Protein Synthesis

Content
• Definition
• Location in the cell
• Requirements
• Steps involved
• Initiation
• Elongation
• Termination
• Antibiotics inhibits the protein synthesis

• The process of protein synthesis translates the codons (nucleotide triplets) of the
messenger RNA (mRNA) into the 20-symbol code of amino acids that build the
polypeptide chain of the proteins
• mRNA translation begins from its 5′-end towards its 3′-end
• The polypeptide chain is synthesized from its amino-terminal (N-end) to its
carboxyl-terminal (C-end).
• no significant differences in the protein synthesis steps in prokaryotes and
eukaryotes, however there is one major distinction between the structure of the
mRNAs – prokaryotes often have several coding regions (polycistronic mRNA),
while the eukaryotic mRNA has only one coding region (monocistronic mRNA

Protein Synthesis Steps involved
• Initiation
• Elongation
• Termination

Protein Synthesis Initiation
• The components involved in the first step of protein synthesis are:
• The mRNA to be translated
• the two ribosomal subunits (small and large subunits)
• the aminoacyl-tRNA which is specified by the first codon in the mRNA
• guanosine triphosphate (GTP), which provides energy for the process –
eukaryotes require also adenosine triphosphate!
• initiation factors which enables the assembly of this initiation complex -
prokaryotes have 3 initiation factors are known (IF-1, IF-2, and IF-3), while
eukaryotes, there have over ten factors designated with eIF prefix.

Two mechanisms are involved in the
recognition sequence
• Shine-Dalgarno (SD) sequence - In
Escherichia coli is observed
sequence with high percentage of
purine nucleotide bases, known as
the Shine-Dalgarno sequence. This
region is located close to 5’ end of
the mRNA molecule, 6-10 bases
upstream of the initiating codon.
The 16S rRNA component of the
small ribosomal subunit possess a
complementary to the SD sequence
near its 3'-end.

Initiating codon (AUG)
• In prokaryotes this event is facilitated by IF-2-GTP, while in eukaryotes by
eIF-2-GTP and additional eIFs.
• The charged initiator transport RNA approaches the P site on the small
ribosomal subunit
• In bacteria (and in mitochondria), a methionine is attached to the initiator
tRNA an subsequently a formyl group is added by the enzyme
transformylase
• in eukaryotes, the initiator transport RNA attaches a non formylated
methionine
• the large ribosomal subunit joins the complex formed by now, and thus a
fully functional ribosome is formed
• This complex has a charged initiating tRNA in the P site, and the A site
empty

Translation Elongation
• During the elongation step the polypeptide chain adds amino acids to the
carboxyl end the chain protein grows as the ribosome moves from the 5' -
end to the 3'-end of the mRNA
• In prokaryotes, the delivery of the aminoacyl-tRNA to ribosomal A site is
facilitated by elongation factors EF-Tu-GTP and EF-Ts, and requires GTP
hydrolysis
• The peptidyl-transferase is an important enzyme which catalyzes the
formation of the peptide bonds
• After the peptide bond has been formed between the polypeptide and the
amino acid, the newly formed polypeptide is linked to the tRNA at the A
site
• Once this step is completed, the ribosome moves 3 nucleotides toward the
3'-end of the mRNA. This process is known as translocation

Termination of Translation
• Termination happens when the A site of the ribosome reaches one of
the three termination codons (UAA, UAG or UGA)
• In prokaryotes, these codons are recognized by different release
factors (abbreviated with RF)
• When these release factors bind the complex, this cause in hydrolysis
of the bond linking the peptide to the tRNA at the P site and releases
the nascent protein from the ribosome