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Chapter 12 Translation

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Title: Chapter 12 Translation


1

Chapter 12Translation
The biochemistry and molecular biology department
of CMU
2

Translation
The synthesis of protein molecules using mRNA as
the template, in other word, to translate the
nucleotide sequence of mRNA into the amino acid
sequence of protein according to the genetic
codon.
3

4
Section 1 Protein Synthetic System
5
  • Protein synthesis requires multiple elements to
    participate and coordinate.
  • mRNA, rRNA, tRNA
  • substrates 20 amino acids
  • Enzymes and protein factors initiation factor
    (IF), elongation factor (EF), releasing factor
    (RF)
  • ATP, GTP, Mg2

6
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1.1 Template and Codon
  • Messenger RNA is the template for the protein
    synthesis.
  • Prokaryotic mRNA is polycistron, that is, a
    single mRNA molecule may code for more than one
    peptides.
  • Eukaryotic mRNA is monocistron, that is, each
    mRNA codes for only one peptide.

8
polycistron
3?
5?-PPP
protein
monocistron
3?
5?-mG -
PPP
protein
Non-coding
ribosomal protein binding site
Coding region
Stop codon
Starting code
9

Genetic codon
  • Three adjacent nucleotides in the 5-3 direction
    on mRNA constitute a genetic codon, or triplet
    codon.
  • One genetic codon codes for one amino acid.

10
Genetic codon
11
  • Three codons for stop signal UAA, UAG, UGA.
  • One codon for start signal AUG. It also codes
    for methionine.
  • 61 codons for 20 amino acids.

12

Properties of genetic codon
  • 1. Commaless
  • A complete sequence of mRNA, from the
    initiation codon to the termination codon, is
    termed as the open reading frame.

13

commaless
  • The genetic codons should be read continuously
    without spacing or overlapping.

spacing
overlapping
14
Frameshift
15
  • 2. Degeneracy

16
  • Except Met and Trp, the rest amino acids have 2,
    3, 4, 5, and 6 triplet codons.
  • These degenerated codons differ only on the third
    nucleotide.

17
  • 3. Wobble
  • Non-Watson-Crick base pairing is permissible
    between the third nucleotide of the codon on mRNA
    and the first nucleotide of the anti-codon on
    tRNA.

18

Base-pair of codon and anticodon

19
  • 4. Universal
  • The genetic codons for amino acids are always
    the same with a few exceptions of mitochondrial
    mRNA.
  • Cytoplasm
  • AUA Ile
  • AUG Met, initiation
  • UAA, UAG, UGA termination
  • Mitochondria
  • AUA Met, initiation
  • UGA Trp
  • AGA, AGG termination

20
1.2 tRNA and AA Activation
tRNA
21

Activation of amino acid
22

23

Activated amino acid
Ala-tRNAAla Ser-tRNASer Met-tRNAMet
24

Summary of AA activation
  • Active form
  • aminoacyl-tRNA
  • Activation site
  • a - carboxyl group
  • Linkage
  • ester bond
  • Activation energy
  • 2 high-energy bonds

25

Protein synthesis fidelity
  • Aminoacyl-tRNA synthetase has the proofreading
    ability to ensure that the correct connection
    between the AA and its tRNA.
  • It recognizes the incorrect AA, cleaves the ester
    bond, and links the correct one to tRNA.

26
Prokaryotic Met-tRNAmet
  • Prokaryotic Met-tRNAmet can be formylated to
    fMet-tRNAimet.

Met-tRNAmet N10-formyl tetrahydrofolate
formyl transferase
fMet-tRNAimet tetrahydrofolate
27

Initiation tRNA
  • For prokaryotes
  • fMet-tRNAimet can only be recognized by
    initiation codon.
  • Met-tRNAemet is used for elongation.
  • For eukaryotes
  • Met-tRNAimet is used for initiation.
  • Met-tRNAemet is used for elongation.

28

1.3 Ribosomes
  • Ribosome is the place where protein synthesis
    takes place.
  • A ribosome is composed of a large subunit and a
    small subunit, each of which is made of ribosomal
    RNAs and ribosomal proteins.

29
Molecular components of ribosome of prokaryotes
30
Ribosome of prokaryotes
31

Three sites on ribosomes
32

A site, P site and E site
33
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34
Section 2 Protein Synthetic Process
35

General concepts
  • The direction of the protein synthesized
    N-terminal?C-terminal
  • The direction of template mRNA 5? 3end
  • The process of Protein
  • initiation
  • elongation
  • termination

36

2.1 Initiation
Prokaryotic initiation
  • Four steps
  • Separation between 50S and 30S subunit
  • Positioning mRNA on the 30S subunit
  • Registering fMet-tRNAimet on the P site
  • Associating the 50S subunit
  • Three initiation factors IF-1, IF-2 and IF-3.

37
Shine-Dalgarno (S-D) sequence
  • -AGGA PuPuUUUPuPu AUG-
  • purine rich of 4-9 nts long
  • 8-13 nts prior to AUG

38

Alignment of 16S rRNA
The 3end of 16s rRNA has consensus sequence UCCU
which is complementary to AGGA in S-D sequence
(also called ribosomal binding site).
39
Initiation 1-2
  • The IF-1 and IF-3 bind to the 30S subunit, making
    separation between 50S and 30S subunit.
  • The mRNA then binds to 30S subunit.

40
Initiation 3
  • The complex of the GTP-bound IF-2 and the
    fMet-tRNA enters the P site.

41
Initiation 4
  • The 50S subunit combines with this complex.
  • GTP is hydrolyzed to GDP and Pi.
  • All three IFs depart from this complex.

