Download Biosynthesis of Protein or Translation

Biosynthesis of Protein or Translation
 Translation is the enzymatic process in which mRNA is decoded or translated into protein
with the help of ribosome and tRNA.
 Translation involves the conversion of a four base code (ATCG) into twenty different
amino acids. A codon or triplet of bases specifies a given amino acid. Most amino acids
are specified by more than one codon.
 The conversion of codon information into proteins is conducted by transfer RNA. Each
transfer RNA (tRNA) has an anticodon which cans base pair with a codon on mRNA.
Some anti-codons have modified bases that can pair with more than one codon, specifying
the same amino; this means that we don't need 61 different tRNA molecules for all 61
codons.
 In
translation, messenger
RNA
(mRNA) produced
by transcription is
decoded
by
the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into
an active protein.
 Translation occurs in the cytoplasm of both prokaryotic (Pr) and eukaryotic (Eu) cells.
 In prokaryotes, ribosome can begin translating the mRNA even before RNA polymerase
completes its transcription.
 In eukaryotes, translation and transcription are completely separated in time and space in
which transcription occur in the nucleus and translation occur in the cytoplasm.
 In Eukaryotes, translation occurs across the membrane of the endoplasmic reticulum in a
process called vectorial synthesis.
 Proteins are synthesized from the amino to the carboxyl terminus.
 Translation proceeds in four phases: activation, initiation, elongation and termination
(1)Phase of Activation
 The amino acids have no direct affinity for mRNA, so tRNA act as an adapter molecule,
which recognizes an amino acid on one end and its corresponding codon on the other, is
required for translation.
 In activation phase, the correct amino acid is covalently bonded to the corresponding
tRNA). The amino acid is joined by its carboxyl group to the 3' OH of the tRNA by a ester
bond. When the tRNA has an amino acid linked to it, it is termed "charged".
 As tRNAs enter the cytoplasm, each combines with its cognate amino acid in a two-step
process called amino acid activation.
 Each type of amino acid is activated by a different amino acyl tRNA synthetase.
 Two high-energy bonds from an ATP are required. .
 The aminoacyl tRNA synthetase transfers the activated amino acid to the 3' end of the
correct tRNA. .
 The amino acid is linked to its cognate tRNA with an energy-rich bond.
 This bond will later supply energy to make a peptide bond linking the amino acid into a
protein.
 Aminoacyl tRNA synthetases have self-checking functions to prevent incorrectly paired
 amino acyl tRNAs from forming.
(2)Phase of Initiation
 Initiation occurs by binding of the 30s subunit to the mRNA.
 In prokaryotes, the 165 rRNA of the small subunit binds to the Shine-Dalgarno sequence
in the 5' untranslated region of the mRNA.
 In eukaryotes, the small subunit binds to the 5' cap structure and slides down the message
to the first AUG.
 The charged initiator tRNA becomes bound to the AUG start codon on the message through
base pairing with its anticodon.
 The initiator tRNA in prokaryotes carries fmet, whereas the initiator tRNA in eukaryotes
carries Met.
 The large subunit binds to the small subunit, forming the completed initiation complex.
 There are two important binding sites on the ribosome called the P site and the A site.
o The peptidyl site (P site) is the site on the ribosome where (f)met-tRNAi initially
binds. After formation of the first peptide bond, the P site is a binding site for the
growing peptide chain.
o The aminoacyl site (A site) binds each new incoming tRNA molecule carrying an
activated amino acid.
(3)Phase of Elongation
 Elongation occurs by successive amidation of the nascent (growing) chain.
 Elongation is a three-step cycle that is repeated for each amino acid added to the protein
after the initiator methionine.
 Each cycle uses four high-energy bonds in which two from the ATP used in amino acid
activation to charge the tRNA, and two from GTP.
 During elongation, the ribosome moves in the 5' to 3' direction along the mRNA,
synthesizing the polypeptide chain from amino to carboxyl terminus.
 The three steps are:
o A charged tRNA binds in the A site. The particular aminoacyl-tRNA is determined by
the mRNA codon aligned with the A site.
o Peptidyl transferase, an enzyme that is part of the large subunit, forms the peptide
bond between the new amino acid and the carboxyl end of the growing polypeptide
chain. The bond linking the growing peptide to the tRNA in the P site is broken, and
the growing peptide attaches to the tRNA located in the A site.
o In the translocation step, the ribosome moves exactly three nucleotides (one codon)
along the message. This moves the growing peptidyl-tRNA into the P site and aligns
the next codon to be translated with the empty A site.
 In eukaryotic cells, elongation factor-2 (eEF-2) used in translocation is inactivated through
ADP-ribosylation by Pseudomonas and Diphtheria toxins.
(4)Phase of Termination
 Termination occurs any of the three stop (termination or nonsense) codons moves into the
A site. This process is facilitated by a releasing factor protein that binds into the ribosomal A
site containing a stop codon.
 With the help of release factor peptidyl transferase hydrolyzes the completed protein from
the final tRNA in the P site.
 Since no tRNA exists with an anticodon complementary to the stop codon, the ribosome
"pauses" until at last it "falls off" the mRNA, and the polypeptide chain terminates.
 The mRNA, ribosome, tRNA, and factors can all be reused for additional protein synthesis.
Fig: Steps in Translation
Pr = Prokaryote Eu= Eukaryote