subunit
What is a ribozyme
-rRNA which catalyses a reaction
23S of large subunit in prokaryotes catalyses peptide formation
Describe what happens after aas added and translocation and the protein mediators
- after peptide bond formed P site tRNA now empty and A site tRNA has peptide chain
- ribosomes moves 3 nucleotides towards 3’ end to next codon
- EF-G-GTP ( prokaryotes)
- EF-2-GTP ( eukaryotes)
How is empty P site tRNA moved to E site and what else moves
- through GTP dependent translocation
- A site tRNA with peptide chain now moves into P site
How does termination occur
-when termination codon moves into A site
What factors recognizes termination codon at A site and what do they do
- RF-1/2 in prokaryotes
- eRF in eukaryotes
-they bind to termination sequence and result in hydrolysis of bond linkage ester of tRNA and peptide leasing nascent chain
What does RT-3-GTP and eRF-3 do
-hydrolytically remove RF-1/2 and eRF from termination signal
Toxins and infections which can affect translation and describe what they do
1 streptomycin - binds to 30S subunit, distorts structure and interferes with protein synthesis
2 tetracyclines - interact with 30S subunit blocking acces of tRNA to A site and inhibit elongation
3 chloramphenicol - inhibits prokaryotic peptidytransferase activity ( aas from P to A tRNA ) high levels inhibit mitochondrial protein synthesis
4 puromycin - bears resemblance to tRNA. Accepts peptide from p site and inhibits elongation. Truncated protein made
5 erythromycin - binds irreversibly to site on 50S subunit blocks tunnel by which peptide leaves and inhibits elongation
6 diphtheria toxin - inactive EF-2 in eukaryotes thereby inhibiting translocation
List elongation factors and what they do
Prokaryotes 1st and eukaryotes 2nd
1 brings all other charged tRNA’s to A site
- EF Tu GTP
- EF 1 alpha
2 guanine exchange factors ( replace GDP with GTP )
- EF Ts
- EF 1 beta gamma
3 translocation of ribosomes along mRNA
- EF G GTP
- EF 2 GTP
List release factors and what they do
Prokaryotic 1st and eukaryotes 2nd
1 recognizes stop codon and cuts peptide form tRNA
- RF 1/2
- eRF
2 releases other RF factors
- RF 3 GTP
- eRF 3 GTP
How do polysomes form
- due to the length of mRNA
- numerous Ribosomes can translate message at the same time
Where is gene expression mainly regulated
-transcription phase
How does translation phase regulate gene expression
- modified eIF ( phosphorylated eIF 2 GTP is inactive )
- proteins can bind to mRNA and inhibit its use by blocking translation or protect its use by protecting it from degradation
What is protein targeting
-addition of amino acid sequences that direct peptide to its final destination
What is co/post translational modifications, its effect and when does it occur
- can occur during translation or after
- covalently modify( adding chemical groups or trimming peptide chain ),folding or ubiquitinating a peptide chain to active or inhibit its function
Describe trimming and its target
- many proteins designated for secretions
- large inactive precursors have some parts removed by endoproteases activating them
Describe covalent attachments
-they activate or inhibit a protein by a variety of covalent attachments
Describe phosphorylation and its target
-addition of phosphate group to OH of serine, Threonine or Tyrosine
What catalyses and reverses phosphorylation
- family of protein kinases
- cellular protein phosphateses
Describe glycosylation and its target
- proteins to become part of membrane or secreted
- sugar added to amide nitrogen of asparagine ( n-linked )
- sugar built on OH group of serine and threonine
Describe hydroxylation ,where done and target
- done In sER
- collagen residues lysine and proline have OH added to them by vitamin C dependent hydroxylases
Describe carboxylation and function and example
- to active proteins
- carboxyl group added by vitamin K dependent to glutamate
-g carboxyglutamate Gla essential for blood clotting proteins
Describe biotin addition
-covalently added to e amino group of lysine
Describe addition of lipids and function and special features
- Farnesyl group help anchor protein at membrane
- can be acetylated at N end
List the covalent additions, the others and the none covalently adding
- glycosylation
- hydroxylation
- phosphorylation
- adding biotin
- adding lipids
- carboxylation
- trimming
- folding
- ubiquitinating
