Question

describe the process of protein synthesis. include the sequences involved, molecules participating and why is this process important. draw structures or molecules involved

Answer 1

Protein synthesis occurs in three stages.

1. Synthesis of messenger RNA (mRNA) or transcription
2. Translation of mRNA into peptide chain
3. Modifications to the peptide chain to give mature protein

The first stage of protein synthesis is transcription. The information required for protein synthesis is present in the nucleus (in case of eukaryotes). Protein synthesis occurs in the cytoplasm since the machinery required for the synthesis is present in the cytoplasm. Therefore, to fill the gap, mRNA is synthesized, which carried the information from nucleus to the site of protein synthesis.

Once the mRNA is ready, it is exported from the nucleus to the cytoplasm. In the cytoplasm, the machinery for protein synthesis, called the ribosomes, are present. Protein synthesis requires the following components:

• mRNA
• the ribosomes (the two ribosomal subunits assemble together)
• the aminoacyl-tRNAs which carries the amino acids
• Guanosine triphosphate (GTP) which provides energy/eukaryotes need ATP
• Initiation factors (IF or eIF)

Protein synthesis is initiated by the initiation codon AUG on mRNA. The codon signals the assembly of the ribosomal subunits, mRNA, in the presence of initiation factors. IN prokaryotes, Shine Dulgarno sequence is present, which facilitates the assembly. In eukaryotes, eIF-4 and cap structure at the 5’ end of mRNA facilitate the assembly.

Once assembly is done, translation begins at the initiation codon AUG. It codes for formyl methionine in prokaryotes whereas in eukaryotes, it codes for methionine. The charged tRNA approaches the P – site on the small ribosomal subunit.

Elongation occurs with elongation of the peptide chain. Based on the codons on the mRNA, the tRNAs bring respective amino acids. New amino acids are added to the carboxyl end of the polypeptide chain, as the ribosome moves from the 5’end to the 3’end of the mRNA.

From the P-site, the initial amino acid moves to the A site, facilitated by elongation factors EF-Tu-GTP and EF-Ts or EF-1?-GTP and EF-1??. Hydrolysis of GTP is required.

Peptide bond between the existing amino acids and the incoming amino acids is formed by the peptidyl-transferase enzyme. The activity is intrinsic to the 23S rRNA of the large ribosomal subunit.

How chain elongation occurs?

The first amino acid binds to the P site of the ribosome. The second amino acid binds to the A site. Peptide bond forms between the two amino acids, and the first amino acid detaches from the tRNA. Now, both the amino acids are bonded, and are bound to the tRNA at the A site. The tRNA devoid of the amino acid moves off or dislocated, and the tRNA at the site, now moves to the P site. It has two amino acids bonded together. Now, the third charged tRNA comes to the A site with its respective amino acid. Peptide bond forms between the second amino acid at the P site and the new amino acid at the A site. The tRNA from the P site is kicked off, and the tRNA from the A site now moves to the P-site, with three amino acids. The same procedure repeats until a stop codon is encountered. Termination codons are UAA, UAG, or UGA.

The codons are recognized by factors called release factors (RF). Binding of release factors cause hydrolysis of the bond linking the peptide to the tRNA at the P site, and releases the nascent peptide from the ribosome. Eukaryotes use eRF for the release of nascent peptide from the ribosome.

After translation, several modifications occur to the nascent polypeptide. They are as follow:

• Reversible phosphorylation of serine, threonine, and/or tyrosine
• Glycosylation or addition of monosaccharides (to asparagine, serine, or threonine)
• Ubiquitination
• S-nitrosylation
• Methylation
• N-acetylation