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MOLGEN > Chapter 22 and 23 - Translation > Flashcards

Chapter 22 and 23 - Translation Flashcards

1
Q

steps of translation

A

initiation
elongation
termination

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2
Q

initiation

A

The steps of translation up to entrance of the first aminoacyl-tRNA into the A site

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3
Q

elongation

A

Repeated rounds of polypeptide chain extension
Addition of one amino acid at a time to the growing polypeptide chain

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4
Q

termination

A

A separate reaction that ends translation by stopping the addition of subunits and stimulating disassembly of the apparatus

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5
Q

a site

A

aminoacyl tRNA

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6
Q

p site

A

peptidyl-tRNA

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7
Q

E site

A

deacetylated tRNA exit

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8
Q

ribozymes

A

ribosome with catalytic site
all of them!

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9
Q

tRNA

A

3’ terminus has the sequence 5’-CCA-3’ that serves as the amino acid acceptor stem
Presence of unusual bases in their structure
All created from standard ribonucleotides post-transcriptionally

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10
Q

tRNA structure

A

Have a characteristic and conserved pattern of single and double stranded regions
Amino acid acceptor arm
ΨU (Pseudouridine) loop
Dihydrouridine (D) loop
Anticodon loop
Variable loop
Often represented as a cloverleaf structure
Actual structure is inverted L-shape

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11
Q

aminoacyl-tRNA synthetase

A

are the family of enzymes that load tRNAs with the correct amino acid

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12
Q

aminoacyl-tRNA synthetase steps

A
  1. An amino acid reacts with ATP to form an aminoacyl adenylate intermediate
    Energy of hydrolysis is trapped in the mixed anhydride linkage of the adenylate
    Pyrophosphate is released
  2. The 2’-OH or 3’-OH of the terminal 3’ nucleotide in the tRNA attacks the carbonyl carbon of the adenylate
  3. An aminoacyl-tRNA and AMP is formed
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13
Q

how do the synthetases detect tRNA differences

A

All tRNAs share the same general tertiary structure, but differ at nucleotide positions of the four arms
Changes in the nucleotide sequences
Subtle differences between the shape of the L-shaped arms
tRNA synthetases discriminate between tRNAs using both direct (nucleotide differences) and indirect (phosphodiester) methods
Most common discriminators are in the anticodon loop and amino acid acceptor arm

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14
Q

how do synthetases detect amino acid differences

A

Primary discriminator is shape of different amino acids
But amino acids are very small, and some are very similar in structure
Those that are similar in structure have different binding efficiencies and free energies

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15
Q

class I tRNA synthetases

A

Primarily monomeric
Aminoacylate tRNA at 2’-OH
Bind tRNA in the minor groove of the amino acid acceptor stem and require hairpin formation
Reaction rate is limited by rate of aminoacyl-tRNA release

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16
Q

class II tRNA synthetases

A

Primarily dimeric
Aminoacylate tRNA at 3’-OH
Bind tRNA in the major groove of the amino acid acceptor stem
Reaction rate is limited by previous chemical steps or active site rearrangement

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17
Q

synthetase kinetic proofreading when correct

A

tRNAs that match the specific nucleotide sequence combination for the synthetase
Properly align their amino acid acceptor stem with the ATP and amino acid in the active site
Quickly trigger aminoacylation reaction

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18
Q

synthetase kinetic proofreading when incorrect

A

Misalignment of acceptor stem in active site
Will not quickly trigger aminoacylation reaction
Dissociates much faster than it can react

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19
Q

isoleucyl synthetase chemical proofreading

A

Isoleucyl-tRNA synthetase cannot effectively distinguish isoleucine from valine using shape of amino acid binding site
Unable to prevent significant levels of valine-tRNAIle synthesis without proofreading
Nine different tRNA synthetases are able to proofread and correct errors once incorrect amino acid has bound to enzyme
Analogous to the 3’to5’ exonuclease proofreading function of DNA polymerases

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20
Q

two forms of chemical proofreading

A

pre-transfer editing
post transfer editing

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21
Q

pre-transfer editing

A

Incorrect aminoacyl-AMP is hydrolyzed after tRNA binding but before charging has occurred

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22
Q

post-transfer editing

A

Amino acid is hydrolyzed from aminoacyl-tRNA after tRNA charging
Uses an editing active site in the synthetase enzyme that is separate from the synthetic/loading active site