42
IF-2
Pi
-GTP
GDP
IF-3
IF-1
One GTP is consumed in initiation course?
43

eukaryotic initiation
  • Four steps
  • Separation between 60S and 40S subunit
  • binding Met-tRNAimet on the 40S subunit
  • Positioning mRNA on the 40S subunit
  • Associating the 60S subunit

44
Eukaryotic initiation factors
45
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46
Process of eukaryotic initiation
47

2.2 Elongation
  • Three steps in each cycle
  • Positioning an aminoacyl-tRNA in the A site---
    Entrance
  • Forming a peptide bond---Peptide bond formation
  • Translocating the ribosome to the next
    codon---Translocation
  • Elongation factors (EF) are required.

48
Step 1 Entrance
  • An AA-tRNA occupies the empty A site.
  • Registration of the AA-tRNA consume one GTP.
  • The entrance of AA-tRNA needs to activate EF-T.

49
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50
GTP
Tu
Ts
Ts
GDP
Tu
51
Step 2 Peptide bond formation
  • The peptide bond formation occurs at the A site.
  • The formylmethionyl group is transferred to aNH2
    of the AA-tRNA at the A site by a peptidyl
    transferase.

52
Peptide bond formation 1
53
Peptide bond formation 2
54
Step 3 Translocation
  • EF-G is a translocase.
  • GTP bound EF-G provides the energy to move the
    ribosome one codon toward the 3 end on mRNA.
  • After the translocation, the uncharged tRNA is
    released from the E site.

55

Translocation
56
fMet
fMet
57
Eukaryotic elongation
  • Elongation factors are EF-1 (EF-T) and EF-2
    (EF-G).
  • There is no E site on the ribosome.

58

2.3 Termination
  • Prokaryotes have 3 release factors RF-1, RF-2
    and RF-3.
  • RF-1 and RF-2 Recognizing the termination codons
  • RF-3 GTP hydrolysis and coordinating RF-1/RF-2
    and rpS
  • Eukaryotes have only 1 releasing factor eRF.

59
Termination 1
  • The peptidyl transferase is converted to an
    esterase.

60
Termination 2
  • The uncharged tRNA, mRNA, and RFs dissociate from
    the ribosome.

61
RF
62
Energy consumption
initiation one GTP (IF-2-GTP) AA activationtwo
P bonds elongation two GTP
(Tu-GTP, EF-G-GTP) termination one GT
P (RF-3) Total at least four high-energy bonds
per peptide bond are consumed.
63
Translation of prokaryotes
64

Polysome
  • Proteins are synthesized on a single strand mRNA
    simultaneously, allowing highly efficient use of
    mRNA.

65
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66
Section 3 Protein Modification and Protein
Targeting
67
3.1 Protein Folding
  • The macromolecules assisting the formation of
    protein secondary structure include
  • molecular chaperon
  • protein disulfide isomerase (PDI)
  • peptide prolyl cis-trans isomerase (PPI)

68
Chaperons
  • A group of conserved proteins that can recognize
    the non-native conformation of peptides and
    promote the correct folding of individual domains
    and whole peptides.
  • Heat shock protein (HSP)
  • HSP70, HSP40 and GreE family
  • Chaperonin
  • GroEL and GroES family

69
Mechanism
  • Protect the unfolded segments of peptides first,
    then release the segments and promote the correct
    folding.
  • Provide a micro-environment to promote the
    correct native conformation of those peptides
    that cannot have proper spontaneous folding.

70
Mechanism
71
3.2 Modification of primary structure
  • Removal of the the first N-terminal methionine
    residue
  • Covalent modification of some amino acids
    (phosphorylation, methylation, acetylation, )
  • Activation of peptides through hydrolysis

72
3.2 Modification of spatial structure
  • Assemble of subunits Hb
  • Attachment of prosthetic groups glycoproteins
  • Connection of hydrophobic aliphatic chains

73
3.4 Protein Targeting
  • The correctly folded proteins need to be
    transported to special cellular compartments to
    exert desired biological functions.
  • AAs sequence on the N-terminus that directs
    proteins to be transported to proper cellular
    target sites is called signal sequence.

74
Signal sequences
75

a. Secretory protein
76

Signal peptide
  • All the secretory proteins have the signal
    peptide.
  • Consist of 13-36 AA in three regions
  • Positively charged AA at N-terminus
  • Hydrophobic core of 10-15 AA in the medial region
  • Small polar AA at C-terminus

77

Signal sequence for ER
Cleavage site
78

Secretory protein into ER
79

b. Mitochondrial protein
  • Mitochondrial proteins in cytosol are present in
    precursor forms.
  • Signal sequence of 20-25 AA at N-terminus are
    rich in Ser, Thr, and basic AA.

80

b. Mitochondrial protein

81

c. Nuclear protein
82
Section 4 Interference of Translation
83
  • The protein synthesis is highly regulated.
  • This process can also be the primary target for
    many toxins, antibiotics and interferons.
  • These interferants interact specifically with
    proteins and RNAs to interrupt the protein
    synthesis.

84

Antibiotics
85

Antibiotics
86

Puromycin
  • It has a similar structure to Tyr-tRNA.
  • It works for both prokaryotes and eukaryotes.

87
Toxins
  • Some toxins, such as plant protein Ricin, is
    among the most toxic substance known, which acts
    on 60s subunits.

88
Diphtheria toxin
89
Interferon
  • Interferons are cytokines produced during immune
    response to antigens, especially to viral
    infections.

90
Interferon
91
mRNA
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