Transcript Biosynthesis of proteins on ribosomes GENETIC

Biosynthesis of
proteins on
ribosomes
GENETIC CODE sequence of
mononucleotides in
mRNA that
specifies the
sequence of amino
acids in peptide
chain
CODON –
mRNA triplet
base sequence
responsible for
1 amino acid
PROPERTIES OF GENETIC CODE
1. Unambiguous. In any organism each codon
corresponds to only one amino acid.
2. Code is degenerate. There are multiple
codons for most amino acids.
3. Universal. Codons are the same for all
organism.
4. Without punctuation. There are no
punctuations between trinucleotides.
5. Nonoverlapping. Codons do not overlap each
other.
ANTICODON –
triplet in tRNA
that can
complementary
bind to codon of
mRNA.
Such base pairing
between codon
and anticodon is
responsible for
the translation of
genetic
information from
mRNA to protein.
Structure of tRNAs
STAGES OF TRANSLATION
• 1. Recognition
• 2. Initiation
• 3. Elongation
• 4. Termination
RECOGNITION
O
R1
CH
COOH + H O
O
P
O
P
OH
NH 2
O
O
OH
P
O Аденозин
OH
O
R1
CH
CO
O
P
O
Аденозин + H4P2O7
OH
NH 2
Aminoacyladenilate
Aminoacyl-tRNA-synthetase
Aminoacyladenilate + tRNA  aminoacyl-tRNA + AMP
Activation of amino acids
Each amino acid has a specific tRNA
There is specific aminoacyl-tRNA-synthetase for each AA
The
structure
of tRNA
Initiation of Translation
• The translation complex is assembled
at the beginning of the mRNA coding
sequence
• Complex consists of:
-Ribosomal subunits
-mRNA template to be translated
-Initiator tRNA molecule
-Protein initiation factors
Initiator tRNA
• First codon translated is usually AUG
• The initiator tRNA recognizes
initiation codons
-Bacteria: N-formylmethionyl-tRNA
-Eukaryotes: methionyl-tRNA
Initiation
of protein
biosynthesis
MethionylтRNA
binds to Pcenter
Sites for tRNA binding in
ribosomes
There are two
centers:
peptidyl (P)
and
aminoacyl (А)
Elongation
1) Positioning of the next
aminoacyl-tRNA
in the A site
2) Formation of the
peptide bound
(enzyme – peptidyl
transferase) between
methionine and AA in Acentre. The residue of
methionine is transferred on
the amino group of another
AA
3) Translocation – shift of
ribosome by one codon.
Methionyl-tRNA is released
from P-centre. DipeptidyltRNA moves from A-centre
to P-centre.
Termination of
Translation
• Ribosome comes to terminal codon UGA,
UAG or UAA
• No tRNA molecules recognize these
codons and protein synthesis stalls
• Protein termination factors F-1, RF-2,
RF-3 split off synthesized polypeptide
from the last tRNA
• Ribosomal complex dissociates
Termination of
Translation
POSTTRANSLATIONAL MODIFICATION
1) Preparing of proteins for different functions
2) Direction of proteins to different locations (targeting)
1. Removing of methionine (formylmethionine)
2. Formation of disulfide and other bonds (secondary,
tertiary structures)
3. Proteolytic cleavage
4. Modification of amino acid residues:
- Hydroxylation
- Glycosilation
- Phosphorilation
5. Joining of prosthetic groups or cofactors
6. Formation of the quaternary structure
Regulation of the Protein Biosynthesis
The operon model (by Jacob and Monod)
Inhibitors of Transcription
Antibiotics
inhibiting protein
synthesis