23
Q

post transfer editing sieve analogy

A

The post-transfer editing pathway can be thought of as an integrated double-sieve
Based on relative sizes of the synthetic and editing sites
The synthetic site is larger than the editing site
The first sieve is the synthetic site
Amino acids larger than correct amino acid will be excluded from the synthetic site
Loading will not occur
The second sieve is the editing site
Amino acids smaller than the correct amino acid will fit into the synthetic site and the editing site
The incorrect amino acid will then be hydrolyzed and removed in the editing site

24
Q

post transfer editing mechanism

A

Amino acids larger than correct amino acid will be excluded from the synthetic site
Loading will not occur
Amino acids smaller than the correct amino acid will fit into the synthetic site and the editing site
The incorrect amino acid will then be hydrolyzed and removed in the editing site
The correct amino acid can fit into the synthetic site, but not the editing site
Will be correctly charged and retained in an aminoacyl-tRNA

25
in bacteria, where does initiation occur
start codon and shine-delgarno sequence
26
bacterial steps of making ribosome
1. 30S subunit binds to mRNA first, aided by initiation factors 2. A 30S subunit carrying several initiation factors binds to an initiation site on mRNA to form an initiation complex 3. All initiation factors are then released and the 50S subunit joins to form the full ribosomal structure
27
IF 3
stabilizes the free 30S subunit and must eventually be released to allow the 50S subunit to join the 30S-mRNA complex also helps the 30S subunit bind to the initiation sites on the mRNA
28
IF 2
aids binding of the initiator tRNA to the complex
29
IF 1
binds to the 30S subunit at the A site and prevents aminoacyl-tRNAs from binding prematurely
30
formyl methionine
tRNAfMet The aminoacyl-tRNA that initiates bacterial polypeptide translation The amino group of the methionine is formylated
31
The methionine is formylated ... it has been loaded onto the initiator tRNA
after
32
The initiator tRNA has unique ... features that distinguish it from all other tRNAs
structural
33
The only initiation complex component that can bind the mRNA is the
small subunit
34
The initiator tRNA is the only tRNA able to bind to the ... contained within the 30S subunit
partial P site
35
The small ribosomal subunit in eukaryotes recognizes the ... of the mRNA and moves to the initiation site
5’ cap
36
Scanning model of eukaryotic initiation
Small subunit binds to the 5’ cap and begins to move 5’3’ down the mRNA As it moves, the small subunit can melt some secondary structures of the mRNA The small subunit stops when it recognizes the start codon and flanking sequences at -4 and +1 Kozak sequence Weak Kozak consensus can lead to “leaky scanning”
37
Uniqueness of eukaryotic initiator tRNA is contained only in its structure
Phosphorylation of 2’-OH on nucleotide 64
38
Entry of an aminoacyl-tRNA into the A site is mediated by
EF-Tu
39
bacterial elongation steps
1. EF-Tu-GTP binds an aminoacyl-tRNA and escorts it to the ribosome 2. The anticodon end of the ternary complex moves into the A site of the 30S subunit and base pairs with the codon 3. The EF-Tu-GTP end of the ternary complex binds to the factor binding center of the large subunit 4. The factor binding center simulates EF-Tu-GTP hydrolysis 5. The aminoacyl end of the aminoacyl-tRNA moved to face the P site tRNA 6. EF-Tu-GDP is released 7. EF-Ts mediates the regeneration of EF-Tu-GTP 8. Peptidyl transferase Creation of a peptide bond between the amino acid on the A site tRNA and the polypeptide on the P site tRNA Attack of the amino group on aminoacyl-tRNA on the acyl linkage of peptidyl-tRNA 9. Upon transfer of the polypeptide to the A site tRNA, the tRNAs move into their hybrid state 10. EF-G-GTP binds to the ribosome and stabilizes the hybrid state 11. Binding of EF-G-GTP to the factor binding center stimulates GTP hydrolysis 12. EF-G-GDP “unlocks” the ribosome by opening the gates between the E, P, and A codon-anticodon binding sites 13. EF-G-GDP also binds to the A site in the codon-anticodon region 14. A site anticodon is pushed to the P site 15. P site anticodon is pushed to the E site 16. The small subunit rotates and the ribosome now has a reduced affinity for EF-G-GDP 17. EF-G-GDP dissociates 18. EF-G has a lower affinity for GDP than GTP, so it will release GDP and rapidly bind a new GTP 19. The system is now ready for another round of elongation
