Eukaryotic Translation,Initiation,Ribosomal Dissociation,Kozak,ElongationFactors,Peptide Bond Formation,Translocation

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	Eukaryotic Translation - Translation, like transcription, is divided into three phases or steps. The mechanism of eukaryotic translation and the molecules involved. Initiation:Initiation of protein synthesis requires an mRNA molecule to be selected for translation by a ribosome. Once the mRNA binds to the ribosome, translation begins. This process involves tRNA, rRNA, mRNA and at least 10 eukaryotic initiation factors (eIFs). Initiation consists of four steps. Ribosomal dissociation Two initiation factors eIF3 and eIF1A, bind to the newly dissociated 40S ribosomal subunit. This delays its reassociation with 60S subunit and allows other translational initiation factors to associate with the 40S subunits.
Formation of the 435 pre-initiation complex The first step in this process involves the binding of GTP by eIF2. This binary complex then binds to met-tRNA, a tRNA specifically involved in binding to the initiation codon AUF. This tertiary complex binds to the 4OS ribosomal subunit to form the 43S pre-initiation complex which is stabilized by association with eIF3 and eIF1A. Formation of the 43S initiation complex The 5 terminals of most mRNA molecules in eukaryotic cells are capped. This methyl-guanosyl triphosphate cap facilitates the binding of mRNA to the 43S preinitiation complex. A cap binding protein complex, (eIF4F) binds to the cap through the 4E protein.
The association of mRNA with the 43S preinitiation complex to form the 48S initiation complex requires A TP hydrolysis. Following the association of the 43S preinitiation complex with the mRNA cap and reduction of the secondary structure near the 5 end of the mRNA, the complex scans the mRNA for suitable initiation codon. Generally this is the 5' most AUG. But the precise initiation codon is determined by the so called "kozak" consensus sequences that surround the AUG in eukaryotes. Formation of the 80S initiation camplex The binding of the 60S ribosomal subunit to the 48S initiation complex involves the hydrolysis of the GTP bound to eIF2 by eIFS. This reaction results in the release of the initiation factors bound to the 48S initiation complex and the rapid association of the 40S and 60S subunits to form the 80S ribosome.
Elongation:Elongation is a cyclic process involving several steps catalysed by proteins called elongation factors eEF. These steps are 1. Binding of aminoacyl-tRNA to the A site 2. Peptide bond formation 3. Translocation Binding of aminoacyl-tRNA to the A site In the complete 80S ribosome formed, during the process of initiation, the A site (aminoacyl or acceptor site) is free. The proper codon recognition-elongation factor eEF-lx-GDP and phosphate. Peptide bond formation The x-amino group of the new aminoacyl-tRNA in the A site carries out a nucleophilic attack on the esterified carboxyl group of the peptidyl tRNA occupying the P site. This reaction is catalysed by a peptidyl transferase, a component of the 28S RNA of the 60S ribosomal subunit.
	Translocation Upon removal of the peptidyl moiety from the tRNA in the P site, elongation factor (eEF2) and GTP are responsible for the translocation of the newly formed peptidyl tRNA at the A site into the empty P site. The GTP required for eEF2 is hydrolysed to GDP and phosphate during the translocation process. The translocation of the newly formed peptidyl tRNA and its corresponding codon into the P site frees, the A site for another cycle of aminoacyl-tRNA codon recognition and elongation. The energy required for the formation of one peptide bond includes the hydrolysis of 2 A TP and 2 GTP molecules. This process occurs rapidly.

A eukaryotic ribosome can incorporate as many as six amino acids per second, whereas 18 amino acids are incorporated per second by prokaryotic ribosomes. Thus, the process of peptide synthesis occurs with great speed and accuracy until a termination codon is reached in prokaryotes.

Termination:In comparison to initiation and elongation, termination is relatively a simple process. Multiple cycles of elongation occur culminating in polymerization of the specific amino acids into a protein molecule. There is no tRNA with an anticodon capable of recognizing such a termination signal.

Releasing factors (eRF) are capable of recognizing termination signal residues in the A site. The releasing factor, in conjugation with GTP and the peptidyl transferases, promotes the hydrolysis of the bond between the peptide and the tRNA occupying the P site. The ribosome dissociates into 40S and 60S subunits.