Download 6-Translation

Protein Synthesis
(Translation)
Proteins Synthesis
Translation

Process by which ribosomes convert the information
carried by mRNA to synthesize new proteins

mRNA is translated from 5 to 3-end producing a protein
(peptide) synthesized from amino to carboxyl end
Requirements
1. Ribosomes
2. Amino acids
3. mRNA
4. tRNA
5. Enzymes
6. Protein factors (initiation, elongation & release factors)
7. ATP & GTP as source of energy
Genetic Code
CODON: a sequence of three adjacent nucleotides (bases)

One or more sequences (codons) for each amino acid
(61 for 20 amino acids)

If there are more than one codon for one amino acid
then these codons are known as synonyms

Starter / initiator codon (always AUG for methionine)

Stop codons or termination codons (UAA, UAG & UGA)
Genetic Code (64 Possible codon sequences)
Phe
UUU, UUC
Leu
CUU, CUC, CUA, CUG, UUA, UUG
ILeu
AAU, AUC, AUA
Met
AUG
Val
GUU, GUC, GUA, GUG
Ser
UCU, UCC, UCA, UCG, AGU, AGC
Pro
CCU, CCC, CCA, CCG
Thr
ACU, ACC, ACA, ACG
Ala
GCU, GCC, GCA, GCG
Tyr
UAU, UAC
If  4 codons for one amino acid; most of these codons differ in 3rd nucleotide
Genetic Code (64 Possible codon sequences)
His
CAU, CAC
Gln
CAA, CAG
Asn
AAU, AAC
Lys
AAA, AAG
Asp
GAU, GAC
Glu
GAA, GAG
Cys
UGU, UGC
Trp
UGG
Arg
CGU, CGC, CGA, CGG, AGA, AGG
Gly
GGU, GGC, GGA, GGG
Proteins Synthesis (Steps)
I. Activation of amino acids
(formation of amino-acyl-tRNAs or charged tRNA)

Amino-acyl-tRNA-synthetases recognize and catalyze
the covalent attachment of a specific amino acid to
CCA arm of tRNA
Amino-acyl-tRNA
Amino acid + tRNA
ATP
AMP + PPi

2Pi
First amino acid is always methionine (met-tRNA)
II. Codon-anticodon recognition

Codon (on mRNA) & anti-codon (on tRNA) are
anti-parallel to one-another
Proteins Synthesis (Steps)
III. Initiation
Proper mRNA is selected by a ribosome ; ribosome attaches
to it and reads the message from 5 to 3 end of mRNA

Ribosome dissociates into 40s & 60s sub-units

Formation of Pre-initiation complex (PIC)
[40 s rRNA + met-tRNA + GTP + initiation factors (eIFs)]

mRNA binds to PIC forming 43 s initiation complex
Proteins Synthesis (Steps)
d. Binding of 43s (IC) with 60s ribosome forms 80s initiation
complex, this complex contains P, A & E sites (“P” for
peptidyl tRNA , “A” for new amino-acyl-tRNA & “E” for exit)

Starter / initiator codon (AUG) is identified by tRNA near
the 5 end (cap end) of mRNA

This is determined by other sequences ,“Kozak
consensus sequences” that surround “AUG”

met-tRNA occupies “P” site and is ready for elongation
Elongation

A cyclic process; one amino acid is added at a time

Sequence determined by the order of codons in mRNA

Requires “Elongation factors” (EFs)
Steps
a. Binding of amino acyl-tRNA to the “A” site
b. Peptide bond formation of this incoming amino acid with the
carboxyl group (end) of met-tRNA (or growing peptide chain)
at “P” site

Catalyzed by “Peptidyl-transferase” – a component of
28s rRNA (ribozyme) of the 60s ribosomal subunit
Elongation
c. This results in attachment of growing peptide to tRNA in
the “A” site
d. Translocation – Now the ribosome advances three
nucleotides (1 codon) towards 3-end of mRNA; results in

Movement of free tRNA (uncharged) from “P” to “E”
site for release

EF2 displaces peptidyl-tRNA from “A” to “P” site

“A” site is now open for another amino-acyl-tRNA for
another cycle of elongation
Termination

Elongation continues until a termination codon is
encountered

Termination occurs when one of the three termination
codons moves into “A” site

These codons are recognized by release factors

Release factors bind to and induce peptidyl transferase
to release the polypeptide from tRNA
Post-translational modifications

Trimming

Covalent modifications

Phosphorylation

Glycosylation

Hydroxylation

Other covalent modifications

Formation of disulfide links

Addition of carboxyl, acetyl group etc.

Addition of prosthetic group