Protein biosynthesis is the process by which cells synthesize proteins. It involves the translation of mRNA into a polypeptide chain based on the genetic code. The main stages are activation of amino acids, initiation of translation at start codons on mRNA, elongation of the polypeptide chain by adding amino acids one by one, and termination when a stop codon is reached. Chaperones assist in protein folding and post-translational modifications further process the protein.

1. PROTEIN BIOSYNTHESIS
SUBMITTED BY:
Maheshwari
III PharmD
Nandha college of pharmacy,Erode -52

2. INTRODUCTION
• In other words, Protein biosynthesis is the
biochemical translation of the genetic
information, from the four-letter language of
nucleic acids into the twenty letter language
of proteins. The process of protein
biosynthesis is called as translation.
• The sequence of amino acids in the protein
synthesized is determined by the nucleotide
base sequence of mRNA.

3. GENETIC CODE
• The genetic code is regarded as a dictionary of nucleotide
bases (A, G, C and U) that determines the sequence of
amino acids in proteins.
• The codons are the three bases sequences in mRNA that
code for specific amino acids.
• The codons consist of the four nucleotide bases, the
purines – adenine (A) and guanine (G), and the pyrimidines
– cytosine (C) and uracil (U)
• The four bases produce 64 different combinations (43) of
three base codons.
• Sixty one codons code for the 20 amino acids found in
protein. The three codons UAA, UAG and UGA do not code
for amino acids. They act as stop signals (termination
codons or non-sense codons) in protein synthesis.
• There is one start codon (initiation codon), AUG, coding
for methionine.

5. PROTEIN BIOSYNTHESIS
The protein synthesis may be divided into the
following stages.
I. Requirement of the components
II. Activation of amino acids
III. Protein synthesis proper
IV. Chaperones and protein folding
V. Post-translational modifications

6. 1] REQUIREMENT OF THE
COMPONENT
Amino acids
• Of the 20 amino acids found in protein structure,
half of them (10) can be synthesized by man.
• About 10 essential amino acids have to be
provided through the diet.
• Therefore, a regular dietary supply of essential
amino acids, should be maintained.
• In prokaryotes, there is no requirement of amino
acids, since all the 20 are synthesized from the
inorganic components.

7. Ribosomes
• The functionally active ribosomes are the centres or
factories for protein synthesis. Ribosomes are huge
complex structures (70S for prokaryotes and 80S for
eukaryotes) of proteins and ribosomal RNAs.
• A ribosome has two binding sites (A site and the P site)
for tRNA.
• During translation, an incoming aminoacyl-tRNA, as
directed by the codon, binds at A site. • This codon
specifies the next amino acid to be added to the
growing peptide chain. • The P site is occupied by the
peptidyl tRNA which carries an upcoming polypeptide
chain.

8. Messenger RNA (mRNA)
Messenger RNA (mRNA) is the carrier of information present in
DNA. Specific information required for a protein synthesis is
present on the mRNA.
Transfer RNAs (tRNAs)
• They carry the amino acids and hand over them to the growing
peptide chain
• The amino acid is covalently bound to tRNA at the 3’-end.
• Each tRNA has a three nucleotide base sequence called
anticodon, which is responsible to recognize the codon
(complementary bases) of mRNA for protein synthesis
• In man, there are about 50 different tRNAs where in bacteria
around 40 tRNAs are found

9. Energy sources
Both ATP and GTP are required as energy
source.
Protein factors
Process of translation involves a number of
protein factors needed for initiation,
elongation and termination of protein
synthesis

10. 2]ACTIVATION OF AMINO ACIDS
• For the incorporation of amino acids into a
polypeptide chain, amino acids are first activated and
get to their appropriate tRNA carriers.
• Linkage of an amino acid to the tRNA requires energy
and is catalyzed by aminoacyl-tRNA synthetase.
• The amino acid is first attached to the enzyme
utilizing ATP to form enzyme-AMP amino acid
complex.
• The amino acid is then transferred to the 3’ end of
the tRNA to form aminoacyl tRNA. A misacylated
tRNA is recognized and removed (deacylated) by
Aminoacyl tRNA synthetase.

