Formylmethionine
= Formylmethionine
\b[Formylmethionine], usually written fMet in bacterial initiation, is <methionine> whose <amino group> bears a <formyl group>, $\mathrm{-C(=O)H}$. The nitrogen modification makes an <amide>; it does not replace the sulfur-containing side chain. The free neutral L-form has <molecular formula> $\mathrm{C_6H_{11}NO_3S}$.<N-formyl-L-methionine in ChEBI>[CHEBI:16552, definition and chemical structure]{ref}
In canonical bacterial initiation, <methionine> is first attached to an <initiator transfer RNA> and then formylated on that carrier. <Bacterial initiation factor IF2> recognizes the formylated end during assembly at the <ribosomal P site>. Free <formylmethionine>, its tRNA-bound form and the initiating residue in a <peptide> are different chemical states.<Large-Scale Movements of IF3 and tRNA during Bacterial Translation Initiation by Tanweer Hussain and others>[Introduction, recognition of the formylmethionyl moiety by IF2]{ref}
An original atom balance clarifies the modification. Replacing one amino-nitrogen hydrogen in free <methionine>, $\mathrm{C_5H_{11}NO_2S}$, by $\mathrm{CHO}$ gives a net addition of $\mathrm{CO}$ and hence $\mathrm{C_6H_{11}NO_3S}$. The free-compound formula does not include the <transfer RNA> or account for its ester linkage.
Formylation alone is not a complete initiation signal. In a primary experiment, anticodon-stem variants of an <Escherichia coli> initiator were purified, aminoacylated and formylated. Some retained IF2 binding but lost efficient P-site binding and initiation. The carrier's <RNA> structure therefore contributes recognition information beyond its attached <formylmethionine>.<Escherichia coli formylmethionine tRNA: mutations in GGGCCC sequence conserved in anticodon stem of initiator tRNAs affect initiation of protein synthesis and conformation of anticodon loop by B. L. Seong and U. L. RajBhandary>[Abstract, separate initiation-factor binding, P-site binding and protein-synthesis assays]{ref}