Question

How would you describe the steps in protein synthesis?

Answer 1

Protein synthesis:

1. Protein synthesis(translation) is the production of a polymer of a chain of amino acids which produces a functioning protein.
2. It involves reading the information from mRNA (messenger RNA) to put together a chain of amino acids.
3. Ribosomes are the structures that synthesize the protein chain.
4. Protein synthesis requires a tRNA (transfer RNA) which helps in the transfer of an amino acid molecule to the growing chain.
5. tRNA has anti-codons for the codons present on the mRNA molecule which helps to recognize the place of single amino acid.
6. There are a total of 20 major common amino acids used in protein synthesis.
7. There are five major steps of protein synthesis.

Major steps of protein synthesis:

A. Activation of amino acids:

1. The activation of amino acids deals with the charging of tRNA and requires the enzyme aminoacyl RNA synthetase.
2. It helps in the formation of the aminoacyl – AMP – enzyme complex.
3. It yields the required energy from adenosine triphosphate (ATP).

B. Transfer of amino acids to tRNA:

1. In this step, the aminoacyl – AMP – enzyme complex gets completely charged by reacting with the specific tRNA.
2. The amino acid is transferred to the specific amino acid.
3. Due to the transfer process, the enzyme aminoacyl tRNA synthetase, and the AMP molecule leave activating the tRNA.
4. Hence the enzyme aminoacyl tRNA synthetase plays a major in the catalysis of covalently linking an amino acid to its cognate tRNA.

C. Initiation of polypeptide chain:

1. The charged activated tRNA moves towards the larger subunit of the ribosome which already is attached to the mRNA strand.
2. The mRNA strand is bound to the SOS sub-unit of ribosomes of 70S type.
3. The ribosome reads the codon on the mRNA and the tRNA has the anticodon complementary to the specific codon on the mRNA.
4. After specific recognition, the charged tRNA molecule enters the A-site (aminoacyl site) of the ribosome.
5. The initiation codon of every polypeptide chain in prokaryotes is always for the amino acid methionine which is coded by the codon AUG or GUG (for valine).
6. Further, the entry of successive new charged tRNA with another amino acid pushes the previously charged tRNA at the A-site to move into the P-site (peptidyl site).
7. There is peptide formation between both amino acids at the P-site and A-site.
8. The amino acid at the P-site gets removed from the tRNA and begins the formation of the peptide chain.
9. The tRNA at the P-site enters into the E-site (exit) of the ribosome to leave the ribosome molecule and create space for the entry of a new charged tRNA molecule at the A-site.
10. The cyclic process is repeated till the peptide chain grows and the mRNA is read until it reaches the termination codon.

D. Chain Termination:

1. Once the ribosome reaches one of the three termination codons, UAG (Amber), UAA (Ochre), and UGA (Opal), it falls off from the mRNA.
2. These codons are known as stop codons.
3. They signal the termination of the growing peptide chain and hence, stop translation.

E. Protein translocation:

1. There are different classes of ribosomes that assist in translation.
2. The free poly-ribosomes directly release the growing peptide chain into the cytoplasm after the complete protein has been synthesized upon the termination.
3. Whereas the membrane-bound polyribosomes do not release the protein chain directly into the cytoplasm, instead they divert the growing polypeptide chain into the lumen of ER for further post-translational modifications.