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A nonpolar sulphur-containing amino acid with the formula CH3S(CH2)2CH(NH2)COOH. Methionine is synthesized from aspartate via the nonprotein amino acid homoserine. Control of methionine synthesis is by feedback inhibition. Methionine is broken down to succinyl CoA, which can be further oxidized in the TCA cycle. The methyl group of methionine is important in the formation of a large number of methylated biomolecules. The actual methyl transferring molecule is not methionine itself but its ATP-activated derivative, S-adenosyl methionine. Methionine has also been implicated in the formation of the growth substance ethylene.
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