11. 3] PROTEIN SYNTHESIS PROPER
• The protein or polypeptide synthesis occurs
on the ribosomes.
• The mRNA is read in the 5’→ 3’ direction and
the polypeptide synthesis proceeds from N-
terminal end to C-terminal end.
• Translation is directional and collinear with
mRNA.
• The prokaryotic mRNAs are polycistronic
where as eukaryotic mRNA is monocistronic

12. • In case of prokaryotes, translation commences
before the transcription of the gene is
completed. This is not so in case of eukaryotic
organisms.
• Translation proper is divided into three stages:
I. INITIATION
II. ELONGATION
III. TERMINATION

13. INITIATION
• During initiation, the mRNA, the tRNA, and the first amino
acid all come together within the ribosome. The mRNA strand
remains continuous, but the true initiation point is the start
codon, AUG. The start codon is the set of three nucleotides
that begins the coded sequence of a gene. The start codon
specifies the amino acid methionine.
• How did methionine get itself to the ribosome? By
attaching to the tRNA that contains the right anticodon. The
anticodon for AUG is UAC. We know that because of the rules
of complimentary base pairing. The tRNA with the anticodon
UAC will automatically match to the codon AUG, bringing the
methionine along for the ride. So, there you have it - mRNA is
attached to tRNA, and tRNA is attached to methionine.

15. 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. In
eukaryotes, the analogous elongation factors are
EF-1α−GTP and EF-1βγ. Both EF-Ts (in
prokaryotes) and EF-1βγ (in eukaryotes) function
as nucleotide exchange factors.

17. • The peptidyl-transferase is an important enzyme
which catalyzes the formation of the peptide
bonds. The enzymatic activity is found to be
intrinsic to the 23S rRNA found in the large
ribosomal subunit. Because this rRNA catalyzes the
polypeptide bound formation reaction, it is named
as a ribozyme.
• The transport RNA at the P site carries the
polypeptide synthesized by now, while on the A site
is located a tRNA, which is bound to a single amino
acid. 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.

18. • Once this step is completed, the ribosome
moves 3 nucleotides toward the 3'-end of the
mRNA. This process is known as translocation
- in prokaryotes, it requires the participation
of EF-G-GTP and GTP hydrolysis, while the
eukaryotic cells use EF-2-GTP and GTP
hydrolysis again. During the translocation, the
uncharged tRNA moves from the P to the E
site and peptidyl-TRNA leaves the A site and
go to the P site. This is an iterative process
that is repeated until the ribosome reaches
the termination codon.

19. TERMINATION
• There is a universal release factor, eRF1, that
recognizes all three stop codons [UAA,UGA
and UAG].
• The protein release factor interacts with the
stop codon to terminate translation.
• The ribosome dissociates and the mRNA is
released and can be used again.

21. 4]CHAPERONS AND PROTEIN
FOLDING
• molecular chaperones are proteins that assist the covalent
folding or unfolding and the assembly or disassembly of other
macromolecular structures. The first protein to be called a
chaperone assists the assembly of nucleosomesfrom folded
histones and DNA and such assembly occur in nucleus.
• the newly-formed peptide chain is folded and processed into
its biologically-active form. At some point of time, during or
after protein synthesis, the polypeptide chain spontaneously
assumes its native conformation by forming sufficient number
of hydrogen bonds and van der Waals, ionic, and hydrophobic
interactions. In this way, the linear (or one dimensional)
genetic message encoded in mRNA is converted into the 3-
dimensional structure of the protein.

22. 5]POST TRANSLATIONAL
MODIFICATION
• N-terminal and C- terminal modification:
The formyl group at the N-terminus of bacterial
proteins is hydrolyzed by a deformylase. One or
more N-terminal residues may be removed by
aminopeptidases.
• Loss of signal sequences: In certain proteins,
some (15 to 30) amino acid residues at the N-
terminus play a role in directing the protein to
its ultimate destination in the cell. Such signal
sequences, as they are called, are ultimately
removed by specific peptidases

23. • Modification of individual amino acids: the
hydroxyl groups of certain serine, threonine,
tyrosine residues of some proteins undergo
enzymatic phosphorylation by ATP. The
phosphate groups add negative charge to
these polypeptides. The functional
significance of this modification varies from
one protein to the other.
• The phosphorylation and dephosphorylation
of the OH group of certain serine residues
regulate the activity of some enzymes, such
as glycogen phosphorylase.

24. • Formation of Di-Sulfide cross linkage:
Some proteins after acquiring native
conformations are often covalently cross-
linked by the formation of disulfide
bridges between cysteine residues .
These cross-links help to protect the
native conformation of the protein
molecule from denaturation in an
extracellular environment that is quite
different from that inside the cell.