Formylmethionine
Formylmethionine (fMet) is a modified form of methionine in which a formyl group has been added to methionine's amino group. fMet is a starting residue in the synthesis of proteins in prokaryotes and, consequently, is located at the N-terminal of the polypeptide. fMet is delivered to the ribosome (30S) - mRNA complex by a specialized tRNA (tRNA.fMet) which has a 5'-CAU-3' anticodon that capable of binding with the AUG start codon located on the mRNA. The addition of the formyl group to methionine is catalyzed by the enzyme transformylase. This modification is done after methionine has been loaded on to tRNA.fMet by aminoacyl-tRNA synthetase.
Note that methionine can be loaded either onto tRNA.fMet or tRNA.Met. Transformylase will catalyze the addition of the formyl group to methionine only if methionine has been loaded onto tRNA.fMet and NOT onto tRNA.Met.
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