Homomethionine is a non-protein amino acid. Homomethionine is synthesized from methionine via chain elongation. Transamination of methionine first forms a 2-oxo acid. The 2-oxo acid is then extended by one methyl group by a condensation reaction, an isomerization reaction, and a oxidative decarboxylation reaction. The newly formed 2-oxo acid can be transaminated to homomethionine or undergo further cycles of condensation, isomerization and oxidative decarboxylation to form di, tri, tetra, penta, and hexahomomethionines. Mono, di, tri, tetra, penta, and hexahomomethionines are precursors for aliphatic glucosinolates biosynthesis in Arabidopsis.

The cytosolic recycled methionine (|FRAME: PWY-5041|), not the plastid-located de novo synthesized
methionine, is believed to be the substrate for methionine chain elongation and glucosinolate biosynthesis|CITS: [17056707]|.
First, the first enzyme in the chain elongation pathway, methionine-oxo-acid transaminase (AT-BCAT4), is
cytosolic. Second, the gene expressions of AT-BCAT4 and the two cytosolic methionine synthase, involved
in the SAM cycle, are strongly co-regulated.

Data source: RiceCyc