Steps Of Protein Synthesis (Translation)

Signal recognition particles (SRPs) recognize the signal sequence on the N-terminal end of proteins destined for the lumen of the endoplasmic reticulum (ER). SRP binding arrests translation and an SRP receptor facilitates import of the SRP, ribosome, and nascent protein into the ER lumen. A signal peptidase removes the signal sequence from the protein, which may remain in the membrane or be routed for secretion.

Common to both eukaryotic and prokaryotic protein synthesis is the requirement for ATP to activate amino acids. The activated aminoacyltRNAs then interact with ribosomes carrying mRNA. Peptidyl transferase catalyzes the formation of peptide bonds between the free amino group of activated aminoacyl-tRNA on the A site of the ribosome and the esterified carboxyl group of the peptidyl-rRNA on the P site; the liberated rRNA remains on the P site.

he entry of the aminoacyl-tRNA into the A site results in the hydrolysis of one GTP to GDP.

Translocation of the newly formed peptidyl-tRNA in the A site into the P site by EF 2 similarly results in hydrolysis of GTP to GDP and phosphate.

Thus, the energy requirements for the formation of one peptide bond include the equivalent of the hydrolysis of two ATP molecules to ADP and of two GTP molecules to GDP, or the hydrolysis of four high-energy phosphate bonds.

Initiation of protein synthesis can be divided into four steps: 

• Dissociation of the ribosome into its 40S and 60S subunits; 
• Binding of a ternary complex consisting of the initiator methionyl-tRNA, (met-tRNAi), GTP, and eIF-2 to the 40S ribosome to form the 43S preinitiation complex; 
• Binding of mRNA to the 40S preinitiation complex to form the 48S initiation complex; and 
• Combination of the 48S initiation complex with the 60S ribosomal subunit to form the 80S initiation complex.

A i.e. AUG

C i.e. Ribosomes

A, B, C i.e. Releasing factor, Stop codon, UAA codon

Ans. a. RNA polymerase (Ref Harper 28/e p362) RNA polymerase enzyme is involved in transcription process, not in translation.

The a-amino group of the new aminoacyl-tRNA in the A site carries out a nucleophilic attach on the esterified carboxyl group of the peptidyl-tRNA occupying the P site (peptidyl or polypeptide site).

Peptidyl transferase: Catalyses two reactions, peptide bond formation between amino acids and together with release factor, peptide release.

A i.e. N formyl Met is the first – RNA to come into action

Ans. B i.e. Rough endoplasmic reticulum

Rough ER

• The surface of the rough endoplasmic reticulum (often abbreviated RER) is studded with protein-manufacturing ribosomes giving it a “rough” appearance (hence its name).