Question

Explain (give at least two reasons) why bacterial protein would have a formly-methionine as their N-terminal amino acid.

Answer 1

• Reason 1
• Formylmethionine is derivative of amino acid Methionine. and it is found in all prokaryotic organisms.
• fMEt is located at N-terminus of the growing polypeptide chain.
• Bacteria uses formlymethionine in the place of  methionine in translation process.
• Methionine is start codon ,coded by AUG but prokaryotes utilize Formylmethionine in the place of methionine on tRNA.
• fMet is delivered to the 30S sub unit of the ribosome of mRNA complex by a specialized tRNA (tRNA^fMet), which has 3'- UAC - 5' as anticodon which can be binded with the start codon 5' - AUG - 3' located on the mRNA.
• Here, formyl group is added to Methionine's amino group, this addition is catalyzed by enzyme transformylase. This modification is done after Methionine is loaded on to tRNA^fMet by amino acyl tRNA synthetase.
• Methionine can be directly loaded onto tRNA^fMet or tRNA^Met.. However transformylase catalyzes addition of formyl group to methionine only if it is loaded onto tRNA^fMet, and not onto tRNA^Met. But this N-terminal fMet is mostly removed from the proteins post translation.
• However, Eukaryotes does not use Formylmethionine for the translation process.
• Reason 2
• Oligopeptides containing f-Met are released from mitochondria of damaged tissues and damaged bacteria can be used as an 'alarm signal'.