40
in bacteria, When there is correct base-pairing between first two positions of the codon and anticodon
16S rRNA forms nonspecific hydrogen bonds with the minor groove of the A site tRNA Correctly base paired tRNAs dissociate from the A site very slowly, but incorrectly base paired tRNAs dissociate quickly
41
EF-Tu as a proofreader
EF-Tu-GTP only interacts with the factor binding center if the aminoacyl-tRNA fully enters the A site Only occurs if there is correct base pairing between the codon and anticodon An incorrect aminoacyl-tRNA will dissociate before EF-Tu-GTP interacts with the factor-binding center and promotes GTP hydrolysis Hydrolysis to EF-Tu-GDP would commit the incorrect amino acid to incorporation into the growing polypeptide The aminoacyl-tRNA initially binds to the A site with the amino acid oriented away from the P site When EF-Tu-GTP is hydrolyzed to EF-Tu-GDP The A site tRNA rotates towards the P site and the peptidyl transferase center Accommodation Correctly base paired tRNA will handle the rotational strain Incorrectly base paired tRNA will be unable to handle the rotational strain Base pairs will break tRNA will dissociate from A site
42
entropic catalysis
Amino group of amino acid on aminoacyl-tRNA placed in close proximity to the carbonyl of the last amino acid added to the peptidyl-tRNA proximity stimulated
43
substrate-assisted catalysis
The rRNA and P site tRNA also directly participate in the reaction in an enzymatic fashion 2’-OH of A2451 in 23S rRNA 2’-OH of P site tRNA and proton shuttle
44
hybrid state
The anticodons of the tRNAs remain in their pre-peptidyl transfer positions The 3’ end of the A site tRNA is now bound to the polypeptide and prefers to bind in the P site The 3’ end of the P site tRNA has now been deacylated and prefers to bind in the E site
45
Termination steps
1. The termination codons are recognized by class 1 protein release factors 2. RF3-GDP binds to the ribosome in the presence of bound class 1 factor 3. Release of the polypeptide by the class 1 factor stimulates a change in the conformation of both the ribosome and the class 1 factor 4. This stimulates RF3 to exchange bound GDP for GTP 5. RF3-GTP will facilitate a transition of the ribosome into the hybrid state 6. The class 1 factor dissociates 7. RF3-GTP now associates with the factor binding center and be hydrolyzed to RF3-GDP 8. RF3-GDP is released 9. RRF binds to the empty A site and recruits EF-G-GTP to the ribosome 10. EF-G-GTP is hydrolyzed to EF-G-GDP 11. Unloaded tRNAs are released from the E and P sites 12. EF-G-GDP, RRF, and the mRNA are released from the ribosome 13. IF3 binds to the small ribosomal subunit, resulting in the dissociation of the subunits
46
RF1
recognizes UAG and UAA
47
RF2
recognizes UGA and UAA
48
eRF1
the only class I termination release factor in eukaryotes
49
RF3
class 2 required GTP
50
Class 2 release factors help ... the class 1 factor from the ribosome
release
51
All three release factors...
have structures similar to that of EF-Tu-tRNA and EF-G All bind GTP All bind to the factor binding center or the A site
52
class 1 release factors
RF1 and RF2 contain a three amino acid peptide anticodon that recognizes and interacts with the stop codons contain a GGQ motif that is placed in the vicinity of the peptidyl transferase center in the large subunit The class 1 factors have structure where the peptide anticodon is on one end of the molecule and the GGQ is on the other Similar to the anticodon and CCA stem in a tRNA The release factors catalyze use of a water molecule as an acceptor of the peptidyl transferase reaction Rather than an aminoacyl-tRNA
53
RRF
Dissociation of the remaining translation components requires the ribosome recycling factor
54
23S rRNA
Interacts with 3’-CCA terminus of P site tRNAs in peptidyl transferase center Removing almost all proteins from the 50S subunit results in a 23S rRNA complex (with protein fragments) that still retains peptidyl transferase activity 23S rRNA alone has a low level of peptidyl transferase activity Archaeal large subunit has only 23S rRNA in the peptidyl transferase center Directly or indirectly involved in removing proton from amino group of peptidyl-tRNA Proton shuttle?

MOLGEN (28 decks)

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