Correct option is B
The correct answer is: (2) Selenium added to a cysteine.
Explanation:
The addition of selenium to a cysteine residue during the synthesis of certain proteins is a co-translational modification. This means that the selenium modification occurs while the protein is being synthesized on the ribosome, as opposed to a post-translational modification that happens after the protein has been fully synthesized. The modification involves the incorporation of selenocysteine, an amino acid similar to cysteine, but with selenium replacing sulfur. This modification is essential for the function of certain enzymes, such as selenoproteins, that play roles in antioxidant defense and other processes.
Information Booster:
Co-translational modifications are modifications that occur while the protein is still being synthesized by the ribosome, often before the peptide chain is fully released.
The incorporation of selenium into cysteine is a co-translational modification specific to the synthesis of selenoproteins.
Selenocysteine is often referred to as the 21st amino acid and is inserted into proteins at specific sites, guided by a unique stop codon (UGA).
This co-translational modification is catalyzed by a specific machinery, including a specialized tRNA and associated proteins that insert selenocysteine at the correct position during translation.
Additional Knowledge:
Phosphate added to a serine (option 1) is a post-translational modification that usually occurs after the protein has been synthesized. It is commonly seen in signal transduction and enzyme regulation.
Acetyl added to a lysine (option 3) is also a post-translational modification that plays a crucial role in regulating protein function, particularly in the context of histone modification.
Methyl added to an arginine (option 4) is another post-translational modification. It is frequently involved in regulating the function of histones and other proteins.
